homogeneous-catalysis-hydrogenation-carbonyls-metathesis-and-polymerisation.pdf

lOMoARcPSD|6077384 Homogeneous catalysis, hydrogenation, carbonyls, metathesis and polymerisation Advanced Inorganic Chemistry and Laboratory (The University of Warwick) Studocu is not sponsored or endorsed by any college or university Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcP...

lOMoARcPSD|6077384 Homogeneous catalysis, hydrogenation, carbonyls, metathesis and polymerisation Advanced Inorganic Chemistry and Laboratory (The University of Warwick) Studocu is not sponsored or endorsed by any college or university Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Week 1 – Adrian Lecture 1:  R.S = lowest energy combination of the catalyst and rxn components – but note the cyclic nature of the reaction  Activation free energy = energy difference between R.S. and T.S. of RDS = highest energy combination of the catalyst and rxn components  R.S. doesn’t have to be at the start of catalytic cycle  Activation energy (Delta G) of catalysed reaction (S-CAT) has to be less than uncatalyzed reaction  Any intermediate can be considered a catalyst and they can: o Can change rate of reaction o Position of egm o Will be eventually consumed  Pre-catalyst – a stable compound from which a ‘catalytically active’ species is generated, i.e. using an activator or by reaction with a substrate or during the reaction/after product formed or an off-cycle species formed reversibly  Pre-catalyst to catalyst is irreversible (usually) and is the induction period where catalyst builds up over time  Off-cycle species – generated from the catalytic intermediates and not directly implicated in the productive cycle  Consider chemoselectivity and regioselectivity Lectures 2-5: - covalent bond and valence electrons  Covalent bond classification (CBC) and electron accountancy o MOELDS + 18 VE rule o Oct complexes  t2g orbitals are non-bonding so EtI  Second order kinetics  Rate increases with leaving group stability R-X: X = OTs > I > Br > Cl >> F  Rate increases with use of electron donating ligands (such as phosphines PR3).  Negative delta S‡ (ordered transition state)  Inversion of stereochemistry at the electrophile carbon centre, trans kinetic product expected  Promoted by polar solvents - Highly polar transition state  Second step is RDS -combination o Characteristics of radical OA  Chain: rate increases with light, radical propagation kinetics, change in OS and CN= +1  Non chain: SET in group 10 metals, change in OS and CN = +2  Reductive elimination reactions o 1x 2e A-B bond formed o 2x 2e M-X bond broken Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 o 2 vacant coordination sites produced o Metal needs to be stable at an OS 2 units lower as change in OS tends to be -2 o Characteristics In oct complexes:  Favours large ligands in oct complexes + non-electron donating (stabilises the sm)  Cis RE – retention of stereochemistry  Eliminating species: hydrides quite favoured, aldehydes  Third row, like Ir metal, probably not going to undergo RE as they form strong bonds o Characteristics in d8 complexes:  Eliminating species needs to be cis  3 pathways: Dissociative (A), direct (B) and associative (C)  Insertion and elimination reactions o 1,1 MI  18ve to 16ve  OS can be -2 or 0  CO moves: Me always cis to 13CO and there will be a product without 13CO  Alkyl moving is the choice however, as there is a 2:1 cis:trans product mixture, which is found experimentally.  Cis processes – forward and backward have to be the same  Stereochemistry at carbon retained in forward and backward o 1,1 elimination  Reverse of 1,1 MI o 1,2-migratory beta insertion and stereochemistry  CN= n + 2 (n amount of ligands)  Planar t.s. o b-elimination  Free coordination site required  Planar t.s.  Prevention: not having beta-H, saturating the coordination site, preventing planarization at the transition state i.e. beta-H within a ring cannot reach around to coordinate to a vacant site, no free coordination site essentially  Sometimes the coordinated CH is an intermediate or in ground state, rather than a TS (agostic interaction) o Agostic interaction:  Intramolecular interaction between CH bond and metal: 3 centre bond with 2e  2x pathways – ground state or intermediate Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384  Strength = 40-65 kmol-1, more than H bond  Weakening carbon bond:  CH elongated by 5-20%, M--H 1.8-2.4A, M-H-C 90-140 0  CH NMR often in hydride region (0ppm or higher field) and J=70-100Hz --- 125Hz for regular C(sp3)  IR CH=2500cm-1 vs normal 2950cm-1 o Frontier orbital picture for 1,2-insertion  Early transition metals, sigma donation mainly as the alkene behaves like a donor – more efficient than later TM as no competition with back bonding  Later transition metals, more pi back bonding o Competition between alkyl migration and b-elimination  Is the agostic interaction more favourable than beta elim in the first step – leading to beta elimation further down the reaction chain Lecture 6-8 – Homogeneous hydrogenation  Activation of diydrogen by transition metals  Mechanisms  Chemo selective and stereoselective reactions The hydrogenation reaction      Favourable thermodynamics Direct addition of H2 is symmetry forbidden Stoichiometric addition of hydride and proton Catalytic activation via TM Transfer hydrogenation Activation of dihydrogen     Very strong and stable molecule – completely non polar H-H bond Bond dissociation/homolytic cleavage : change in enthalpy = 436 kjmol-1 Deprotonation/heterolytic cleave: pka = 36, very weak acid, Would require a very reactive low coordinate transition metal to activate dihydrogen o 4X Outcomes possible: no reaction, stable n2 – H2 complex (TS), bis-hydride complex (homolytic cleavage), mono hydride complex (heterolytic cleavage) Dihydrogen complexes – kubas  Identification? o Bond lengths: longer H2 bonds due to bonding to a metal o This can be shown via NMR (reduced coupling constant) o Neutron diffraction Bonding in dihydrogen complexes, MO diagram  Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384  Dihydrogen is an L donor  Both the HOMO and LUMO of h2 form an interaction with frontier MOs of the TM, resulting in population of the sigma star H-H orbital and H-H bond elongation  i..e back bonding and sigma donor  No known early TM complexes (d0) (not enough electron density)  Often have low O.S in stable dihydrogen complexes H-H Bond length elongation (activation)            Pi backbonding results in reduction of H-H bond order/lengthening of the bond Complete donations results in the breaking of H-H bond/OA Metal ligand composition: Closer in energy of the d pi orbital with the sigma* H-H, the better overlap and the more effective back bonding Hence, with larger and diffuse orbitals, with a low O.S state (positive charge will lower the d pi orbital energy), electron donating groups and trans influence group, there will be more effective bonding of the metal to the H-H Examples: 4 has the highest charge (2+), Ru is second row and there is EWG attached making it give the H2 the lowest bond length due to ineffective backbonding 5 has Ru again, with H bonded which doesn’t really add anything to the system 1 is 2nd row again, low charge but is slightly higher than 5 due to the EDG, Cl2 is very similar to 1 but in this case we have Os, which is 3rd row – more diffuse orbitals so more effective overlap Finally, 3 gives the largest H-H bond length due to Os, Cl- EDG Reactivity of dihydrogen complexes  Homolytic vs heterolytic: Wilkinsons pre-catalyst           Primary kinetic pathway, common for group 9 metals 1. Wilkinsons catalyst, 16ve, Rh(1), d8 square planar 2. Rh(1) – 14 VE 3. Rh(3) – 16VE, d6, oct, 5 coordinate, LBN=5: formed by rapid OA 4. Rh(3) – 18ve, d6, LBN =6. R.S. most stable: formed by alkene coordination/association 5. Rh(3), 16ve, d6, LBN=5: formed by RDS, the 1,2 MI step. Non rigid, so can then easily change geometry to give the product Rate law: dp/dt = kobs [Rh] Rh(1)  must be facile for fast catalyst Dissociation of PPh3 is required to generate the active catalyst – Vashas complex (Ir) doesn’t require this Alternative H2 activation pathways and catalyst deactivation Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Mono-hydride mechanism  alkene to alkane       Single hydride ligand (rather than 2) is present prior to coordination of the alkene 1. Ru(2), 16ve d6 – agnostic =18ve 2. Ru(2), 14ve, 4 coordination: formed by removal of pph3 3. Ru(2), 16ve, 6 coordination: formed by alkene coordination 4. Ru(2), 14ve: formed by 1,2 migratory insertion 5. Ru(2/), 16ve: formed by coordination of H2, and leads to the product releasing Olefin hydrogenation mechanism  alkene to alkane  Common for pre-catalysts with bidentate ligands and alkenes containing an EDG  i.e. [Ru(diphosphine)(diene)]+ - best with a weakly donating solvent as this will stabilise the catalyst  dp/dt = kobs [Rh][H2] – catalyst rate dependent  1. Rh(1), 16ve – pre catalyst, irreversible formaton of the active catalyst  2. Rh(1), 12ve: formed by removal of cyclooctane via addition of H2  3. Rh(1), 16ve, resting state: formed by addition of alkene deuterium  4. Rh(3), 18ve: formed by addition of H2, OA – This is the RDS as we prefer 3 coordinate OA  5. Rh(3), 16ve, LBN=5: formed by 1,2 MI – stereochemical flexible so that can release the product Asymmetric hydrogenation catalyst  Selectivity reported as enantiomeric excess or ee% = %major/%minor  Examples:  Re vs Si face addition Control of enantioselectivity  Use chiral, enantiopure metal catalyst  Curtin-Hammett principle: Whereby if M-substrate A and M-substrate B are in fast equilibrium (small energy barrier between the two) with each other then the product composition is dependent on delta delta Ga transition state/B transition state rather than delta delta GA/B  i.e. that the difference in transition state energies is the RDS element (stability of the TS) -basically the Ea/absolute barrier, rather than the stability of the starting materials Reaction profiles 1. 2. 3. 4. Same barrier of the TS, small energy conversion so 50:50 products R.S= A due to lower product energy. Barrier is smaller so we get more Pa than Pb Identical so we get a 50:50 product The starting materials are not in fast interconversion with each other and so principle doesn’t apply. Therefore we get a mixture of A and B Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Chiral chelating ligands         To activate metal catalysts, we need chiral chelating ligands. Examples: Visualise the steric effect using a quadrant diagram shown on the right Depicts the steric bulk around coordination pocket For R,R-Duphos: High steric bulk in the upper left region Low steric bulk in the upper right region Asymmetric environment for the coordination of pro chiral substrate Case study: enamide hydrogenation  Re face  Donating group is an amide, catalyst shows selective C-C hydrogenation vs carbonyl – discuss further on  Generate the R product – adding the hydrogen to the Si face, which makes sense as it goes to the back  Alkene must bind via si face to the metal centre  We can then look at the quadrants for this to understand why thermodynamically the Re face is more favoured as a starting material.  Calculated energy profile shows that the transition state for the S product is much higher in energy, and therefore is less favoured than the R product  We can understand this as the Re-face, major diastereoisomer, is forced into steric clash after reacting with H2 and the minor diastereoisomer has minor changes in conformation – with a lower absolute barrier.  So although the starting materials sees Re as the lower energy, in the end it gives the minor product as the transition state is much higher in energy Lecture 8 Hydrogenation of carbonyl compounds  Inner sphere o Coordination of the unsaturated substrate o Alkenes> ketones o Strongly coordinating double bond  Outer sphere o No direct coordination of the unsaturated substrate o Ketones> alkenes o Polar double bond Outer sphere hydrogenation – ionic mechanism:  1. W(2), 18VE  2. W(2), 16VE Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384  3.W(4), 18VE: Formed via oxidative addition of H2  4. W(2), 18VE: formed via heterolytic cleavage of dihydrogen/ketone deprotonates the dihydride as it is basic enough  4 goes onto release the product, secondary alcohol, following deprotonation making ketone more electrophilic Outer sphere hydrogenation – bifunctional mechanism 1. 2. 3. 4. 5. 6. Want to say that Ru remains as Ru(2) the whole time, concerted TS, Ru(2), 18ve – pre catalyst, deprotonation yields amino group but remains Ru(2) Ru(2), 18ve Ru(2), 18ve: formed by heterolytic cleavage of dihydrogen = RDS Ru(2), 18ve Transition state: concerted outer sphere hydride and proton transfer with LX functionality Difference between bifunctional and ionic mechanisms  Ionic: We don’t have the amido ligand here, instead we see deprotonation of the carbonyl group by one of the hydrogens, making it more electrophilic and then the other hydrogen attacks.  Bifunctional: has an amido ligand that cleaves the dihydrogen. Transition state is seen between carbonyl and amido ligand that stabilises the T.S. Transfer hydrogenation of ketones  Reversible  Outer sphere mechanism  Usually in 2-proponal or formic acid Summary  Activation of H2 generally requires a transition metal – Extent depends on the metal-ligand composition – Homolytic vs. heterolytic cleavage Alkene hydrogenation – Typically inner sphere: hydride, mono-hydride, and olefin mechanisms that exploit strong  alkene coordination and MI reactions of hydride ligands. – Importance of pre-catalyst activation by ligand dissociation Carbonyl hydrogenation – Most effective process mediated in the outer sphere sphere: exploit polarity of the C=O bond  and concerted hydride and proton transfer reactions, following heterolytic H2 activation. – Bifunctional ligands Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 – Transfer hydrogenation reactions possible (reversible reactions under thermodynamic control) Inner sphere vs. outer sphere and chemoselectivity Stereoselectivity possible using enantiopure catalysts – Case study: use of chiral diphosphine ligands that lead to diastereomeric Malkene  intermediates (can invoke Curtin-Hammett principle) – Case study: use of chiral diamine ligands that lead to diastereomeric transition states Lecture 9-11 – Carbonyl containing products  Hydroformylation of alkenes  Methanol carbonylation: The Monsanto and Cativa Process  The Wacker Process Hydroformylation of alkenes         Elements of formaldehyde H2CO added to an alkene Co catalysed process developed in 1930s. Rh developed in 1970s Key issues: Linear:branched (normal:iso) selectivity – mixtures expected, linear most valuable When internal alkenes i.e. 2-hexene, linear aldehydes can Bulky ligands promote the formation of a linear metal alkyl intermediate Selectivity for hydroformylation over hydrogenation Rh process has similar catalyst to Wilkinsons catalyst Co-Catalysed Hydroformylation 1. Pre-catalyst 2. Co(1), MXL4, 18VE: a. Formed by radical OA + H2. b. Is the resting state - determined by comparison of 1 with known spectrum of 2 c. 5 coordinate, even though we expect 23 to be dissociative. This is to avoid 20VE 3. Co(1), 18VE: formed by alkene substitution and removal of CO 4. (n-4): Co(1), 18VE: formed by i) 1,2 MI, ii) CO coordination a. I-4 goes on to form the branched aldehyde via i-5, not shown b. Selectivity is 3:1 (n-4:i-4) normal:iso c. This is because of sterics, i-4 has more steric bulk so is less stable d. And electronics: more hyperconjugation in i-4 so the negative charge destabilises i-4 e. Therefore, sterics and electronics cause a higher Ea for the branched pathway Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 5. 6. 7. 8. f. Higher CO pressure improves selectivity for the i-4, occurring after 34, but we know that rate is inversely proportional to CO so we will have a lower reactivity. g. Using bulkier ligands improves selectivity of linear product h. OA addition of H2 to form an alkane Co(1), 16VE: Formed by 1,1 MI Co(3), MX3L3, 18VE: Formed by OA and H2 addition a. Consider that there could be direct RE forming [CoH(CO)3) - Stereo chemically rigid – higher absolute barrier and 7 is more favourable product, see below Co(3), 16VE: Formed by dissociation of CO, RDS. Could be the source of -1 order in CO a. 7 is more labile, 5 coord good for RE Formed by reductive elimination of our aldehyde, alternative RDS  Rate equation: d[aldehyde]/dt = kobs [alkene][Co][H2] / [CO]  The RDS is listed above (hydrolysis of acyl), taking into consideration of the factors in the rate equation Alkene Isomerisation     Alkene feedstocks tend to be mixtures of isomers Alkene isomerisation or chain walking is v. common Processes involving alkene hydrides like 3, are subject to undergo isomerisation Internal alkenes are the more stable but primary alkyls undergo faster 1,1 MI Phosphine-modified Co Hydroformylation  8’ is more stable than 8 in the mechanism seen above  Conditions governed by catalyst decomposition kinetics: If T increases to increase the overall rate, the CO pressure needed to prevent catalyst death becomes high. This is because 8 2 needs to be fast – but we know that CO pressure reduces the rate  Shell workers show that addition of phosphines gave much higher catalyst stability but lower activity.  Higher stability comes from P sigma-donation leading to more MCO back bonding and stronger M-CO bonds.  Lower activity probably arises because CO loss is a key step for the formation of 7’, and so if its harder to lsoe  less activity  Reductive elimination not helped by the presence of phosphine, which stabilises higher OS. Although maybe the RDS has changed  Phosphines cause hydrogenation to become an important side-reaction  Linear selectivity is enhanced – bulkier system promotes linear product in 1,2insertion step. Rh Catalysed hydroformylation  Higher catalytic activity than Co by 1000 times  Lower T regies Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384      1. 2. 3. 4. 5. 6. 7. 8. More stable than Co Gives more tuneable regioselectivity Less hydrogenation Rate equation under low [PPH3] and high [H2] are as follows: d[aldehyde]/dt = kobs[Rh][Alkene]/[CO] RDS is either step 3 or 45 – no longer the hydrolysis step Dimer - At low [H2], formation of dimers, is important Rh(1), ML4X, 18VE, RS Rh(1), 16VE Rh(1), 18VE: formed by alkene coordination - RDS Rh(1), 16VE: formed by 1,2 MI -RDS RH(1), facile: formed by CO coordination Rh(1), 16VE: formed by 1,1 MI Rh(3), 18VE, ML3X3 Methanol carbonylation: Important feedstocks (L10) Monsato process:  D[CH3COI]/dt = kobs[Rh][MeI]  Anioinc catalyst promotes OA  Feedstock/substrate is MeOH, converted by co-catalyst HI (v. reactive, causes corrosion) to MeI  MeOH cant undergo OA i.e. breaking of the C-O bond  MeI> MeBr as I is a better leaving group  HI probably responsible for co-production of acetaldehyde and formation of catalytically inactive, 6  causes corrosion and so need to use expensive alloy or glass-lined reactors  High pressure of CO helps stabilise catalyst and so limits catalyst death 1. Rh(1), 16VE: formed by RE of CH3COI (from 4). – promoted by neutral charge of intermediate, 5. Resting state. In the WGSR process, formed by 8 – removal of HI and CO2 2. Rh(3), 18VE: Formed by nucleophilic OA of Ch3I – RDS (slower than 23) 3. Rh(3), 16VE, 5-coord (-ve charge stabilised by high oxidation state): formed by 1,1 MI (fast) – alkyl moves mechanism, rather than anything else 4. Rh(3), 18VE, anionic, octahedral (rigid): formed by CO coordination 5. Neutral, 5 coordinate complex. – formed by loss of iodide from 4 WGSR Monsato  second stage 6. Rh(3), 18VE: formed by addition of HI. We can use 6 with high conc. of water to regenerate the catalyst. (otherwise forms insoluble sludge, RhI3)  WGSR keeps catalyst alive 7. Rh(3), 18VE: formed by coordination of CO and loss of iodide  attack at CO by water occurs here rather than 6 as 7 is more electrophilic than 6. Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 8. Formed by water attacking CO bonded to Rh complex, 6. 7 is more electrophilic than 6 so this happens here 9. Formed by oxidative addition of HI to 1  similar as to before with MeI  Net of the WGSR monsato process  overall is the water gas shift reaction, catalysed by 1 or 6  OR HI catalysed reduction of 6, helping 6 go back into the reactive catalytic mechanism. This will consume CO though and therefore limit our substrate Cativa Process  Is similar to Monsanto, HI is used as a co-catalyst although not shown In mechanism  Rate: d[CH3COI]/dt = kobs [Ir][CO]/[I-]  Issue: iodide is essential for the catalyst stability, and forming the anionic nucleophile 1 but it slows down the RDS  Need a species that will remove free iodide but not compete with the active species – can see in the mechanism that we can still make CH3COI  Peak activity comes at low water content as it is hard to separate water from acetic acid 1. Ir(1), 16VE, negatively charged 2. Ir(3), 16VE 2. 2’: resting state, 18VE, formed by OA of iodide – 150x faster than for Rh due to the stability of higher os for third row metal, and low electronegativity of iodide. However the 1,1, MI step after this is 105 slower than Rh 3. Formed by coordination of CO, RDS 4. Formed by 1,1 MI, RDS  700x faster than 2’ due to increased electrophilicty of CO in 3, compared to the anionic complex 2’ Monsanto vs Cativa  Both use HI as the co-catalyst and formation of organometallic intermediate by OA of MeI  Monsanto (Rh) and nucleophilic OA of MeI is the RDS  This is faster in Ir (Cativa) and so the RDS is actually the 1,1 MI of an alkyl carbonyl intermediate Wacker Process  Overall: C2H4 + ½ O2  CH3CHO – favourable product, used in lots of processes Nucleophilic addition  When metal has a low electorn count and free site, then the second mechanism occurs (inner)  Water added to alkene ligand – outer sphere hydroxypalladation Wacker Cycle: Outline (Schmidt) Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384  Favoured at low Cl This mechanism is controversial due to the process of hydroxypalladation: as both the outer-sphere anti mechanism and inner-sphere syn mechanism is consistent with the rate laws  Recent computational analysis favours the outer-sphere mechanism  Rate= -d[C2H4]/dt = k[PdCl42-][C2H4]/[H3O+][CL-]2  Step 23 or 3’ is governed by the trans effect. Order is: Halogen NHC promotes cycloaddition, rather than dissocation of the pcy3  difference is now 4 orders of magnitude, much better deciding factor than dissociation  Hence we can visualise here that the pcy3 is a strong sigma donor, weak pi backbonder – more like a fischer carbene for grubbs 1 Improvements on the grubbs type catalyst  Piers 14VE  Grubbs-Hoveyda  Comparison: Alkyne metathesis      Less developed compared to alkene metathesis Group 6 carbyne complexes i.e. Mo, W Similar mechanism of the [2+2] cycloaddition reactions and cycloreverison Reactants: [(tBuO)3W=-CME] Higher o.s. of W=6 Polyolefins from coordination polymerisation  Polyethene -PE  Polypropylene -PP Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384  Most produced by heterogeneous Zigler-Natta Process  Stereochemistry names: isotatic, synchrotatic, random Synchro=syndio Heterogeneous Ziegla-Natta     Reactants: TiCl3/4, AlEt3, 25-50 degrees Celsius, 1 bar – mild Early TM i.e. Ti Stereoselecitivty favours Ipp  isotatic propylene 104 gpe PE formation -HDPE Cossee-Arlman mechanism (enantiomorphic site control)  PP formation isotactic  1,2 MI, coordination of alkenne, 1,2-MI Chain termination – different ways of terminating an alkene     1) Beta elimination forming a 4-mem planar transition state – most important 2) Zimmermann traxler transition state 3) transmetallation reactions 4) H2 added for control, sigma-blend metathesis Metallocenes General polymerisation cycle  Most active polymerisation catalysts have an extra empty coordination site after alkene coordination  Can see that the agnostic interaction is adding to the extra site hence sp3 alkyl is tilted towards the alkene, promoting 1,2 MI. Can see this in the transition state  Gamma agostic interaction in the product can be a problem for slowing down beta elimination – termination step Metallocenes activators: MAO  AlMe3 + water  Partial hydrolysis of AlMe3 forms co-catalyst, MAO – methylaluminoxane MAO and the alkyl cation:  Expensive and wasteful i.e. 200-1000 eqv required  Leaves Al residues in the polymers without expensive workup Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384   1. 2. 3. 4. Pyrophoric Functions of MAO activator/co-catalyst: Alkylate the metal chloride precatalyst Abstract Me-, leaving a highly electrophilic cation Provide a counter-anion that doesn’t interfere with the electrophile Scavenge the system for protic impurities Stoichmetric activators  B(C6F6)3  relatively low catalysis, not fully dissociated, partially coordinated, free coordination in cis  [Ph3C][B(C6F5)4] ‘trityl barf’  weakly coordinating, forms an active catalyst and first. Negative charge is distributed across the whole molecule  [HNME2Ph][B(C6F5)4]  partially coordinates to active catalyst, slows down reaction Chain end vs enantiomorphic site control  Enantiomorphic site control is preferable and the domain of homogeneous catalysts Ansa-metallocene catalysts:  Cp rings have been connected together  Growing polymer chain will orient itself to avoid steric clashes, and the alkene (propylene) will avoid steric clashes with the ligands and polymers  This is because the rings are no longer free to rotate, and so chiral C2 metallocense such as Brintzinger compounds can be made by freely adding substituents where you want.  Leading to tacicity control in e.g. PP  tetrahydroindenyl ligans  Bridge pulls back Cp rings and then opens the wedge a little  increase in reactivity  Constrained geometry catalysts are fast but give poor tacicity control Iso and syndio specific PP mechanisms Downloaded by Anushaya Jeyabalan ([email protected])