ch7.pdf

7 INSERTION AND ELIMINATION Oxidative addition and substitution allow us to introduce a variety of 1e and 2e ligands into the coordination sphere of a metal. With insertion, and its reverse, elimination, we can combine and transform these ligands, ultimately to expel these transformed ligands to giv...

7 INSERTION AND ELIMINATION Oxidative addition and substitution allow us to introduce a variety of 1e and 2e ligands into the coordination sphere of a metal. With insertion, and its reverse, elimination, we can combine and transform these ligands, ultimately to expel these transformed ligands to give useful products, often in the context of a catalytic cycle. In this way, organometallic catalysis can convert organic reagents into organic products with regeneration of the metal species for subsequent reaction cycles. 7.1 INTRODUCTION By insertion, a π-bound 2e ligand, A=B, inserts into an M–X bond to give M–(AB)–X, where AB has formed a new bond with both M and X. There are two main types of insertion, either 1,1 (Eq. 7.1) or 1,2 (Eq. 7.2). In 1,1 insertion, M and X end up bound to the same atom of AB, but in the 1,2 type, M and X end up on adjacent atoms of AB. The type of insertion in any given case depends on the nature of A=B. For example, CO gives only 1,1 insertion where both M and X end up bound to CO carbon. On the other hand, ethylene gives only 1,2 insertion, where the product, MCH2CH2X, has M and X on adjacent atoms of the ligand. The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc. 185 186 Insertion and Elimination In general, η1 ligands give 1,1 insertion, and η2 ligands give 1,2 insertion. SO2 is the only common ligand that can give both types of insertion and accordingly, SO2 can either be an ηl (S) or η2 (S,O) ligand. (7.1) (7.2) In principle, insertion is reversible, and reversibility is indeed seen experimentally,1 but just as we saw for OA and RE in Chapter 6, in many cases, only the thermodynamically favored direction is ever observed. For example, SO2 commonly inserts into M–R bonds to give alkyl sulfinate complexes, but these rarely eliminate SO2. Conversely, N2 readily eliminates from diazoarene complexes, but the reverse is not seen. M−R + SO2 → M−SO2 R (7.3) M−N=N−R → M−R + N 2 (7.4) Both the 1e and 2e ligands normally need to coordinate to the metal before insertion. This means that a 3e set of ligands in the intermediate converts to a 1e ligand in the insertion product, so that a 2e vacant site (□) is generated (Eq. 7.2). Binding of an external 2e ligand can trap the insertion product (Eq. 7.5). Conversely, the elimination requires a 2e vacant site, so that an 18e complex cannot undergo the reaction unless a ligand first dissociates. The insertion also requires a cis arrangement of the 1e and 2e ligands, while the elimination generates a cis arrangement of these ligands. The formal oxidation state does not change during the reaction. (7.5) In one useful picture of insertion, the X ligand migrates with its M–X bonding electrons (e.g., as H− or Me−) to attack the π* orbital of the A=B ligand. In this intramolecular nucleophilic attack on A=B, the 187 CO Insertion migrating group, R, retains its stereochemistry. This picture also justifies the term “migratory insertion,” often applied to these reactions, in that the X migrates to the A=B group. A component of M–(A=B) bonding is back donation, in which an M dπ electron pair is partially transferred to the A=B π*; in an insertion, an M–X bonding electron pair is fully transferred to the A=B π*. 7.2 CO INSERTION CO shows a strong tendency to insert into metal–alkyl bonds to give metal acyls, a reaction that has been carefully studied for a number of systems. Although the details may differ, most follow the pattern set by the best-known case: (7.6) The usual mechanism of migratory insertion is shown in Eq. 7.7. The alkyl group in the reagent (Rgt) undergoes a migration to the CO to give an acyl intermediate (Int.) that is trapped by added ligand, L, to give the final product (Pdct).     (7.7) The kinetics are reminiscent of dissociative substitution (Section 4.4) except that the 2e site is formed at the metal in the migratory step, not by loss of a ligand. Using the usual steady-state method, the rate is given by Eq. 7.8. Rate = −d[Rgt] k1k2 [L][Rgt] = dt k−1 + k2 [L] (7.8) There are three possible regimes,2 each of which can be found in real cases: 1. If k−1 is very small relative to k2[L], [L] cancels and Eq. 7.8 reduces to Eq. 7.9. 188 Insertion and Elimination Rate = −d[Rgt] = k1[Rgt] dt (7.9) Because k−1 is small, L always traps the intermediate; this means the rate of the overall reaction is governed by k1, and we have a first-order reaction. 2. If k−1 is very large relative to k2[L], then Eq. 7.8 reduces to Eq. 7.10. Rate = −d[Rgt] k1k2 [L][Rgt] = dt k−1 (7.10) In this case, the intermediate almost always goes back to the starting reagent, and the second step, attack by L, governs the overall rate, so we have second-order kinetics. 3. If k−1 is comparable with k2[L], then the situation is more complicated and the equation is usually rewritten as Eq. 7.11, where a new term, kobs, is defined by Eq. 7.12. −d[Rgt] = kobs [Rgt] dt k1k2 [L] kobs = k−1 + k2 [L] Rate = (7.11) (7.12) The intermediate is now trapped by L at a rate that is comparable with the reverse migration. This is handled by plotting l/kobs versus 1/[L] to find 1/k1 from the intercept and k−1/(k1k2) from the slope (Eq. 7.13). Dividing the slope by the intercept gives k−1/k2, which tells us how the intermediate partitions between the forward (k2) and back (k−1) reactions. (7.13) When the incoming ligand in Eq. 7.7 is 13CO, the product contains only one labeled CO, cis to the newly formed acetyl. This suggests that the acetyl group is initially formed cis to a vacant site in the intermediate. The labeled CO can be located in the product by NMR and IR spectroscopy. In an example of a useful general strategy, we can learn about any forward process by looking at the reverse reaction—here, α elimination of CO from Me13COMn(CO)5 (Eq. 7.14; C* = 13C). We can easily label the acyl carbon with 13C by reaction of [Mn(CO)5]- with Me13COCl and find that after α elimination of CO, the label ends up in a CO cis to the methyl in the product. CO Insertion      189 (7.14) By microscopic reversibility, the forward and reverse reactions of a thermal process must follow the same path. In this case, if the labeled CO ends up cis to Me in the elimination direction, the CO to which a methyl group migrates in the insertion direction must also be cis to methyl. We are fortunate in seeing the kinetic products of these reactions. If a subsequent scrambling of the COs had been fast, we could have deduced nothing. We now know that Me and CO must be mutually cis to insert, but we do not yet know if Me migrates to the CO site or vice versa. It is also possible to use reversibility arguments to show that it is Me, not CO, that moves.To do this, we look at CO elimination in cis-(MeCO)Mn(CO)4(13CO), in which the labeled CO is cis to the acetyl. If the acetyl CO migrates during the elimination, then the methyl in the product will stay where it is and so remain cis to the label. If the methyl migrates, then it will end up both cis and trans to the label, as is in fact observed (Eq. 7.15). (7.15) 190 Insertion and Elimination This implies that the methyl also migrates in the insertion direction. The cis-(MeCO)Mn(CO)4(13CO) required for this experiment can be prepared by the photolytic method discussed in Section 4.7. This migration of Me not CO is one feature of migratory insertion that does not reliably carry over to other systems, where the product acyl is occasionally found at the site originally occupied by the alkyl. Consistent with this mechanism, any stereochemistry at the alkyl carbon is retained both on insertion and on elimination. Enhancing Insertion Rates Steric bulk in the Ln ligand set of LnM(Me)(CO) accelerates insertion, no doubt because the acetyl in the LnM(COMe) product, occupying one coordination site, is far less bulky than the alkyl and carbonyl, occupying two sites in the starting complex, LnM(Me) (CO). Lewis acids such as AlCl3 or H+ can increase the rate of migratory insertion by as much as 108-fold, where k2 is the slow step.3 Metal acyls (7.1) are more basic at oxygen than are the corresponding carbonyls by virtue of the resonance form 7.2. By binding to the oxygen, the Lewis acid would be expected to stabilize the transition state and speed up trapping by L and therefore speed up the reaction. Polar solvents such as acetone also significantly enhance the rate. Another important way of promoting insertion is oxidation of the metal.4 Cp(CO)2FeIIMe is normally very slow to insert, but 1e oxidation at −78°C in MeCN electrochemically or with Ce(IV), gives the 17e, Fe(III) acyl [CpFeIII(MeCN)(CO)(COMe)]+, in which the solvent plays the role of incoming ligand. As we saw in Chapter 4, 17e complexes can be very labile, but another factor here may be the increased electrophilicity (decreased π basicity) of the oxidized metal enhancing the partial positive charge on the CO carbon. The migration of Me− to a now more electron-deficient CO carbon is expected to be faster. Early d0 metals are Lewis acids that prefer O-donor ligands (for the oxophilicity of d0 metals, see Section 3.2); they can therefore act as their own Lewis acid catalysts for insertion, the product being an η2-acyl (Eq. 7.16). CO Insertion 191 (7.16) By altering the thermodynamics in favor of the adduct, this effect is even sufficient to promote the normally unfavorable CO insertion into an M–H bond, as shown in Eq. 7.17. (7.17) In each of these reactions, the formation of an intermediate carbonyl complex is proposed but d0 Zr(IV) and Th(IV) are both poor π bases, so these intermediates must be very unstable; in compensation, the limited back bonding makes the CO much more reactive for insertion, however. In rare cases, CO inserts directly into an M–R bond without first binding to the metal, as seems to be the case for a Re(V) oxo alkyl where the high valent Re is poorly adapted to bind CO.5 Apparent Insertions An insertion that appears to be migratory can in fact go by an entirely different route (Eq. 7.18). Since MeO− is a good π donor bound to a d6 π-donor metal, the MeO− group easily dissociates to give an ion pair with a 2e vacancy at the metal. The free CO present then binds to this 2e site and is strongly activated toward nucleophilic attack at the CO carbon owing to the positive charge on the metal. The product is the interesting metalloester shown in Eq. 7.18. (7.18) Genuine migratory insertions into M–O bonds are also possible. For trans-[Pt(Me)(OMe)(dppe)], CO inserts into the Pt–OMe bond, while for [Ni(Me)(O-p-C6H4CN)(bipy)], CO inserts into Ni–Me. For nickel, the M–Me bond is significantly stronger than M–OMe, but migratory insertion with M–Me is marginally preferred owing to the weaker C–O bond of the aryloxycarbonyl. For platinum, M–Me and M–OMe bonds are equally strong, so the stronger methoxycarbonyl C–O bond favors reaction with the M–OMe bond.6 192 Insertion and Elimination Double Insertion Given that the methyl group migrates to the CO, why stop there? Why does the resulting acyl group not migrate to another CO to give an MeCOCO ligand? To see why, we can treat [Mn(CO)5]− with MeCOCOCl to give [MeCOCOMn(CO)5], which easily and irreversibly eliminates CO to give MeCOMn(CO)5. This means that the double-insertion product does not form because it is thermodynamically unstable with respect to MeCOMn(CO)5 + CO. The –CHO and CF3CO– groups also eliminate CO irreversibly to give M–H and M–CF3 complexes, implying that these insertions cannot occur thermally. Thermodynamics drives these eliminations because the M–COMe, M–H, and M–CF3 bonds are all distinctly stronger than the M–CH3 bond that is formed in CO elimination from the acetyl. In contrast to CO, isonitriles can undergo repeated migratory insertion to give R(CNR)mM polymers, with m as high as 100. The instability of R(CO)mM is associated with having successive δ+ carbonyl carbons mutually adjacent; =NR being less electronegative than =O, the problem is less severe for RNC than for CO. We look at 1,1 insertions involving carbenes in Chapter 11. 7.3 ALKENE INSERTION The insertion of coordinated alkenes into M–H bonds leads to metal alkyls and constitutes a key step in a variety of catalytic reactions (Chapter 9). For example, the commercially important alkene polymerization reaction (Chapter 12) involves repeated alkene insertion into the growing polymer chain. As η2-ligands, alkenes give 1,2 insertion in the reverse of the familiar β elimination (Eq. 7.19). Some insertions give agostic (7.3) rather than classical alkyls, and species of type 7.3 probably lie on the pathway for insertion into M–H bonds. The position of equilibrium depends not only on whether an incoming ligand, L in Eq. 7.19, is available to trap the alkyl, but also very strongly on the alkene and the insertion thermodynamics. For simple alkenes, such as ethylene (Eq. 7.18), the equilibrium tends to lie to the left and the alkyl prefers β elimination, but for alkenes such as C2F4, which form strong M–R bonds, insertion is preferred and the product alkyl LnMCF2CF2H does not β-eliminate. (7.19) Alkene Insertion 193 The transition state for insertion, 7.4, resembles 7.3 in having an essentially coplanar M–C–C–H arrangement, and this implies that both insertion and elimination also require the M–C–C–H system to become coplanar. We have seen in Section 3.1 how we can stabilize alkyls against β elimination by having a noncoplanar M–C–C–H system. The same principles apply to stabilizing alkene hydride complexes. Compound 7.5 undergoes insertion at least 40 times more rapidly than 7.6, although the alkene and M–H groups are cis in both cases, only in 7.6 is there a noncoplanar M–C–C–H arrangement. Regiochemistry of Insertion In hydrozirconation of alkenes by Cp2ZrHCl,7 terminal alkenes insert in the anti-Markovnikov direction to give a stable 1° alkyl. Internal alkenes, such as 2-butene, insert to give an unstable 2° alkyl, that β-eliminates to give 1- and 2-butene. The 1-butene can now give a stable 1° alkyl that is the final product. This is particularly noteworthy because the free terminal alkene is less stable than the internal alkene. The outcome arises because the 1° alkyl is thermodynamically more stable than a 2° alkyl for steric reasons. The 1° alkyl, R, can subsequently be functionalized in a number of ways to give a variety of RX derivatives. Hydrozirconation is also effective with less reactive substrates, such as nitriles, where addition of Zr–H across the C≡N bond is possible.8 For ArCH=CH2, the preferred LnM–H insertion product tends to have the metal bound at the benzylic position in spite of the resulting steric disadvantage; not just Ph but electron-withdrawing groups in general prefer to locate at the α-carbon on insertion. Equilibration of the two regioisomers (Eq. 7.20)9 also favors 7.8, showing that this is indeed the thermodynamic product. Traditionally, this outcome of insertion has been ascribed to the new M–C bond being stronger in 7.8 than in 7.7, but Jones10 has called attention to the strength of the newly formed C–H′ bonds as a key factor. In 7.7, the new C–H′ bond, being benzylic, is weak, while in 7.8, the new C–H′ bond is no longer benzylic, so much stronger. The new benzylic M–C bond in 7.8 is typically weaker than the M–C bond in 7.7, not stronger as once thought. Breaking the 194 Insertion and Elimination M–C bond in 7.8 homolytically gives a stabilized C radical so is easier than breaking the M–C bond in 7.7, where the resulting radical is not specially stabilized. 7.8 is nevertheless preferred as product, probably because its M–C bond is a little stronger than might be expected without back donation from metal dπ orbitals into the C–Ar σ*, less favorable in 7.7, where the M–C bond has no electronegative substituent. The same arguments probably apply to a variety of other electronegative substituents, such as –CN, –F, and –CHO. This reflects the general principle that we must consider all the bonds broken and formed in order to successfully interpret reactivity trends. (7.20) Simple α-olefins, where the two ends of the C=C bond are not well differentiated electronically, may give insertion with a mixed regiochemistry, although steric effects can bias the outcome in suitable cases.11 Syn versus Apparent Anti Insertion In the usual syn insertion, the stereochemistry at both carbons is retained. This is best seen for alkynes, where the vinyl product can preserve the syn disposition of M and H. If the initially formed cis-vinyl complex remains 16e, it can rearrange to the sterically less hindered trans isomer, via an 18e η2-vinyl. This can lead to an apparent anti addition of a variety of X–H groups (Eq. 7.21) to alkynes.12 (7.21) 195 Alkene Insertion TABLE 7.1 Comparison of Barriers (kcal/mol) for Insertion in [Cp*{(MeO)3P}MR(C2H4)]+ for R = H and R = Et11 M R = Ha R = Etb Difference Rh Co 12.2 6–8 (est.) 22.4 14.3 10.2 6–8 (est.) a ±0.1 kcal/mol. ±0.2 kcal/mol. b (7.22) Insertion into M–H versus M–R For thermodynamic reasons, CO insertion generally takes place into M–R, but not into M–H bonds. Alkene insertion, in contrast, is common for M–H, but much less common for M–R. The thermodynamics still favor the reaction with M–R, so its comparative rarity must be due to kinetic factors. Brookhart and Templeton13 have compared the barriers for insertion of ethylene into the M–R bond in [Cp*{(MeO)3P} MR(C2H4)]+, where R is H or Et and M is Rh or Co. The reaction involving M–H has a 6- to 10-kcal/mol lower barrier (Table 7.1). This corresponds to a migratory aptitude ratio kH/kEt of 106–108. As we have seen before, reactions involving M–H are almost always kinetically more facile than reactions of M–R. This means that an alkene probably has less intrinsic kinetic facility for insertion than does CO. Looking at the reverse reaction (Eq. 7.22), elimination, we see that this implies that β-H elimination in an alkyl will be kinetically very much easier than β-alkyl elimination, and it will also give a thermodynamically more stable product, so it is not surprising that β-alkyl elimination is extremely rare. In those cases where it is observed, there is always some special factor that modifies the thermodynamics or the kinetics or both. For example, for f-block metals M–alkyl bonds appear to be comparable in strength, or stronger than M–H bonds, and both β-H and β-alkyl elimination is seen. Strain, or the presence of electronegative substituents on the alkene, or moving to an alkyne are some of the other factors that can bias both 196 Insertion and Elimination the thermodynamics and the kinetics in favor of insertion, as shown in Eq. 7.23 for a strained bridgehead alkene. (7.23) Radical Pathways Styrene can insert into the M–M bond of [Rh(OEP)]2 (OEP = octaethylporphyrin) via initial M–M bond homolysis to give the 15e metalloradical [Rh(OEP)]. This adds to the alkene to give [PhCH( )CH2Rh(OEP)], stabilized by benzylic resonance, followed by the sequence of Eq. 7.24. [Rh(OEP)]2 also initiates radical photopolymerization of CH2=CHCOOR, where the intermediate C radicals add repetitively to acrylate rather than recombine with a metalloradical as in Eq. 7.24. (7.24) As we saw in Sections 5.2–5.3, butadiene and allene react with a variety of hydrides by 1,2 insertion, but butadienes also react with HMn(CO)5 to give an apparent 1,4 insertion. Since this 18e hydride has no vacant site and CO dissociation is slow, an indirect mechanism is proposed: H atom transfer to give a 1,1-dimethylallyl radical that is subsequently trapped by the metal (Eq. 7.25). Only substrates such as a 1,3-diene that form particularly stable radicals, such as an allyl, can react in this way; CIDNP effects (Chapter 10) arising from the radical pathway are sometimes seen in the NMR spectra of the reacting mixture. (7.25) Insertion of O2 into (dipy)PdMe2 to give (dipy)PdMe(OOMe), has highly irreproducible rates because a radical chain is initiated by trace Outer Sphere Insertions 197 impurities, but addition of the radical initiator AIBN gives reproducible rates.14 Alternating CO/Alkene Insertion [(phen)PdMe(CO)]+ can copolymerize CO and ethylene to give a strictly alternating copolymer, (CH2CH2CO)n.15 This is of practical interest because its carbonyl functionality permits useful chemical modification. The polymerization reaction is also of mechanistic interest because of the essentially perfect alternation of alkene and CO insertions.    (7.26) Of the possible erroneous insertions, double CO insertion is forbidden for the thermodynamic reasons discussed in Section 7.2, and double alkene insertion is rare because of its much slower intrinsic rate and the high affinity of the catalyst for CO, together amounting to a rate enhancement of 2000 versus CO insertion into M–R. 7.4 OUTER SPHERE INSERTIONS In some cases, the A=B bond does not need to coordinate to the metal prior to insertion and can undergo the reaction with an 18e complex. The weakly binding ligand, CO2, can insert into an M–H bond in this way. The nucleophilic hydride first attacks the carbon of free CO2 to give a 16e M+ unit and free HCOO−. The formate then binds to the metal to give the 1,2-insertion product, M–OCHO. Sulfur dioxide is a much stronger electrophile than CO2 and also needs no vacant site. If SO2 electrophilically attacks the α carbon of an 18e alkyl from the side opposite the metal, an alkyl sulfinate ion is formed with inversion at carbon. Since the anion has much of its negative charge on the oxygens, it is not surprising that the kinetic product of ion recombination is the O-bound sulfinato complex. On the other hand, the thermodynamic product is usually the S-bound sulfinate, as is appropriate for a soft metal binding. This sequence constitutes a 1,2 (O bound sulfinate) or a 1,1 insertion of SO2 (S bound). 198 Insertion and Elimination      (7.27) As expected for this mechanism, the reactivity falls off for bulky alkyls and electron attracting substituents. A crossover reaction of a mixture of RS and SR isomers of [CpFe*(CO)L{CH2C*H(Me)Ph}], chiral at both Fe and the β-carbon, forms very little of the crossover products, the R,R and S,S isomers of the sulfinate complex. This shows both that the intermediate must stay ion-paired, and that the intermediate iron cation must have stereochemical stability. Ion pairing is very common in organic solvents of relatively low polarity, such as CH2Cl2, and ion pairs can have a well-defined solution structure, and such pairing can affect reaction outcomes.16 O2 can insert into M–H to give M–O–O–H; in some cases, an H atom abstraction mechanism by O2 via M and O–O–H has been identified.17 Insertions of CO2 are discussed in Section 12.3. 7.5 α, β, γ, AND δ ELIMINATION β Elimination Continuing the discussion of β elimination from Section 3.1, we now look at the kinetics. An 18e complex has to lose a ligand to open up a site for elimination, but this may or may not be rate limiting. In either case, the addition of an excess of ligand can inhibit the reaction by quenching the open site. A significant kinetic isotope effect kH/kD in the elimination rate of LnMC2H5 versus LnMC2D5 suggests that the elimination itself is rate limiting since C–H(D) bond breaking must be important in the slow step. In 16e complexes, a 2e site is usually available for β elimination. For example, 16e d8 trans-[PdL2Et2] complexes (L = PR3), can decompose by β elimination via an 18e transition state, but PR3 dissociation is still required for elimination in trans-[PtL2Bu2], where the preference for 16e over 18e structures is more marked than for Pd(II).18 The related metalacycle 7.9 β-eliminates 104 times more slowly than [PtL2Bu2], α, β, γ, AND δ ELIMINATION 199 presumably because a coplanar M–C–C–H arrangement is much harder to achieve (Eq. 7.28).     (7.28) In a series of analogous nickel complexes in the presence or absence of excess phosphine, three different decomposition pathways are found, one for each of the different intermediates, 14e, 16e, and 18e, that can be formed (Eq. 7.29).      (7.29) An alkyl and its alkene hydride elimination product can occasionally be seen in equilibrium together (Eq. 7.30).19 (7.30) Alkoxide complexes β eliminate readily to give ketones or aldehydes, accounting for the ability of basic isopropanol to reduce many metal halides to hydrides with formation of acetone by the pathway of Eq. 3.27. β Elimination of amides and amines to imines also occurs but tends to be slow.20 α Elimination Common for alkyls that lack β hydrogens, this is the reverse of 1,1 insertion (e.g., Eq. 7.14). β elimination being impossible, LnM–Me can only undergo an α elimination to give LnM(=CH2)H. While any β process gives an alkene, a stable species that can dissociate from the metal, an alkylidene ligand from an α elimination is unstable in the free state and cannot dissociate. Methylene hydride complexes are therefore rarely seen because they are thermodynamically unstable with 200 Insertion and Elimination respect to the corresponding methyl complex, but α elimination can still occur reversibly in a reaction sequence. For this reason, the α process is less well characterized than β elimination. Isotope exchange studies on both Mo and Ta alkyls suggest that α elimination can be up to 106 times faster than β elimination even in cases in which both α- and β-H substituents are present.21 A coordinatively unsaturated methyl complex can be in equilibrium with a methylene hydride,22 that can be trapped either by nucleophilic attack at the carbene carbon (Eq. 7.31) or by removing the hydride by reductive elimination with a second alkyl (Eq. 7.32): (7.31) (7.32) Other Eliminations A great variety of other ligands may lack β-Hs but possess γ- or δ-H’s and can thus undergo γ or δ elimination to give cyclic products (Eq. 7.33). (7.33) 1,1-Insertion occurs for ηl ligands such as CO and 1,2 insertion occurs for η2 ligands such as C2H4. In each case an X ligand migrates from M to L (Eq. 7.1 and Eq. 7.2). Insertions are kinetically favored for X = H over X = R, but for CO, insertion into M–H is thermodynamically disfavored (Eq. 7.1 and Eq. 7.2). Problems 201 REFERENCES 1. A. J. Pontiggia, A. B. Chaplin, and A. S. Weller, J. Organometal. Chem., 696, 2870, 2011. 2. A. Derecskei-Kovacs and D. S. Marynick, J. Am. Chem. Soc., 122, 2078, 2000. 3. K. Fukumoto and H. Nakazawa, J. Organometal. Chem., 693, 1968, 2008; M. Rubina, M. Conley, and V. Gevorgyan, J. Am. Chem. Soc., 128, 5818, 2006. 4. Z. X. Cao, S. Q. Niu, and M. B. Hall, J. Phys. Chem., A 104, 7324, 2000. 5. C. P. Lilly, P. D. Boyle, and E. A. Ison, Organometallics, 31, 4295, 2012. 6. S. A. Macgregor and G. W. Neave, Organometallics, 23, 891, 2004. 7. Y. H. Zhang, R. J. Keaton, and L. R. Sita, J. Am. Chem. Soc., 125, 8746, 2003. 8. C. Lu, Q. Xiao, and P. E. Floreancig, Org. Lett., 12, 5112, 2010. 9. J. Vela, S. Vaddadi, T. R. Cundari, J. M. Smith, E. A. Gregory, R. J. Lachicotte, C. J. Flaschenriem, and P. L. Holland, Organometallics, 23, 5226, 2004. 10. M. E. Evans, T. Li, A. J. Vetter, R. D. Rieth, and W. D. Jones, J. Org. Chem., 74, 6907, 2009; G. Choi, J. Morris, W. W. Brennessel, and W. D. Jones, J. 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Shapley, Organometallics, 19, 761, 2000. PROBLEMS 7.1. Predict the structures of the products (if any would be expected) from the following: (a) CpRu(CO)2Me + PPh3, (b) Cp2Zr­ HCl + butadiene, (c) CpFe(CO)2Me + SO2, and (d) Mn(CO)5CF3 + CO. 202 Insertion and Elimination 7.2. Me2NCH2Ph reacts with PdCl2 to give A; then A reacts with 2,2-dimethylcyclopropene and pyridine to give a mixture of C and D. Identify A and explain what is happening. Why is it that Me2NPh does not give a product of type A, and that A does not insert ethylene? 7.3. In the pyrolysis of TiMe4, both ethylene and methane are observed; explain. 7.4. Suggest mechanisms for the following: 7.5. The reaction of trans-PdAr2L2 (A, Ar = m-tolyl, L = PEt2Ph) with MeI gives 75% of m-xylene and 25% of 3,3′-bitolyl. Explain how these products might be formed and list the possible Pd-containing products of the reactions. When the reaction of A was carried out with CD3I in the presence of d0-PdMeIL2 (B), both d0- and d3xylene were formed. A also reacts with B give m-xylene and 3,3′-bitolyl. How does this second result modify your view of the mechanism? 7.6. [Cp*Co{P(OMe)3}Et]+ has an agostic interaction involving the β-H of the ethyl group. Draw the structure. It reacts with ethylene to form polyethylene. How might this reaction proceed? RhCl3/ EtOH and other late metal systems usually only dimerize ethylene to a mixture of butenes. Given that a Rh(I) hydride is the active catalyst in the dimerization, what mechanism would you propose? Try to identify and explain the key difference(s) between the two systems. 7.7. Design an alkyl ligand that will be resistant to β elimination (but not the ones mentioned in the text; try to be as original as pos- Problems 203 sible). Design a second ligand, which may be an alkyl or an arylsubstituted alkyl, that you would expect to be resistant to β elimination but have a high tendency to undergo β–C–C bond cleavage. What products are expected? 7.8. Given the existence of the equilibrium shown: how would you change L, M, and the solvent to favor (a) the right-hand side and (b) the left-hand side of the equation? 7.9. trans-PtCl(CH2CMe3){P(C5H9)3}2 gives 1,1-dimethylcyclopropane on heating. What mechanism is most likely, and what Pt-containing product would you expect to be formed? If the neopentyl group is replaced by –CH2Nb (Nb = 1-norbornyl), then CH3Nb is formed instead. What metal complex would you expect to find as the other product? 7.10. In mononuclear metal complexes, β elimination of ethyl groups is almost always observed, rather than α elimination to the ethylidene hydride LnM(=CHCH3)H. In cluster compounds, such as HOs3(CO)10(Et), on the other hand, α elimination to give the bridging ethylidene H2Os3(CO)10(η1,μ2-CHCH3) is observed in preference to β elimination. Suggest reasons for this difference. 7.11. Consider the three potential rate-accelerating effects on CO insertion mentioned in Section 7.2: steric, Lewis acid, and oxidation. For each effect, discuss whether an acceleration of the overall reaction rate is to be expected if the reaction in question is (a) first order, (b) second order, (c) an intermediate case, and (d) an apparent insertion of the type shown in Eq. 7.18.