Homogenous Catalysis Notes PDF

Introduc)on to Catalysis: Catalysis refers to the process of increasing the rate of a chemical reac)on by adding a substance known as a catalyst. Catalysts are not consumed during the reac)on and can con)nue to accelerate successive chemical transforma)ons. (page 1) Homogeneous vs Heterogeneous Cata...

Introduc)on to Catalysis: Catalysis refers to the process of increasing the rate of a chemical reac)on by adding a substance known as a catalyst. Catalysts are not consumed during the reac)on and can con)nue to accelerate successive chemical transforma)ons. (page 1) Homogeneous vs Heterogeneous Catalysis: In homogeneous catalysis, the catalyst and reactants are in the same phase, usually both being in the liquid phase. In heterogeneous catalysis, the catalyst is in a different phase from the reactants - typically a solid catalyst is used with liquid or gas phase reactants. (page 26) Key concepts in catalysis include reac)on profiles, cataly)c mechanisms, cataly)c cycles, and catalyst deac)va)on. Precatalysts are oJen used that convert to the ac)ve cataly)c species. Off-cycle reac)ons can lead to catalyst deac)va)on over )me. (pages 1-2) Electron coun)ng and coordina)on numbers in organometallics refer to 18-electron rule and how ligands coordinate to the metal center in complexes. This affects their reac)vity. (pages 26-27) Reac)vity of organometallics involves oxida)ve addi)on, where a bond is broken and the atoms bond to the metal, and reduc)ve elimina)on, where atoms bonded to the metal form a new bond and are eliminated. Other reac)ons include inser)on, where a ligand inserts into a metal-ligand bond, and elimina)on. (pages 26-27) Reac)vity of organometallics: Oxida)ve addi)on and Reduc)ve Elimina)on Oxida)ve Addi)on (pages 23-25): Involves breaking of a two-electron A-B bond and forma)on of two new "M-X" bonds Requires two vacant coordina)on sites on the metal Increases the metal's oxida)on state and coordina)on number by two Generally occurs when the metal can achieve a more stable oxida)on state two units higher Reduc)ve Elimina)on (pages 23-25): The reverse of oxida)ve addi)on Involves breaking of two "M-X" bonds and forma)on of a new two-electron A-B bond Decreases the metal's oxida)on state and coordina)on number by two Generally occurs when the metal can achieve a more stable oxida)on state two units lower Both reac)ons involve a change in the metal's oxida)on state and coordina)on number. Oxida)ve addi)on increases them while reduc)ve elimina)on decreases them. Reac)vity of organometallics: Inser)on and Elimina)on Inser)on (pages 33-34): Involves a ligand migra)ng into a metal-ligand σ-bond Two types - 1,1-inser)on and 1,2-inser)on 1,1-inser)on changes coordina)on number by 0, 1,2-inser)on changes it by -1 OJen occurs through a planar transi)on state with the migra)ng ligand and ligand being replaced becoming coplanar Elimina)on (pages 33-34): Reverse of inser)on Involves breaking of a metal-carbon σ-bond and reforma)on of the ini)al ligandligand π-bond Changes coordina)on number by +1 Specifically: 1,1-Inser)on: Ligand inserts between metal and ligand in metal-ligand σ-bond (pages 38-42) 1,2-Inser)on: Ligand inserts into an adjacent metal-ligand σ-bond. Competes with β-H elimina)on (pages 46-50) β-Elimina)on: Ligand eliminates with hydride transfer (pages 46-50) The mechanism and factors influencing inser)on vs. elimina)on are important aspects of organometallic reac)vity. Hydrogena)on: Ac)va)on of dihydrogen using transi)on metals (pages 51-55): Dihydrogen is ac)vated at a transi)on metal center by σ-dona)on and πbackbonding Forms metal dihydrogen complexes which weaken and ac)vate the H-H bond Factors like oxida)on state, ligand environment affect binding mode and reac)vity Mechanisms of alkene hydrogena)on (pages 60-65): Oxida)ve addi)on of H2 to metal Migratory inser)on of alkene into metal-hydride bond Hydride delivery from same face or opposite face Steric and electronic factors influence stereoselec)vity Mechanisms of ketone hydrogena)on (pages 65-70): Can occur through metal hydride mechanism or via transfer hydrogena)on Transfer hydrogena)on uses sacrificial alcohol or formate as H+ source Allows use of less reac)ve catalysts under milder condi)ons Asymmetric hydrogena)on (pages 70-75): Uses chiral, enan)opure metal catalysts Differing energies of transi)on states for re and si-face ahack of H2 Can give high enan)oselec)vi)es (>99% ee) Homogeneous vs. Heterogeneous catalysis (pages 80-85): Homogeneous uses molecular metal complexes in solu)on Heterogeneous uses insoluble metal par)cles or surfaces Each have advantages depending on process needs key points about hydrogena)on and ac)va)on of dihydrogen: The hydrogena)on reac)on is thermodynamically favorable due to strong C=C and C=O pi bonds being broken. Direct addi)on of H2 to the unsaturated bond is symmetry forbidden. Hydrogena)on can occur through stoichiometric addi)on of hydride and proton or cataly)cally via transi)on metal ac)va)on of H2. Transfer hydrogena)on uses a sacrificial hydrogen source like an alcohol. Dihydrogen is a very stable molecule with a strong, non-polar H-H bond. Homoly)c cleavage requires a high bond dissocia)on enthalpy of 436 kJ/mol. Heteroly)c cleavage/deprotona)on has a very low pKa of 36, making H2 a very weak acid. This requires a reac)ve low coordinate transi)on metal to facilitate ac)va)on. Four outcomes are possible when H2 interacts with a transi)on metal: no reac)on, stable eta-2 H2 complex, bis-hydride complex forming from homoly)c cleavage, or mono-hydride complex from heteroly)c cleavage. Factors like the metal center oxida)on state, coordina)on number, ligand environment influence how dihydrogen binds and whether it undergoes homoly)c or heteroly)c cleavage. Regarding dihydrogen complexes like Kubas complexes: Iden)fica)on can be done through several methods: Bond lengths - The H-H bond is elongated/lengthened compared to free H2 due to bonding to the metal center. This is evidence of ac)va)on. NMR spectroscopy - The H-H coupling constant is reduced compared to free H2. This indicates weakening of the H-H bond through interac)on with the metal. Neutron diffrac)on - Can directly observe the posi)ons of hydrogen atoms and show an elongated H-H bond consistent with bonding to the metal in a dihydrogen complex. Specifically for Kubas complexes, the H-H bond length is around 0.80-0.90 Angstroms, significantly longer than 0.74 Angstroms in free H2. 1H NMR spectroscopy shows reduced coupling constants around 30-50 Hz compared to 164 Hz for H2 gas, indica)ng weakened and ac)vated dihydrogen bonding to the metal center. So in summary - longer H-H bond lengths from techniques like neutron diffrac)on and reduced 1H NMR coupling constants provide evidence of dihydrogen complex forma)on through metal-hydrogen bonding. Dihydrogen acts as both a σ-donor and π-acceptor ligand in dihydrogen complexes. The HOMO of H2 (the σ bond) par)cipates in σ-dona)on to the metal, dona)ng electron density. The LUMO of H2 (the σ* an)bonding orbital) par)cipates in π-backbonding from the metal, accep)ng electron density. This simultaneous σ-dona)on and π-backbonding results in popula)on of the σ*HH an)bonding orbital. This weakens the H-H bond by decreasing its order and elonga)ng the bond length. Early transi)on metals (d0 complexes) are not known to form stable dihydrogen complexes because they lack sufficient electron density for effec)ve π-backbonding interac)ons. Stable dihydrogen complexes oJen feature low oxida)on state transi)on metals. This favors π-backbonding through having more electrons in the metal's d-orbitals to donate into the σ*HH orbital. So in summary - dihydrogen acts as both a σ-donor and π-acceptor ligand through simultaneous σ-dona)on and π-backbonding. This requires sufficient electron density in the metal, explaining why only late transi)on metals form stable dihydrogen complexes. Regarding bis-hydride and mono-hydride complexes formed upon dihydrogen cleavage: "Homoly)c cleavage of H2 results in a bis-hydride complex containing two hydride ligands on the metal center" (L6 annotated.pdf, page 4) "Heteroly)c cleavage involves protona)on of one hydride ligand to give a monohydride complex containing an M-H bond and a protonated ligand" (L6 annotated.pdf, page 4) Factors influencing whether cleavage is homoly)c or heteroly)c include: "The metal's ability to stabilize nega)ve charge on the ligand following heteroly)c cleavage" (L6 annotated.pdf, page 4) "The metal's ability to stabilize an even electron configura)on following homoly)c cleavage" (L6 annotated.pdf, page 4) "Steric and electronic proper)es of the suppor)ng ligands" which can favor one mode of cleavage over the other (L6 annotated.pdf, page 4) So in summary, the metal and ligand environment determine whether dihydrogen cleavage occurs through homoly)c produc)on of a bis-hydride complex or heteroly)c produc)on of a mono-hydride complex. Expand on the steps: 1. Ru(II) complex is 16 VE due to agos)c H interac)on (p. 60) 2. Phosphine dissocia)on forms 14 VE Ru(II) complex (p. 60) 3. Alkene coordina)on to Ru forms 16 VE π-complex (p. 60) 4. 1,2 migratory inser)on of alkene into Ru-H bond forms 14 VE alkyl complex (p. 61) 5. Oxida)ve addi)on of H2 to 14 VE alkyl complex reforms 16 VE Ru hydride, with concomitant elimina)on of the alkane product. (p. 61) The mono-hydride mechanism involves key steps of alkene coordina)on, migratory inser)on, then H2 re-addi)on to regenerate the Ru-hydride complex and release the fully hydrogenated alkane. Key steps in Wilkinson's catalyst cataly)c cycle based on the informa)on provided: 1. Wilkinson's catalyst is 16 VE square planar Rh(I) (p. 62) 2. Dissocia)on of PPh3 ligand forms 14 VE Rh(I) (p. 62) 3. Rapid oxida)ve addi)on of H2 forms 16 VE octahedral Rh(III) hydride (p. 62) 4. Alkene coordina)on forms 18 VE complex (p. 62) 5. Rate determining 1,2 migratory inser)on step forms 16 VE product bound Rh(III) (p. 62) 6. Rapid dissocia)on of product allows catalyst turnover (p. 62) Rate law depends on [Rh] concentra)on as the catalyst (p. 62) Facile interconversion between Rh(I) and Rh(III) is necessary for fast catalysis (p. 62) PPh3 dissocia)on required to generate ac)ve catalyst species (p. 62) Detailed explana)ons of the Wilkinson's catalyst cataly)c cycle: Wilkinson's Catalyst Primary Kine)c Pathway (p. 62-63): 1. Rh(I)Cl(PPh3)3 is the pre-catalyst, with a square planar 16 VE d8 configura)on 2. Dissocia)on of one PPh3 ligand forms the 14 VE ac)ve catalyst species [Rh(PPh3)2Cl]+ 3. Rapid oxida)ve addi)on of H2 occurs to form the 16 VE octahedral Rh(III)-hydride complex 4. Alkene coordinates trans to chloride to form an 18 VE intermediate 5. Rate-determining migratory inser)on of alkene into the Rh-H bond forms a 16 VE Rh(III) alkyl complex 6. Product dissociates, regenera)ng the ac)ve 14 VE Rh(III) species Facile interconversion between Rh(I) and Rh(III) oxida)on states, as well as lability of PPh3, allows for fast turnover. Alterna)ve H2 Ac)va)on (p. 63): Pathways include σ-bond metathesis and oxida)ve addi)on to ligand π-systems, depending on ligand proper)es. Catalyst Deac)va)on (p. 63): Can occur through decomposi)on, poisoning by substrate impuri)es, or forma)on of inac)ve 16 VE Rh(III) complexes. Mono-hydride mechanism The full cycle: 1. Ru(II) complex is 16 VE due to agos)c H interac)on 2. Removal of PPh3 ligand forms 14 VE Ru(II) complex 3. Alkene coordinates to Ru, forming 16 VE π-complex 4. 1,2 migratory inser)on of alkene into Ru-H bond forms 14 VE alkyl complex 5. Coordina)on of H2 to 14 VE alkyl complex forms 16 VE Ru hydride 6. Migratory inser)on of hydride into Ru-alkyl bond forms Ru-alkane complex 7. Coordina)on of second H2 molecule followed by migratory inser)on forms fully hydrogenated alkane and regenerates star)ng Ru hydride complex Detailed explana)on of the mono-hydride cataly)c cycle for alkene hydrogena)on: 1. The star)ng catalyst is a 16 VE Ru(II) complex with an agos)c C-H interac)on, stabilizing the intermediate (L6 annotated.pdf, p. 60). 2. Dissocia)on of one PPh3 ligand forms a 14 VE Ru(II) complex, crea)ng a coordina)on site for alkene binding (p. 60). 3. The alkene coordinates to the metal center, forming a 16 VE π-complex (p. 60). 4. The rate-determining step is 1,2 migratory inser)on of the alkene into the Ru-H bond. This forms a 14 VE alkyl complex (p. 60). 5. Coordina)on of H2 to the 14 VE alkyl complex forms an 18 VE intermediate (p. 60). 6. Rapid migratory inser)on of the hydride into the Ru-alkyl bond forms a 16 VE Rualkane complex (p. 60). 7. Coordina)on of a second H2 molecule followed by migratory inser)on fully hydrogenates the alkane. This regenerates the star)ng Ru hydride catalyst (p. 60). The facile interconversion between oxida)on states and lability of ligands like PPh3 allows for rapid turnover of the catalyst (p. 60). Key steps in the olefin hydrogena)on mechanism: 1. 16 VE pre-catalyst irreversibly forms the ac)ve catalyst (p. 61) 2. Removal of bidentate ligand forms 12 VE catalyst via H2 addi)on (p. 61) 3. Addi)on of alkene forms 16 VE res)ng state complex (p. 61) 4. Rate determining step is addi)on of H2 and oxida)ve addi)on to form 18 VE Rh(III) intermediate. Sterically less hindered than alterna)ve pathways. (p. 61) 5. Stereochemically flexible 1,2 migratory inser)on forms 16 VE Rh(III) product complex. This flexibility allows facile product dissocia)on. (p. 61) Some addi)onal notes: Bidentate ligands and alkenes with EDGs promote this pathway (p. 61) Rate depends on [Rh] and [H2] concentra)ons (p. 61) Weakly dona)ng solvent helps stabilize 14e intermediate (p. 61) Detailed explana)on of the olefin hydrogena)on mechanism: 1. The pre-catalyst is the 16 VE square planar Rh(I) complex (L6 annotated.pdf, p. 61). 2. Removal of the bidentate cyclooctane ligand via H2 addi)on forms the 12 VE ac)ve catalyst species (p. 61). Bidentate ligands promote this pathway. 3. Coordina)on of the alkene substrate to the 12 VE Rh forms a 16 VE res)ng state complex (p. 61). Alkenes with EDGs also promote this mechanism. 4. The rate determining step is oxida)ve addi)on of H2 to the 16 VE Rh-alkene complex. This forms an 18 VE Rh(III) intermediate (p. 61). We prefer the 3 coordinate intermediate as it is less sterically hindered. 5. A stereochemically flexible 1,2 migratory inser)on of the alkyl group occurs, forming a 16 VE Rh(III) alkyl complex (p. 61). This flexibility allows facile release of the hydrogenated alkane product. 6. The rate depends on concentra)ons of Rh and H2 according to the rate law dp/dt = kobs[Rh][H2] (p. 61). 7. A weakly dona)ng solvent helps stabilize the 14 VE intermediate species (p. 61). Detailed explana)on of asymmetric hydrogena)on: Selec)vity is reported as enan)omeric excess (ee%), defined as %major - %minor enan)omer (p. 70). Chiral, enan)opure metal catalysts are used to differen)ate between pro-R and pro-S transi)on states for re-face vs si-face ahack of H2 (p. 70). Differing energies of these transi)on states leads to preferen)al forma)on of one enan)omer (p. 70). Example 1 (p. 71): Hydrogena)on of a ketone shows re-face addi)on of H2 to the alkene, installing the R configura)on at the newly formed stereocenter. Example 2 (p. 71): Hydrogena)on of an imine shows si-face addi)on of H2, installing the S configura)on at the stereocenter. The CIP priority rules determine whether R or S nomenclature applies based on ligand subs)tuents (p. 71). High enan)oselec)vi)es over 99% ee can be achieved by tuning the chiral catalyst to favor one transi)on state over the other (p. 70). Further explana)on of asymmetric hydrogena)on: The goal of asymmetric hydrogena)on is to selec)vely add H2 to a prochiral alkene or imine substrate to form one enan)omer of the product over the other. This is achieved through the use of a chiral, enan)opure transi)on metal catalyst. The catalyst controls the stereochemistry of product forma)on by differen)a)ng between the transi)on states for re-face vs si-face ahack of H2 during migratory inser)on. Due to its chiral environment, one transi)on state (TS) will be lower in energy than the other. For example, in the hydrogena)on of a ketone or aldehyde, re-face ahack of H2 during 1,2inser)on preferen)ally forms the R-product. This is because the re-face TS is favored due to steric and electronic interac)ons between the substrate and catalyst ligands. In contrast, for imines the favored si-face ahack of H2 forms the S-product. This is because in imines, the si-face TS is stabilized to a greater degree by the catalyst. The degree of selec)vity is reported as %ee, which is dependent on the difference in energies between the two TSs. Well-designed chiral catalysts can stabilize one TS significantly more than the other, achieving >99% ee. The CIP rules are then used to assign absolute stereochemistry (R or S) based on the catalyst ligand subs)tuents. How enan)oselec)vity is controlled in asymmetric hydrogena)on reac)ons. To summarize the key points: A chiral, enan)opure transi)on metal catalyst is used to differen)ate between the pro-R and pro-S transi)on states. The Cur)n-Hammeh principle applies when the substrate-catalyst intermediates A and B are in rapid equilibrium, as is oJen the case. In this situa)on, the product composi)on/selec)vity is determined predominantly by the difference in energies of the transi)on states (ΔΔG‡A-B), not the star)ng materials (ΔΔGA-B). In other words, the selec)vity/enan)oselec)vity is governed by the rela)ve stability of the transi)on states, i.e. the difference in their ac)va)on energies (Ea), rather than the ground states. By tuning the chiral environment of the catalyst, one transi)on state can be stabilized significantly more than the other, leading to high enan)oselec)vi)es. Detailed explana)on of enan)oselec)vity control in asymmetric hydrogena)on: Chiral, enan)opure metal complexes are used as catalysts to differen)ate between the pro-R and pro-S transi)on states (L6 annotated.pdf, p. 70). The Cur)n-Hammeh principle applies as the substrate-catalyst intermediates are in rapid equilibrium (p. 70). This means the selec)vity/enan)oselec)vity is determined by the difference in ac)va)on energies (ΔΔG‡) of the transi)on states rather than the ground states (p. 70). For example, in Noyori's Ru(II) catalyst system, the si-face transi)on state is stabilized rela)ve to the re-face by the bulky BINAP ligand (p. 72). This preferen)ally stabilizes approach of H2 to the si-face, leading to high Sselec)vity/ee in imine hydrogena)ons (p. 72). By tuning ligand sterics/electronics, the difference in transi)on state energies can be op)mized to achieve >99% ee (p. 70). Olefin hydrogena)on mechanism 1. The pre-catalyst Rh(I) complex is 16 VE (p. 63) 2. Removal of the cyclooctane ligand by addi)on of H2 forms the 12 VE ac)ve catalyst (p. 63). 3. Addi)on of the alkene substrate forms the 16 VE res)ng state complex (p. 63). 4. Addi)on of H2 and oxida)ve addi)on forms the 18 VE Rh(III) intermediate. This is the rate determining step since we prefer the more sterically accessible 3 coordinate intermediate (p. 63). 5. A stereochemically flexible 1,2 migratory inser)on forms the 16 VE Rh(III) product complex. This flexibility allows facile release of the hydrogenated alkane product (p. 63). More detailed explana)on of the olefin hydrogena)on mechanism 1. The pre-catalyst is commonly a 16 VE square planar Rh(I) complex with bidentate diphosphine ligands, as seen in [Rh(diphosphine)(diene)]+. This structure is best stabilized by a weakly dona)ng solvent (L7 annotated.pdf, p. 63). 2. Removal of the diene ligand by addi)on of H2 forms a 12 VE ac)ve catalyst intermediate (L7 annotated.pdf, p. 63). 3. Coordina)on of the alkene substrate to the 12 VE complex reforms the 16 VE res)ng state complex (L7 annotated.pdf, p. 63). 4. Addi)on of H2 leads to oxida)ve addi)on, forming an 18 VE Rh(III) intermediate. This step is rate determining as the 3-coordinate intermediate is most accessible (L7 annotated.pdf, p. 63). 5. A 1,2 migratory inser)on occurs in a stereochemically flexible manner, allowing facile product release upon forma)on of the 16 VE Rh(III) product complex (L7 annotated.pdf, p. 63). The rate equa)on dp/dt = kobs[Rh][H2] indicates the reac)on is first order in both [Rh] and [H2], as expected for a cataly)c cycle (L7 annotated.pdf, p. 63). Please let me know if you require any part of the explana)on to be further expanded or clarified based on informa)on in the provided PDFs. Detailed explana)on of asymmetric hydrogena)on ci)ng informa)on from the provided resources: Selec)vity is reported as enan)omeric excess (ee%), defined as %major enan)omer %minor enan)omer (L6 annotated.pdf, p. 70). Chiral, enan)opure transi)on metal complexes are used as asymmetric hydrogena)on catalysts. The catalyst controls the stereochemistry of product forma)on (L6 annotated.pdf, p. 70). In the hydrogena)on of a prochiral ketone substrate, re-face addi)on of H2 to the carbon-carbon double bond occurs preferen)ally. This installs the R configura)on at the newly formed stereocenter (L6 annotated.pdf, p. 71). By contrast, in the hydrogena)on of a prochiral imine, si-face addi)on of H2 is favored. This results in the S configura)on being installed at the stereocenter (L6 annotated.pdf, p. 71). The Cur)n-Hammeh principle explains how the chiral catalyst environment can significantly stabilize one transi)on state diastereoselec)vely, leading to high ee values over 99% in some cases (L6 annotated.pdf, p. 70). Detailed explana)on of the control of enan)oselec)vity through the use of chiral catalysts and the Cur)n-Hammeh principle, ci)ng informa)on from the provided resources: A chiral, enan)opure transi)on metal catalyst [M] is used to control the stereochemistry (L6 annotated.pdf, p. 65). When a prochiral substrate reacts with the catalyst, two diastereomeric substratecatalyst intermediates can form: [M]-substrate (A) and [M]-substrate (B) (L6 annotated.pdf, p. 65). The Cur)n-Hammeh principle applies when A and B are in rapid equilibrium, which is oJen the case as there is a small energy barrier between the two intermediates (L6 annotated.pdf, p. 65). Under these condi)ons, the product ra)o is determined predominantly by the difference in energies of the transi)on states (ΔΔG‡A-B), not the star)ng materials [M]-substrate (A) and [M]-substrate (B) (L6 annotated.pdf, p. 65). In other words, the selec)vity is governed by the rela)ve stability of the transi)on states (TS), represented by the ac)va)on energy barrier Ea, rather than the rela)ve stability of the star)ng materials (L6 annotated.pdf, p. 65). This is because the system has sufficient )me at the TS to dis)nguish between diastereotopic faces, leading to high enan)oselec)vity (L6 annotated.pdf, p. 65). Detailed explana)ons of the different reac)on profiles: 1. Same barrier height for transi)ons states A and B, and a small energy difference between the products. Therefore, the system has equal probability to pass through either pathway, resul)ng in a 50:50 product ra)o (L6 annotated.pdf, p. 66). 2. Pathway A has a lower product energy than pathway B. Addi)onally, the barrier for pathway A (ΔG‡A) is smaller than for pathway B (ΔG‡B). Therefore, more of the system will pass through pathway A, resul)ng in a predominant forma)on of product PA over PB (L6 annotated.pdf, p. 66). 3. of product PA over PB (L6 annotated.pdf, p. 66). 4. The transi)on states and products for pathways A and B are iden)cal in energy. Therefore, the system has an equal probability to pass through either pathway, resul)ng again in a 50:50 ra)o of products PA and PB (L6 annotated.pdf, p. 66). 5. The star)ng products PA and PB (L6 annotated.pdf, p. 66). 6. The star)ng materials [M]-substrate A and [M]-substrate B are not in fast interconversion, viola)ng the Cur)n-Hammeh assump)on. This means the rela)ve stabili)es of A and B determine the product ra)o, instead of just the transi)on states. Therefore a mixture of products PA and PB is obtained. (L6 annotated.pdf, p. 66) Chiral chela)ng ligands (p. 58): Chiral ligands are needed to ac)vate metal catalysts in an enan)oselec)ve manner. Examples given include BINAP, DIOP, and Duphos ligands. Visualizing steric effects with a quadrant diagram (p. 58): The diagram depicts the steric bulk around the coordina)on pocket For R,R-Duphos: High steric bulk is in the upper leJ region, lower steric bulk in the upper right region This creates an asymmetric environment for coordina)on of prochiral substrates Case study: enamide hydrogena)on (p. 71): The amide group is electron dona)ng, allowing selec)ve C=C hydrogena)on over carbonyl Hydrogen is added to the Si face, genera)ng the R product as this approach is to the back of the complex The alkene must bind via the Si face to the metal center Looking at quadrants to understand thermodynamic preference for Re face (p. 71): Calculated profiles show the transi)on state for the S product is much higher in energy than the R product The Re face is forced into steric clash aJer H2 addi)on, while the Si face has only minor conforma)onal changes, lowering its absolute barrier So in summary, while the Re face is ini)ally thermodynamically preferred, the Si face transi)on state has a lower barrier, making it the kine)cally favored (minor) product (p. 71). Hydrogena)on of carbonyl compounds can proceed by either an inner-sphere or outersphere mechanism (p. 72): Inner-sphere: Involves direct coordina)on of the unsaturated substrate to the metal center (p. 72) Alkenes are reduced more readily than ketones due to their stronger coordina)ng double bond (p. 72) Outer-sphere: There is no direct coordina)on of the unsaturated substrate to the metal (p. 72) Ketones are reduced more readily than alkenes because the carbonyl group polarizes the C=O double bond, making it more suscep)ble to nucleophilic ahack (p. 72) Detailed explana)on for the outer sphere hydrogena)on ionic mechanism ci)ng informa)on from L8 annotated.pdf: 1. W(2), 18 VE complex is the res)ng state of the catalyst (p. 73) 2. Oxida)ve addi)on of H2 forms the W(2), 16 VE dihydride complex via loss of 2 electrons (p. 73) 3. The dihydride undergoes heteroly)c cleavage to form a W(4), 18 VE ca)onic dihydride complex (p. 73) 4. The ketone substrate is basic enough to deprotonate the acidic dihydride, forming a W(2), 18 VE alkyl complex (p. 73) 5. Deprotona)on makes the ketone more electrophilic, allowing it to undergo migratory inser)on with the alkyl group to form the secondary alcohol product (p. 73) 6. Product release then regenerates the res)ng state catalyst (p. 73) Please let me know if any part of this outer sphere ionic mechanism needs further explana)on or clarifica)on based on the provided text. Further explana)on of the outer sphere hydrogena)on ionic mechanism: Tungsten is used as the catalyst due to its ability to stabilize high oxida)on states The res)ng state W(2) complex is 18 VE, allowing it to undergo oxida)ve addi)on of H2 Oxida)ve addi)on is facilitated by W's vacant d-orbitals, which accept electrons from the breaking H-H σ bond This forms the 16 VE dihydride complex, with hydrides in bridging posi)ons Heteroly)c H-H cleavage occurs because W favors dona)ng electron density to hydrides over dihydrogen This generates a ca)onic W(4) dihydride complex, with the hydrides now terminally bonded The ketone substrate acts as a proton acceptor, with its lone pair dona)ng into the vacant d-orbitals of W This forms an 18 VE alkyl complex and regenerates the electrophilic ketone substrate The ketone can then insert into the W-alkyl bond, facilitated by its polariza)on and electrophilic nature Product release and protona)on returns the catalyst to its ini)al 18 VE state to con)nue the cycle Detailed explana)on of the outer sphere hydrogena)on bifunc)onal mechanism: 1. The Ru remains as Ru(II) throughout, with a concerted transi)on state (p. 74) 2. The precatalyst is a Ru(II), 18 VE complex containing an amine ligand. Deprotona)on of the amine yields an amino group but Ru remains as Ru(II) (p. 74) 3. This forms the res)ng state Ru(II), 18 VE complex (p. 74) 4. Heteroly)c cleavage of dihydrogen is the rate-determining step, forming a Ru(II), 18 VE intermediate (p. 74) 5. The transi)on state involves concerted outer sphere hydride and proton transfer from the Ru center and amine ligand respec)vely (p. 74) 6. This is facilitated by the bifunc)onal ligand (LX) containing both amine and phosphine func)onali)es (p. 74) 7. Products are released regenera)ng the Ru(II) star)ng material (p. 74) Addi)onal explana)on of the outer sphere hydrogena)on bifunc)onal mechanism: Ru(II) is used as it can stabilize mul)ple oxida)on states, facilita)ng oxida)ve addi)on The amine ligand acts as a proton relay group in the transi)on state Upon heteroly)c H-H cleavage, hydrides are terminally bonded to low-valent Ru(II) This generates two vacant coordina)on sites on Ru, making it electrophilic The adjacent basic amine ligand deprotonates to form an amino group In the concerted transi)on state, the amino group and hydride transfer occur simultaneously This is facilitated by the bifunc)onal phosphine-amine ligand holding the groups in close proximity The phosphine moiety stabilizes the 16e Ru(II) intermediate through sigma dona)on The substrate coordinates to Ru through the vacant site and amino group This posi)ons it for stereospecific hydride and proton transfer in the transi)on state Regenera)on of the Ru(II) star)ng material readies it for another cataly)c cycle Ru(II) promotes heteroly)c cleavage and the ligand facilitates the concerted outer sphere mechanism Key differences between the bifunc)onal and ionic hydrogena)on mechanisms: Bifunc)onal mechanism: Involves a bifunc)onal ligand (e.g. phosphine-amine) that facilitates H-H cleavage Heteroly)c cleavage of H2 at the metal center forms a metal hydride Amino ligand acts as a proton relay group Transi)on state involves concerted outer sphere hydride and proton transfer between metal hydride and amino group Amino ligand stabilizes the transi)on state geometry Ionic mechanism: Does not involve an amino ligand Homoly)c cleavage of H2 forms separate hydrogen ca)ons One hydrogen ca)on deprotonates the carbonyl group, making it more electrophilic The other hydrogen ca)on then ahacks the electrophilic carbon Transi)on state involves direct ahack of a free hydrogen ca)on on the carbonyl carbon No amino ligand to stabilize transi)on state geometry So in summary, the bifunc)onal mechanism u)lizes heteroly)c H2 cleavage and a transi)on state stabilized by an amino ligand, while the ionic mechanism proceeds through homoly)c cleavage and lacks an amino ligand to stabilize the transi)on state. Detailed summary of the key points: Transi)on metal catalysts are required to ac)vate H2 via heteroly)c or homoly)c cleavage (L6 annotated.pdf, p. 65) The metal and ligand composi)on determines the extent and mechanism of H2 ac)va)on (L6 annotated.pdf, p. 65) Alkene hydrogena)on typically proceeds through inner sphere pathways exploi)ng strong alkene coordina)on (L6 annotated.pdf, p. 70) Mechanisms include hydride, mono-hydride and oxida)ve addi)on/reduc)ve elimina)on pathways involving MI reac)ons of hydride ligands (L6 annotated.pdf, p. 70) Carbonyl hydrogena)on is most effec)vely mediated through outer sphere pathways, exploi)ng the polarity of the C=O bond (L6 annotated.pdf, p. 72) This involves concerted hydride and proton transfer following heteroly)c H2 cleavage (L6 annotated.pdf, p. 72) Bifunc)onal ligands can facilitate this through a proton relay effect (L6 annotated.pdf, p. 72) Transfer hydrogena)ons are possible under thermodynamic control (reversible reac)ons) (L6 annotated.pdf, p. 81) Inner vs outer sphere coordina)on influences chemoselec)vity (L6 annotated.pdf, p. 81) Stereoselec)vity can be achieved using chiral ligands like diphosphines or diamines to give diastereomeric intermediates or transi)on states respec)vely (L6 annotated.pdf, p. 81) The Cur)n-Hammeh principle can then determine selec)vity (L6 annotated.pdf, p. 81) Hydroformyla)on of alkenes: The process adds the elements of formaldehyde (H2CO) to an alkene substrate (p. 83) Cobalt-catalysed hydroformyla)on was first developed in the 1930s, with rhodium catalyst systems developed later in the 1970s (p. 83) Key issues are linear to branched (normal to iso) selec)vity, as mixtures are typically formed - the linear aldehyde is usually the more valuable product (p. 83) For internal alkenes (e.g. 2-hexene), the linear aldehyde products can isomerize to the branched isomer (p. 83) Regarding the ques)ons: Bulky ligands promote the forma)on of a linear metal alkyl intermediate. This is because bulky ligands favor the 1,2-inser)on step that leads to the linear product (p. 83) Rhodium catalyst systems have similar selec)vity for hydroformyla)on over hydrogena)on compared to Wilkinson's catalyst, as they possess a similar 16e square planar catalyst (p. 83) Detailed explana)on of Rh-catalyzed hydroformyla)on The Rh catalyst system has higher cataly)c ac)vity than Co, approximately 1000 )mes more ac)ve (p. 105). Rh catalysts are also much more stable and "greener" without need for a phosphine ligand (p. 105). The ac)ve catalyst is similar to Wilkinson's catalyst, containing a 16e square planar Rh(I) complex (p. 83). The mechanism proceeds through a metal carbonyl hydride intermediate formed by oxida)ve addi)on of H2 and CO (p. 84). Migratory inser)on of the alkene substrate into the Rh-H bond forms an alkyl-Rh species (p. 84). A second migratory inser)on of CO then forms the acyl-Rh intermediate (p. 84). Reduc)ve elimina)on releases the aldehyde product and regenerates the Rh-hydride catalyst (p. 84). Bulky phosphine ligands can be used to control selec)vity, favoring the linear isomer through steric effects (p. 83). Detailed explana)on of cobalt-catalyzed hydroformyla)on: The precatalyst is a 16e octahedral Co(I) carbonyl hydride complex (species 1 on p. 83) This converts to the ac)ve 18e octahedral Co(III) alkyl hydride complex (species 2 on p. 83), which is the res)ng state as determined by IR spectroscopy. The rate equa)on shows the reac)on is first order in alkene and CO concentra)on, and inverse first order in H2 concentra)on (p. 83). Based on the rate equa)on, the migratory inser)on of the alkene into the Co-H bond is likely the rate-determining step (p. 83). Increasing CO pressure improves selec)vity but the exact reason is unclear - it may favor forma)on of the ac)ve 18e Co(III) complex over decomposi)on (p. 84). Ligand modifica)on can be used to control selec)vity - bulky phosphines favor the linear isomer in the 1,2-inser)on step (p. 83). Off-cycle reac)ons can occur through alkene isomeriza)on or migratory inser)ons of CO or alkenes (p. 87). Analysis of the Co-catalyzed hydroformyla)on mechanism including iden)fying the ratedetermining step (RDS): 1. The precatalyst is a 16e Co(I) carbonyl hydride complex. 2. Radical oxida)ve addi)on of H2 forms an 18e Co(III) dihydride complex, which IR data indicates is the res)ng state. 3. Alkene subs)tu)on occurs to form a 18e alkyl-hydride complex. 4. Migratory inser)on of the alkene into the Co-H bond forms a 16e alkyl complex. This is iden)fied as the RDS based on: The rate equa)on showing first order dependence on alkene concentra)on Migratory inser)ons typically being slower than other steps 5. CO coordina)on forms an 18e alkylcarbonyl complex. 6. Dissocia)on of CO could also be contribu)ng to the inverse dependence on [CO] in the rate equa)on, making it a possible alterna)ve RDS. 7. Reduc)ve elimina)on of the aldehyde product regenerates the res)ng state. In summary, based on kine)c data and typical reac)on step kine)cs, the most likely RDS is the 1,2-migratory inser)on of the alkene into the Co-H bond in step 4. Dissocia)on of CO in step 6 may also contribute to the observed rate dependence on factors. Alkene isomeriza)on is a key considera)on for hydroformyla)on and other alkene func)onaliza)on reac)ons. Alkene feedstocks are commonly mixtures of isomers, with internal alkenes predomina)ng over primary alkenes (p. 87). However, primary alkyl intermediates formed from the less stable primary alkenes undergo migratory inser)on faster than those from internal alkenes (p. 87). This is because internal alkenes are the thermodynamically more stable isomers, but primary alkyls can adopt a conforma)on in the transi)on state for migratory inser)on with less steric hindrance (p. 87). Many processes involving alkylhydride intermediates formed from alkenes, like the hydroformyla)on of 3, are subject to alkene isomeriza)on or "chain walking" between primary and internal forms (p. 87). This isomeriza)on competes with desired func)onaliza)on reac)ons and can limit selec)vity for linear over branched products (p. 87). Addi)onal details on alkene isomeriza)on: Alkene isomeriza)on occurs through a hydride migra)on mechanism, involving migratory inser)on of an alkyl ligand into a metal-hydride bond to form a new alkylmetal species (p.87) This allows the alkene subs)tuents to rearrange posi)ons in a "1,2 shiJ" along the carbon chain (p.87) The rate of isomeriza)on is dependent on factors like sterics and subs)tu)on pahern of the alkene - more subs)tuted/branched alkenes isomerize more slowly (p.87) Isomeriza)on competes with the desired func)onaliza)on reac)on (e.g. hydroformyla)on) for coordina)on to the metal center (p.87) It reduces selec)vity for linear products by allowing branched isomers to form, which then also react to give more branched aldehyde products (p.87) Sterically hindered catalysts/ligands can slow isomeriza)on rates and improve selec)vity by disfavoring the transi)on states involved (p.87) Temperature and pressure condi)ons can also influence the rates of isomeriza)on vs. desired reac)ons (p.87) Key details regarding phosphine-modified Co hydroformyla)on catalysts from the provided text: Shell researchers found adding phosphine ligands gave much higher catalyst stability but lower ac)vity (p. 88). Higher stability arises because phosphines par)cipate in sigma dona)on to the metal center, enhancing back-bonding from the metal to CO. This strengthens the metal-CO bonds (p. 88). Lower ac)vity likely results because dissocia)on of CO is a key step in forming the 7' intermediate from 8, and phosphines stabilize the higher oxida)on state which disfavors CO loss (p. 88). Reduc)ve elimina)on forming the aldehyde product may not be favored since phosphines stabilize the higher oxida)on state as well (p. 88). The rate determining step could change as a result of phosphine modifica)on. Phosphines cause hydrogena)on to become a more important side reac)on (p. 88). Linear selec)vity is enhanced because bulkier ligands promote forma)on of the linear metal alkyl intermediate in the 1,2-inser)on step (p. 88). Expanded explana)on of phosphine-modified Co hydroformyla)on catalysts ci)ng more details from the provided text: Phosphines like trialkylphosphines are commonly used as ligands (p. 88) They stabilize intermediate 8 by sigma dona)on into the low-lying Co d-orbitals, strengthening Co-CO bonds (p. 88) This makes 8 kine)cally more stable compared to the 16e hydridocobaltcarbonyl 2, slowing its forma)on rate (p. 88) Dissocia)on of the first CO from 8 is believed to form the 14e 7' intermediate necessary for alkene binding - phosphines hinder this key step, lowering ac)vity (p. 88) Reduc)ve elimina)on of aldehyde may proceed through 7' but phosphines disfavor its forma)on, slowing this turnover-limi)ng step under normal condi)ons (pp. 88-89) However, under higher pressures phosphine-modified catalysts show beher stability as CO compe))ve binding is reduced (p. 89) Bulky phosphines like tri-o-tolylphosphine impart steric hindrance, promo)ng linear 1,2-inser)on over branched isomeriza)on (p. 89) This enhances selec)vity for normal aldehydes over branched isomers (p. 89) Hydrogena)on competes more due to phosphine bulk hindering alkene coordina)on and func)onaliza)on (p. 89) Detailed explana)on of the Rh-catalyzed hydroformyla)on mechanism based on the provided informa)on: 1. At low H2 pressures, Rh(I) dimers form which are inac)ve (p. 94). 2. The precatalyst is a 18 VE square planar Rh(I) complex with bidentate phosphine ligands (p. 94). 3. Dissocia)on of one phosphine ligand forms the 16 VE ac)ve catalyst species Rh(I)(PPh3)2X (p. 94). 4. Coordina)on of alkene forms an 18 VE intermediate. This step is rate determining at low phosphine concentra)ons and high H2 (p. 94). 5. A 1,2 migratory inser)on forms the 16 VE Rh(alkyl) intermediate. This step is rate determining at higher phosphine concentra)ons (p. 94). 6. Rapid and reversible CO coordina)on forms the 18 VE Rh(acyl) species (p. 94). 7. Reduc)ve elimina)on of the aldehyde regenerates the 16 VE Rh(I) catalyst (p. 94). 8. Higher oxida)on state 18 VE Rh(III) species also form via 1,1 migratory inser)on (p. 94). Some key advantages of Rh catalysts include higher ac)vity, lower temperatures needed, more tunable regioselec)vity and less hydrogena)on side reac)ons (p. 94). More details on each step of the Rh-catalyzed hydroformyla)on mechanism: 1. At low H2 pressures, inac)ve dimeric species [(Rh(PPh3)2X)2] form via bridging hydrides. Increased H2 breaks this dimer (p. 94). 2. Precatalyst is square planar Rh(I)(PPh3)2X. PPh3 ligands are labile but stabilize the 16e state (p. 94). 3. Dissocia)on of one PPh3 forms the 14e ac)ve catalyst Rh(PPh3)X. This favors alkene binding over CO (p. 94). 4. Alkene coordinates in an η2-fashion to Rh(PPh3)X forming an 18e intermediate. Steric effects of PPh3 influence regioselec)vity (p. 94). 5. A 1,2 migratory inser)on of the alkene into the Rh-H bond forms the alkyl-Rh intermediate. This step competes with β-H elimina)on or isomeriza)on (p. 94). 6. Rapid and reversible CO binding occurs to the 16e alkyl-Rh species to form an acyl-Rh complex via backbonding (p. 95). 7. Reduc)ve elimina)on of the aldehyde occurs readily due to the electronwithdrawing acyl group stabilizing the 16e state (p. 95). 8. Alterna)vely, a 1,1 migratory inser)on can occur with internal alkenes to form an alkyl-Rh intermediate which can isomerize (p. 95). 9. Higher valent 18e Rh(III) hydrido species also form via oxida)ve addi)on of H2 or dissocia)on of CO/alkene (p. 95). Monsanto Process Details: Rate law shows first order dependence on both [Rh] and [MeI], consistent with the proposed mechanism (L10 annotated.pdf, p. 101) 1. The res)ng state is 16 VE Rh(I) formed by reduc)ve elimina)on of CH3COI from 4 (p. 101) 2. Nucleophilic oxida)ve addi)on of methyl iodide to Rh(I) forms an 18 VE Rh(III) intermediate. This is the rate determining step (slower than OA) (p. 101) 3. Rapid 1,1 migratory inser)on of the alkyl group forms the 16 VE Rh(III) product (p. 101) 4. CO coordina)on to 3 forms an 18 VE, octahedral Rh(III) acyl complex (p. 101) 5. Loss of iodide from 4 reforms the neutral 5 coordinate res)ng state complex (p. 101) High CO pressures help stabilize the catalyst against decomposi)on, limi)ng forma)on of inac)ve Rh(III) species (p. 101) The iodide anion promotes the oxida)ve addi)on step by facilita)ng electron transfer (p. 101) More detailed steps for the Monsanto process mechanism ci)ng informa)on from L10 annotated.pdf: 1. The 16e square planar Rh(I) complex is the res)ng state catalyst (p. 101) 2. Oxida)ve addi)on of methyl iodide (generated from MeOH and HI) occurs to the Rh(I) center. This is facilitated by the iodide anion and forms an 18e octahedral Rh(III)-CH3I intermediate (p. 101). This step is rate determining. 3. Rapid 1,1-migra)on of the methyl group occurs, facilitated by the electron withdrawing acyl group. This forms the 16e 5-coordinate Rh(III)-acetyl intermediate (p. 101). 4. CO coordinates in the axial posi)on to form the 18e octahedral Rh(III)-acyl complex. This stabilizes the intermediate (p. 101). 5. Iodide dissociates to reform the 16e square planar Rh(I) res)ng state and release acetyl iodide (p. 101). High CO pressures (30-70 atm) help limit decomposi)on of the catalyst to inac)ve Rh(III) species during turnover (p. 101). More detailed explana)on of the second stage of the WGSR-Monsanto process mechanism ci)ng informa)on from L10 annotated.pdf: 6. Rh(III) 18VE complex is formed by oxida)ve addi)on of HI. Using high concentra)ons of water helps regenerate the catalyst by reversing this step. Otherwise insoluble RhI3 sludge forms. (p. 102) 7. CO coordinates to 6, forming the 18VE Rh(III)-acyl complex. This makes the carbon more electrophilic than in 6. (p. 102) 8. Water then ahacks the CO-bonded carbon in 7. This is the rate-determining step of the water-gas shiJ. (p. 102) 9. Oxida)ve addi)on of HI to the resul)ng formyl-Rh(I) complex regenerates 6, restar)ng the cataly)c cycle. (p. 102) The net reac)on is the water-gas shiJ reac)on, with the Rh catalyst catalyzing both the forward CO + H2O reac)on and reverse H2 + CO2 reac)on via intermediates 1 and 6. (p. 102) Consuming CO to form 7 would limit the amount of substrate available. Maintaining a high CO par)al pressure would help drive the equilibrium towards CO coordina)on. Expanded explana)on of the second stage of the WGSR-Monsanto process mechanism: 6. Rh(III) 18VE complex is formed by oxida)ve addi)on of HI. This converts the catalyst to its inac)ve form. Using high concentra)ons of water helps regenerate the ac)ve Rh(I) catalyst by reversing this deac)va)on step through reduc)ve elimina)on of HI. Otherwise, insoluble RhI3 sludge forms, permanently deac)va)ng the catalyst. 7. CO rapidly and reversibly coordinates to the 18VE Rh(III) center in 6, forming an octahedral acyl-Rh(III) complex. The electron-withdrawing acyl group pulls electron density away from the metal center, making the carbon bound to Rh more electrophilic and suscep)ble to ahack compared to the carbon in 6. 8. Water then performs a nucleophilic ahack on the polarized carbon-oxygen π-bond of the coordinated CO ligand. This forms an Rh-bound formyl intermediate and is the rate-determining step of the overall water-gas shiJ reac)on. 9. Oxida)ve addi)on of HI to the 16e formyl-Rh(I) complex formed in step 8 regenerates the original 18e Rh(III)-HI complex 6, restar)ng the cataly)c cycle. Maintaining high CO and H2O par)al pressures helps drive the reversible equilibrium towards the forma)on of reac)ve intermediates like 7, while regenera)ng the Rh catalyst, allowing the WGSR to proceed to comple)on. Ca)va Process 1. 16 VE Ir(I) res)ng state complex, stabilized by nega)ve charge (p. 107) 2. Oxida)ve addi)on of CH3I forms the 16 VE Ir(III) intermediate (p. 107) 3. CO coordina)on forms an 18 VE Ir(III)-acyl complex. This is the rate determining step (p. 107) 4. Rapid 1,1 migratory inser)on of the methyl group occurs, forming the 16 VE Ir(III) acetyl complex. This step is 700x faster than the previous step due to increased electrophilicity of CO in the 3 complex vs the anionic 2' complex. (p. 107) The key advantages of the Ca)va process are using CH3I instead of HI to avoid iodide inhibi)on, and performing hydroacyla)on in one step vs the separate hydroformyla)on and condensa)on in the Monsanto process. Maintaining low water concentra)ons helps drive the equilibrium to products by favoring hydrolysis of ace)c acid over unreacted aldehyde. Expanded details on the Ca)va process mechanism: 1. The res)ng state is a 16 electron, square planar Ir(I) complex stabilized by the nega)ve charge of the halide ligand (typically chloride or iodide). This electron-rich complex favors oxida)ve addi)on. 2. Oxida)ve addi)on of methyl iodide occurs rapidly, forming an 18e octahedral Ir(III)CH3 intermediate complex (species 2). The Ir center is now more electrophilic and suscep)ble to nucleophilic ahack. 3. CO coordinates to the vacant site on Ir(III), displacing the halide and forming an octahedral Ir(III)-acyl complex (species 3). This step of CO coordina)on is now ratedetermining, as the acyl group further increases the electrophilicity of the Ir center. 4. Rapid 1,1-migratory inser)on of the methyl group into the Ir-C(O) bond occurs, facilitated by the strong electron-withdrawing nature of the acyl ligand. This forms the 16e Ir(III)-acetyl intermediate complex and releases the halide (species 4). 5. Hydrolysis promotes C-C bond cleavage and releases ace)c acid, regenera)ng the ac)ve 16e Ir(I) complex to restart the cataly)c cycle. Iodide plays a dual role, facilita)ng OA/MI steps but also inhibi)ng via halide poisoning, necessita)ng its removal. The key advantages over Monsanto are the use of CH3I to avoid halide inhibi)on and performing hydroacyla)on in a single step vs separate steps. Detailed comparison of the Monsanto and Ca)va processes: Monsanto Process (Rh catalyst): Uses HI as a co-catalyst and involves nucleophilic oxida)ve addi)on (OA) of MeI to form an organometallic Rh intermediate. This OA step of MeI to the 16e Rh(I) complex is the rate determining step (RDS), as it is rela)vely slow. Subsequent 1,1-migratory inser)on (1,1-MI) of the alkyl group into a carbonyl ligand is faster. Ca)va Process (Ir catalyst): Also uses HI as a co-catalyst and forms organometallic intermediates through OA of MeI. However, the OA of MeI to the 16e Ir(I) complex is approximately 150x faster than for Rh due to Ir's higher oxida)on state and the greater nucleophilicity of iodide. As a result, the normally faster 1,1-MI step of an alkylcarbonyl intermediate now becomes the RDS, being over 100,000x slower than the ini)al OA step for Ir. In summary, while both involve HI and OA of MeI, the significantly faster OA for Ir in Ca)va shiJs the RDS to the subsequent 1,1-MI, whereas OA of MeI remains rate limi)ng for Monsanto's Rh catalyst. Wacker Process (p. 101-102): The overall reac)on is: C2H4 + 1/2O2 → CH3CHO The reac)on involves nucleophilic addi)on of O2 to the alkene π-bond to form an alkylhydroperoxide intermediate. Subsequent protonolysis leads to the thermodynamically favorable acetaldehyde product. When the metal has a low electron count and free site, the second mechanism of inner sphere hydroxypallada)on can occur (p. 102): Water can add to the alkene ligand in an inner sphere fashion, where the substrate coordinates directly to the metal center. This is contrasted with the outer sphere hydroxypallada)on mechanism described on p. 102-103 that does not involve direct substrate coordina)on. Key details about the Wacker process mechanism: The outer sphere an) mechanism and inner sphere syn mechanism are both consistent with the rate laws, making the mechanism controversial. Recent computa)onal analysis favors the outer sphere mechanism where water adds in an an) fashion to the ethylene ligand, which is trans to Pd. The rate law is: Rate = -d[C2H4]/dt = k[PdCl42-][C2H4]/[H3O+][Cl-]2 In step 2→3 or 3', the trans effect governs. The order of ligand strength is: Halogen < O < N < C. Therefore, 3 is favored where O is trans to Cl, both kine)cally (lower barrier) and thermodynamically (more stable by 60 kJ/mol). This allows the inner sphere mechanism to occur depending only on kine)cs if the barrier is low enough for water to add in the cis posi)on to ethylene. The key steps that allow cataly)c turnover are: 1. Rapid β-elimina)on in step 6 to regenerate the ac)ve Pd(II) species 2. Rapid dissocia)on of aldehyde product in step 8 rather than slower β-elimina)on 3. Rapid subs)tu)on of Cl- for aldehyde in step 9 to reform the ac)ve Pd(II) complex Oxida)on of Pd(0) back to Pd(II) by Cu(II) prevents decomposi)on to palladium black. Please let me know if any part of the mechanism or cataly)c cycle needs more explana)on! Key steps of the Wacker process mechanism: 1. Forma)on of the ac)ve Pd(II)-ethylene complex (species 1): Oxida)ve addi)on of O2 to Pd(0) forms Pd(II)-peroxo which then loses 2 Cl- ligands. Ethylene then coordinates trans to the peroxo ligand. 2. Outer sphere hydroxyla)on (species 2 and 3): Ethylene subs)tu)on followed by rapid nucleophilic ahack of water trans to ethylene in an SN2-like fashion. Water is a beher ligand than Cl due to trans effect. 3. Cis hydroxyla)on (species 3'): May compete via an inner sphere mechanism where water adds cis to ethylene, but is higher in energy. 4. Hydroxypallada)on (species 4): Nucleophilic addi)on of hydroxide to the alkene π-bond. A key step in the cataly)c cycle. 5. β-hydride elimina)on (species 5 to 6): Elimina)on of H-OH regenerates the ac)ve Pd(II)-ethylene complex and aldehyde product. A rapid step that drives the reac)on forward. 6. Product dissocia)on and regenera)on of Pd(0) (species 7 to 9): Aldehyde readily dissociates followed by reduc)ve elimina)on of HCl and oxida)on back to Pd(II) to restart the cataly)c cycle. Outer sphere: Species 3 is neutral, so there is less pi backbonding than in anionic species 2. This makes the carbon atom more electrophilic and suscep)ble to nucleophilic ahack. (p. 103) Inner sphere: Direct coordina)on of the alkene increases the electrophilicity of the carbon via pi dona)on into the empty d-orbitals of Pd. (p. 103) The addi)on of hydroxyl and deprotona)on occur in a concerted step, with a single transi)on state. (p. 103) Computa)onal results show the barrier for this concerted inner sphere process is around 125 kJ/mol higher than the stepwise outer sphere pathway. (p. 103) Even though thermodynamically favorable due to alkene coordina)on, the high kine)c barrier means the inner sphere process only competes effec)vely if subsequent steps in the cataly)c cycle are also fast. Beta elimina)on: Releases the aldehyde product and regenerates the ac)ve Pd(II) complex, driving the reac)on forward kine)cally. (p. 103) Oxida)ve addi)on: OA of O2 to Pd(0) is facilitated by the vacant coordina)on site trans to Cl. This allows O2 to bind in a side-on η2 fashion. Forma)on of the ini)al Pd(II)-peroxo intermediate weakens the O-O bond for heteroly)c cleavage. Ethylene coordina)on: Ethylene binds trans to the peroxo ligand due to its weaker trans influence compared to Cl. This displaces one O atom to form the ac)ve 16e Pd(II)-ethylene-hydroperoxo species 1. Outer sphere hydroxyla)on: The SN2-like ahack of H2O occurs with inversion of configura)on at the reac)ng carbon. Water is a beher nucleophile than Cl due to its higher polarizability and ability to u)lize an empty d-orbital to stabilize the transi)on state. Hydroxypallada)on: Nucleophilic addi)on of hydroxide occurs with reten)on of configura)on, forming a 5-membered palladacycle intermediate 4. This ring strain provides the driving force for the subsequent β-hydride elimina)on step. Reduc)ve elimina)on: Loss of HCl from intermediate 7 occurs with reforma)on of the Pd(0) oxida)on state. This step is reversible and favors aldehyde dissocia)on due to Le Chatelier's principle. Oxida)ve addi)on (L10 annotated.pdf p. 104): OA of O2 occurs in a side-on η2 fashion, facilitated by the vacant site trans to Cl. This allows the O-O σ* orbital to overlap with the empty Pd dz2 orbital, lowering the barrier. Ethylene coordina)on (L10 p. 104): Ethylene binds trans to the peroxo due to its weaker trans influence vs. Cl. This is evidenced by IR data showing a shiJ in the O-O stretch frequency upon ethylene binding in a related system.1 Hydroxyla)on (L10 p. 104): DFT calcula)ons show the SN2-like transi)on state has an energy barrier of ~20 kcal/mol, consistent with experimental kine)cs.2 The polar pro)c H2O is beher able to stabilize the buildup of nega)ve charge on the alkene π* orbital through hydrogen bonding interac)ons in the TS. Hydroxypallada)on (L10 p. 104-105): The 5-membered ring intermediate is higher in energy due to puckering strain, providing driving force for β-hydride elimina)on as evidenced by computa)onal studies.3 Alkene metathesis: Alkene metathesis requires a transi)on metal catalyst, most commonly Ru or Mo complexes (L12 annotated.pdf, p. 118). Cross metathesis allows for the interchange of alkylidene subs)tuents between two different alkenes (L12 p. 118). Ring-opening metathesis polymeriza)on (ROMP) uses ring-strained cyclic alkenes that release ring strain upon polymeriza)on (L12 p. 118). Ring-closing metathesis (RCM) forms new carbon-carbon bonds to create cyclic alkene products from acyclic precursors (L12 p. 118). Reac)ons are promoted by relieving ring strain through cycloalkane forma)on or loss of ethylene to form more stable alkylidene complexes (L13 annotated.pdf, p. 124). Alkene metathesis mechanisms: The key cataly)c cycle involves forma)on of a metal alkylidene species through dissocia)on of the alkene ligand (L12 p. 119). This ac)ve alkylidene complex can then: Undergo homoly)c cleavage of the M=C bond to generate a metal-alkyl and alkylidene carbene intermediate. These can recombine with another alkene in a [2+2] cycloaddi)on to perform cross-metathesis (L12 p. 119). Insert into another alkene in an [2+2] cycloaddi)on to form a metallacyclobutane intermediate. Elimina)on then regenerates the alkylidene and produces the new alkene products (L12 p. 119). ROMP occurs similarly but the ring-strained cyclic alkene rapidly inserts to form a growing metallocycle un)l termina)on (L13 p. 124). RCM involves forma)on of a smaller metallacycle that can then eliminate to form the new cyclic alkene product (L13 p. 124). Mechanism of olefin metathesis: Chauvin proposed the first mechanis)c model involving forma)on of a stable fourmembered metallacyclobutane ring intermediate (L12 annotated.pdf, p. 117). The double bonds (C=C π bonds) are broken exclusively rather than weaker C-C σ bonds, which seems thermodynamically unfavorable given bond energies (C=C 610630 kJ/mol vs C-C 350-360 kJ/mol) (L12 p. 117). However, forma)on of the stable four-membered metallacyclobutane ring intermediate makes the process overall favorable by stabilizing the transi)on state (L12 p. 117). The intermediate allows for stereospecific crossover to occur between the two alkene π-systems (L12 p. 117). N-heterocyclic carbene (NHC) ligands like IMes are commonly used as they favor the forma)on of stable 16e σ-alkylidene complexes in the cataly)c cycle (L12 p. 117). Addi)onal details on the Chauvin mechanism of olefin metathesis: The ac)ve catalyst is a 16-electron σ-alkylidene complex formed by dissocia)on of one of the alkene π-bonds from the parent metal alkylidene (L12 p. 117) Coordina)on of another alkene occurs with regio- and stereoselec)vity dictated by the metal center and ligand environment (L12 p. 117) A [2+2] cycloaddi)on then takes place between the coordinated and bound alkenes to form the key four-membered metallacyclobutane ring intermediate (L12 p. 117) This ring closes in a suprafacial manner, stabilizing the transi)on state through conjuga)on between the metal d-orbitals and the π-system of the ring (L12 p. 117) Reduc)ve elimina)on then occurs, breaking the metal-carbon bonds to regenerate the alkylidene catalyst and form the new alkene products (L12 p. 117) Stereochemical informa)on is retained throughout due to the concerted nature of the transforma)on in the cyclic transi)on state (L12 p. 117) Subsequent cycles can then catalyze further alkene conversions like cross-metathesis (L12 p. 119) Homogeneous alkene metathesis catalysts: Shrock Catalyst: Well-defined alkylidene complexes based on Mo and Ta (L12 p. 118) Very high ac)vity but commercially expensive (~£550/mmol for Mo) (L12 p. 118) Modest func)onal group tolerance due to oxophilic nature of Mo (L12 p. 118) Very air and moisture sensi)ve complexes (L12 p. 118) Grubbs Catalyst 1st Genera)on: Ru-based alkylidene complex stabilized by PCy3 ligands (L12 p. 118) More affordable than Shrock (~£60/mmol) (L12 p. 118) Improved func)onal group tolerance vs Shrock (L12 p. 118) Air and moisture stable to some extent (L12 p. 118) Grubbs Catalyst 2nd Genera)on: Ru-based complex with NHC ligands like IMes instead of PCy3 (L12 p. 118) Even more ac)ve and tolerant than 1st gen Grubbs (~£290/mmol) (L12 p. 118) Most stable in air and moisture due to chela)ng NHC ligands (L12 p. 118) Details on homogeneous alkene metathesis catalysts: Shrock Catalyst: Well-defined 16-electron alkylidene complexes of Mo and Ta in the +6 oxida)on state (L12 p. 118) Features chela)ng alkyl ligands like 2,2'-bipyridine that stabilize the low coordina)on number required for reac)vity (L12 p. 118) However, the oxophilic nature of Mo and Ta means they are suscep)ble to degrada)on by pro)c impuri)es (L12 p. 118) Extremely air and moisture sensi)ve due to the lability of alkylidenes and propensity of Mo/Ta to form oxo complexes in the presence of O2 or H2O (L12 p. 118) Grubbs 1st Genera)on: Ru-based complex with PCy3 ligands that stabilize the 14e Ru center while retaining reac)vity of the alkylidene (L12 p. 118) More func)onal group tolerant than Shrock likely due to less oxophilic, more electron-rich Ru metal center (L12 p. 118) However, PCy3 ligands can be displaced by pro)c impuri)es or air/moisture over )me (L12 p. 118) Grubbs 2nd Genera)on: NHC ligands like IMes provide stronger σ-dona)on and are not as readily displaced as PCy3 (L12 p. 118) This enhances stability while maintaining high ac)vity through stabiliza)on of the 16e Ru alkylidene intermediate (L12 p. 118) Shrock Carbene Complexes: Feature triplet carbene ligands complexed to early TMs like Mo, Ta in high oxida)on states (L12 p. 112) Carbene carbon is bound to the metal via two covalent σ-bonds, resembling dihydrogen complexes (L12 p. 112) R groups are typically hydride or alkyl, stabilizing the triplet state through induc)ve dona)on (L12 p. 112) Nucleophilic reac)vity due to vacant p-orbital on carbene carbon (L12 p. 112) Fischer Carbene Complexes: Singlet carbene ligands complexed to late TMs like Rh, Ir in lower oxida)on states (L12 p. 112) Carbene carbon is stabilized by π-backbonding and possesses a vacant p-orbital for da)ve bonding (L12 p. 112) R groups are oJen aryl, alkoxy, amino to favor π-dona)on over π-backbonding (L12 p. 112) Electrophilic reac)vity due to par)al posi)ve charge on carbene carbon (L12 p. 112) Grubbs Catalysts: Ru alkylidene is bonded head-on to the alkene ligand, resembling both Shrock and Fischer types (L12 p. 112) Ru favors π-backbonding to stabilize the 16e intermediate (L12 p. 112) Key steps in the Grubbs-type catalyst mechanism with more details: 1. The pre-catalyst is a 14e Ru complex with PCy3 ligands (L12 p. 114) 2. Dissocia)on of one PCy3 ligand forms the reac)ve 4-coordinate 14e Ru species (L12 p. 114) 3. Coordina)on of an alkene substrate generates the 16e Ru alkylidene species (L12 p. 114) 4. A [2+2] cycloaddi)on between the Ru=C double bond and another alkene forms a metallacyclobutane, regenera)ng the 14e Ru (L12 p. 114) 5. Cycloreversion of the metallacyclobutane, favored by π-backbonding from Ru, forms the new R-subs)tuted alkene and regenerates the 16e Ru alkylidene (L12 p. 114) 6. Product alkene dissociates, releasing ethene and regenera)ng the 14e Ru to begin another cycle (L12 p. 114) 7. Coordina)on of THF solvent molecule prevents irreversible coordina)on of alkenes or phosphines, maintaining high turnover (L12 p. 114) More details on the individual steps of the Grubbs catalyst mechanism: 1. The Ru pre-catalyst is a 14e square planar complex with PCy3 ligands in the cis posi)ons. 2. Dissocia)on of one PCy3 creates a coordina)on site, genera)ng the reac)ve 14e Ru species. This occurs rapidly and is reversible. 3. Coordina)on of the alkene substrate to the vacant site forms a 16e π-complex. This step is also rapid and reversible. 4. A [2+2] cycloaddi)on between the Ru=C double bond and the π-bond of another alkene substrate forms a 7-membered metallacyclobutane ring. This step is slower. 5. Cycloreversion of this metallacyclobutane ring, facilitated by π-backbonding from the Ru, generates the new R-subs)tuted alkene product. It also regenerates the 16e Ru alkylidene species. 6. Dissocia)on of the alkene product releases it from the metal, reforming the 14e Ru intermediate. 7. Coordina)on of THF prevents irreversible binding of reactants or phosphines, maintaining catalyst turnover. It does not par)cipate directly in the mechanism. The key reversible steps are ligand dissocia)on/associa)on and alkene coordina)on/dissocia)on. The rate-determining step is the [2+2] cycloaddi)on/cycloreversion. Expand on the points about the ini)a)on steps: The ethoxy subs)tuent is less π-dona)ng than phenyl due to the heteroatom. This makes the ethoxy-carbene more electrophilic/Fischer-like and less prone to [2+2] cycloaddi)on. For both Grubbs 1 and 2, the first step is reversible dissocia)on of one PCy3/NHC ligand to form the 14e reac)ve species. The k1 rate constants for this step are similar between Grubbs 1 and 2, indica)ng it is not rate-determining. The key difference is in k-1/k2, the ra)o of rates of the [2+2] cycloaddi)on vs alkene dissocia)on. For Grubbs 1, the PCy3 ligands are strong σ-donors but weak π-backbonders, disfavoring cycloaddi)on. This makes k-1/k2 larger. For Grubbs 2, the NHC is a beher π-acceptor that stabilizes the 16e intermediate and promotes cycloaddi)on via the metal. This makes k-1/k2 smaller. More detail on the ini)a)on steps of Grubbs-type catalysts: 1. Both Grubbs 1 and 2 exist as 16e complexes with the PCy3/NHC ligands in the solid state. 2. The first step is rapid, reversible dissocia)on of one ligand (PCy3 for Grubbs 1, NHC for Grubbs 2) to form a 14e reac)ve species. o For Grubbs 1: k1 = 10-2 M-1s-1 o For Grubbs 2: k1 = 10-4 M-1s-1 o Rates are similar, so this step is not rate determining. 3. Coordina)on of the alkene substrate forms a 16e π-complex. o This step (k2) is also rapid and reversible. 4. The π-complex can either: A) Dissociate the alkene (k-2), regenera)ng the 14e species B) Undergo [2+2] cycloaddi)on (k3) 5. The ra)o k-1/k3 determines whether cycloaddi)on or alkene dissocia)on occurs. o For Grubbs 1: k-1/k3 = 102 M-1 due to weak π-acceptor PCy3 ligands For Grubbs 2: k-1/k3 = 10-2 M-1 due to beher π-acceptor NHC stabilizing 16e state Therefore, the NHC in Grubbs 2 promotes cycloaddi)on by stabilizing the 16e π-complex, making it the rate-determining step. Let me know if any part needs more details! More detail on improvements to the Grubbs-type catalyst: Piers 14VE catalyst: Features a bulky tris(o-biphenyl)phosphine ligand which prevents dimeriza)on and increases ac)vity/stability (L12 p. 116). Grubbs-Hoveyda catalyst: Contains an alkylidene linkage between the Ru center and an alkylidene carbon, blocking one face of the metal and favoring monomer forma)on over dimeriza)on. Also increases func)onal group tolerance. Comparison: Piers 14VE and Grubbs-Hoveyda catalysts show higher ac)vity than 2nd gen Grubbs due to preven)ng undesirable dimeriza)on pathways. Grubbs-Hoveyda has beher func)onal group tolerance than Piers 14VE due to electronic effects of the alkylidene linkage blocking one face of Ru. Both are more stable and selec)ve than 2nd gen Grubbs, making them preferable for industrial applica)ons and reac)ons of sensi)ve func)onalized alkenes. o More detail on alkyne metathesis: Alkyne metathesis is less developed compared to alkene metathesis due to the higher reac)vity of alkynes. It is catalyzed by group 6 carbyne complexes containing Mo or W. These complexes feature a metal-carbon triple bond. The mechanism proceeds via a similar [2+2] cycloaddi)on/cycloreversion pathway to alkene metathesis: 1. Coordina)on of the first alkyne substrate to the metal-carbon triple bond. 2. [2+2] Cycloaddi)on between the coordinated alkyne and the metal-bound alkyne ligand generates a metallacyclopentadiene intermediate. 3. Cycloreversion leads to dissocia)on of one alkyne and reforma)on of the metalcarbon triple bond, ready to catalyze further transforma)ons. A representa)ve catalyst is [(tBuO)3W≡C-C(Me)=CPh] which features the tungsten carbene. The higher oxida)on state of W(6) compared to Ru(0) stabilizes the metal-carbon triple bond and promotes the [2+2] cycloaddi)on step. More details on the mechanism of alkyne metathesis: The key steps are: 1. Coordina)on of the first alkyne substrate to the metal-carbon triple bond of the catalyst. This forms a 16-electron π-complex. 2. [2+2] Cycloaddi)on between the coordinated alkyne and the metal-bound alkyne ligand. This generates a 14-electron metallacyclopentadiene intermediate with a 5membered ring. 3. The metallacycle can undergo one of two pathways: i) Cycloreversion - Direct reforma)on of the metal-carbon triple bond and dissocia)on of one alkyne product. This regenerates the 16e catalyst. ii) Cycloisomeriza)on - Carbon-carbon bond rota)on within the metallacycle, followed by reforma)on of the metal-alkyne bond and dissocia)on of an isomerized alkyne product. 4. The rate of each step and the stability of intermediates/products determines whether the reac)on proceeds by cross-metathesis or self-metathesis. Steric and electronic factors around the metal center influence the equilibrium between these pathways. Bulkier ligands generally favor cross-metathesis by disfavoring cycloisomeriza)on. More detail notes on the stereochemistry of polyolefins produced by coordina)on polymeriza)on: Polyethylene (PE) produced by Ziegler-Naha polymeriza)on is atac)c, having random stereochemistry Polypropylene (PP) can exhibit three main types of stereochemistry: 1. Isotac)c: All methyl subs)tuents on the chiral carbons are on the same side of the polymer backbone. Produced using chiral catalysts. 2. Syndiotac)c: Alterna)ng placement of methyl groups on opposite sides of the backbone. 3. Atac)c: Random placement of methyl groups with no stereoregularity, similar to PE. The terms "isotac)c" and "syndiotac)c" refer to the same stereochemical arrangement, with "syndio" being the IUPAC nomenclature. Isotac)c PP has the highest crystallinity and mel)ng point due to the regular packing of methyl groups on the same side. Syndio/atac)c have lower crystallinity. Ziegler-Naha catalyst design aims to control the stereochemistry, with chiral ligands favoring isotac)city through steric effects in the transi)on state. More detail notes on heterogeneous Ziegler-Naha polymeriza)on: Reactants: TiCl4 supported on MgCl2, AlEt3 or AlR3 co-catalyst, 25-50°C, 1-5 bar ethylene or propylene pressure (L13 p. 121) Early transi)on metal catalysts are used, most commonly )tanium (Ti) The heterogeneous catalysts are supported on chlorided magnesium (MgCl2), which helps dispersing the ac)ve Ti species (L13 p. 121) Stereoselec)vity favors forma)on of isotac)c polypropylene (iPP) due to the chiral, asymmetric ac)ve sites (L13 p. 121) Produc)vity of up to 104 g PP per gram catalyst has been achieved with op)mized systems (L13 p. 121) Polyethylene forma)on produces high-density polyethylene (HDPE) with a high degree of branching using Ziegler-Naha (L13 p. 121) The Al-alkyl acts as a co-catalyst, ac)va)ng the Ti-Cl sites through alkyl group transfer to form an ac)ve Ti-alkyl species (L13 p. 121) More detail on the Cossee-Arlman mechanism for isotac)c polypropylene forma)on: The mechanism involves enan)omorphic site control using a chiral, asymmetric metal catalyst (L13 p. 122) Propylene coordinates to the metal center in a η3-bonding mode (L13 p. 122) 1,2-metallate ion (MI) forma)on occurs by migratory inser)on of the metal-alkyl bond into the metal-carbon π-bond of the coordinated alkene (1,2-MI) (L13 p. 122) This generates a chiral center at the α-carbon of the growing PP chain (L13 p. 122) Subsequent monomer inser)ons and β-hydride elimina)ons propagate the chain while maintaining the same chiral orienta)on due to the enan)omorphic sites on the metal (L13 p. 122) Steric clashes between the methyl group and incoming alkene/alkyl chain favor one orienta)on over the other, locking in isotac)c stereochemistry (L13 p. 122) Later itera)ons of the mechanism incorporated agos)c interac)ons of the methyl group to beher explain stereocontrol (L13 p. 122) More detail notes on the different ways a polymeriza)on chain can terminate in ZieglerNaha polymeriza)on: 1. Beta-hydride elimina)on - the most important pathway. It forms a planar 4membered transi)on state and eliminates an ethylene molecule, termina)ng the chain (L13 p. 123). 2. Zimmerman-Traxler transi)on state - involves 1,2-alkyl shiJ followed by beta-hydride elimina)on. Less common termina)on pathway (L13 p. 123). 3. Transmetalla)on reac)ons - transfer of the growing polymer chain to another metal center, such as from Ti to Al (L13 p. 123). 4. As you men)oned - hydrogen can be added to induce chain termina)on by betahydride elimina)on or sigma-bond metathesis where H displaces the polymer chain (L13 p. 123). This controls molecular weight by limi)ng chain growth. Other less common pathways include reduc)ve elimina)on or migratory decarbonyla)on but the above four capture the major termina)on mechanisms involved (L13 p. 123). More detail on metallocenes and MAO: Metallocenes: General polymeriza)on cycle involves alkene coordina)on to the metal center, leaving an empty coordina)on site (L13 p. 126) The agnos)c interac)on of the alkyl group )lts it towards the incoming alkene, favoring the geometry for migratory inser)on (1,2-MI) into the metal-carbon bond (L13 p. 126) This can be seen in the transi)on state structure (L13 p. 126) The gamma-agos)c interac)on of the product alkyl group with the metal center can slow beta-hydride elimina)on and chain termina)on (L13 p. 126) MAO: Par)al hydrolysis of AlMe3 produces MAO (methylaluminoxane) as the co-catalyst (L13 p. 127) MAO contains aluminoxane polymers, rings, clusters and some free AlMe3 formed from the reac)on of AlMe3 with water (L13 p. 127) It is not fully understood but thought to alkylate the metallocene chloride precursor, abstract a "methide" anion to form a ca)onic metal center, and provide a noninterfering counterion to promote reac)vity (L13 p. 127) More details on the func)ons of MAO and issues with it: MAO is expensive and wasteful, requiring 200-1000 equivalents which leaves excess Al residues in the polymer without an expensive workup (L13 p. 127). MAO is also pyrophoric due to the presence of AlMe3 (L13 p. 127). The key func)ons of MAO as an ac)vator/co-catalyst are (L13 p. 127): 1. Alkylate the metal chloride precatalyst 2. Abstract a "methide" anion (Me-) from the metal center, leaving a highly electrophilic alkyl ca)on 3. Provide a non-interfering counterion (from the aluminoxane polymers/rings/clusters) that does not interfere with the electrophilic alkyl ca)on 4. Scavenge the system for any pro)c impuri)es that could compete with or poison the catalyst So in summary, MAO alkylates the metal, forms a ca)onic ac)ve site, stabilizes it with a counterion, and removes impuri)es - but it is expensive and leaves aluminum residues without special workup. Considerable research has focused on finding cheaper, more welldefined alterna)ves to MAO. More details on stoichiometric ac)vators: B(C6F6)3 (L13 p. 127) leads to rela)vely low catalysis because it is not fully dissociated from the metal center and is par)ally coordinated, leaving only one free coordina)on site in the cis posi)on for substrate binding. [Ph3C][B(C6F5)4] (also known as "trityl borate" or "tritylborate", L13 p. 127) is a more weakly coordina)ng anion that forms a more ac)ve ca)onic catalyst upon reac)on with the metal chloride. The nega)ve charge is distributed across the whole anion molecule rather than localized. [HNMe2Ph][B(C6F5)4] (L13 p. 127) par)ally coordinates to the ac)ve catalyst, which has the effect of slowing down the reac)on rate compared to [Ph3C][B(C6F5)4] as it occupies a coordina)on site. In terms of chain end vs. enan)omorphic site control (L13 p. 130): Enan)omorphic site control is preferable and is the domain of homogeneous catalysts where the chiral environment can be closely controlled. Heterogeneous catalysts rely more on chain end control since only the surface is chiral and bulk effects can reduce selec)vity. More details on ansa-metallocene catalysts: Ansa-metallocenes have the cyclopentadienyl rings connected together by a bridging group (L13 p. 129). This prevents free rota)on of the rings, meaning chiral C2-metallocenes can be readily synthesized by adding subs)tuents to specific posi)ons on the rings to control stereochemistry (L13 p. 129). Examples include Brintzinger compounds with tetrahydroindenyl ligands, which lead to high isotac)city in PP polymeriza)on due to the constrained chiral environment (L13 p. 129). The bridge pulls the Cp rings back and opens the coordina)on site slightly, which increases the reac)vity of the metal center for olefin inser)on (L13 p. 129). Constrained geometry catalysts without chirality give fast polymeriza)on but poor tac)city control due to the symmetric environment (L13 p. 129). In terms of isotac)c and syndiotac)c PP mechanisms (L13 p. 132): Isotac)c PP results from inser)on occurring on the same side of the growing chain, maintaining the same chiral center. Syndiotac)c PP occurs when the monomer inserts on alternate sides, giving a racemic mixture of chiral centers along the chain. Steric effects and steric matching of the catalyst/monomer control the tac)city.