M.C. White, Chem 153 Hydroformylation -204- Week of october 29. 2002 hydride catalysts Hydroformylation proceeds with RhCI(PPh3), however there is an induction period. It was shown that in the presence of a base the induction period is removed. What is the role of the base? Indicate the active catalyst for this process and specify how it is formed under the reaction conditions. (Question 3, Q&A pg 200) Ph3p Rh C3H7 H2CO(11,100am,70°C 16 h, >99%conversion linear: branched (2.7: 1) drive the rxn forward by forming insoluble amine hydroch Ph3p uMPH CO PPh3 &+H Rh(COXPPh3) PhPRh@3 cO Addition of CO, a strong T-acid, enhances the a-bond metathesis commercially at electrophilic character of the Rh promoting st a dihydride catalyst may result in reductive elimination of the PPh3 intermediate metal alkyl with a second hydride(favored over hydrogenation product active monohydric
M.C. White, Chem 153 Hydroformylation -204- Week of October 29, 2002 Monohydride catalysts H2 Rh(CO)(PPh3)n Cl H H δ− δ+ HCl σ-bond metathesis external amine base may drive the rxn forward by forming insoluble amine hydrochloride salts Ph3P Rh(I) Ph3P PPh3 Cl CO Addition of CO, a strong π-acid, enhances the electrophilic character of the Rh promoting heterolytic σ-bond metathesis with H2. Rh(I) PPh3 PPh3 Cl CO Ph3P Rh(I) PPh3 PPh3 H CO Ph3P commercially available hydroformylation precatalyst PPh3 CO Rh(I) PPh3 PPh3 H CO OC PPh3 OC Rh(I) H PPh3 CO active monohydride catalyst A dihydride catalyst may result in reductive elimination of the intermediate metal alkyl with a second hydride (favored over CO insertion). This would result in undesired hydrogenation product. Hydroformylation proceeds with RhCl(PPh3)3, however there is an induction period. It was shown that in the presence of a base the induction period is removed. What is the role of the base? Indicate the active catalyst for this process and specify how it is formed under the reaction conditions. (Question 3, Q&A pg. 200). Ph3P Rh(I) Ph3P PPh3 Cl C3H7 C3H7 O H cat. H2:CO (1:1, 100 atm), 70oC 16 h, >99% conversion C3H7 O + H linear: branched (2.7:1)
M.C. White Chem 153 Hydroformylation -205- Week of october 29. 2002 Isomerization/hydroformylation referentially. Provide a detailed mechanism for this transformation. Rationalize the effect of unc u/ yo des Ligand % I-nonanal igand in the observed regioselectivity. ( note: Question 5, Q&A pg 202) 92 90% t-B R Ligand, I mol% H branched aldehydes Hy/CO(20 atm),80C, toluene n Leeuwen ACIEE 1999(38)336 18h H sigma-bond note: formation of the dihydride would result in R R RhO R H c0 R R Rh(CO r P R P-R ChOCO R R
M.C. White, Chem 153 Hydroformylation -205- Week of October 29, 2002 Isomerization/hydroformylation A rhodium hydroformylation catalyst was reported that hydroformylates internal alkenes to produce linear aldehydes preferentially. Provide a detailed mechanism for this transformation. Rationalize the effect of the bulky phosphine ligand in the observed regioselectivity. (note: Question 5; Q&A pg. 202) O H C5H11 C5H11 Rh(I) CO CO O O R R 0.1 mol% Ligand, 1 mol% H2/CO (20 atm), 80oC, toluene 18h n + mixture of branched aldehydes iso O t-Bu t-Bu O P O P 1 Ligand PPh3 1 n:iso 0.9 9.2 % 1-nonanal 46% 90% H2 Rh(I) H CO CO R O OH R sigma-bond metathesis? 1 O t-Bu t-Bu R R P R R P Rh(I) H CO note: formation of the dihydride would result in competitive hydrogenation R R P R R P Rh(I) H R' R R P R R P Rh(I) R' H R R P R R P Rh(I) R' H R R P R R P Rh(I) OC R R R P R R P Rh(I) OC O R R CO H2 n-aldehyde bulky ligand may promote hydrometallation to form less sterically hindered primary Rh alkyl. Proposed mechanism CO CO CO CO via Rh(III) dihydride van Leeuwen ACIEE 1999 (38) 336
M C. White, Chem 153 Hydroformylation-206- Week of october 29. 2002 One-Pot Hydroaminomethylation F3c CF3 (BF4) 0. 1 mol% H: CO(50 atm: 10 atm ), piperdine(I eg), tol/THE, R C 120°C.24h R= Me, 97% yield n:iso(90:10) Non-cvclic 1°and2° amines als hydroaminomethylated producer resulted in high yields of lin C served Beller science 2002(297)1676 de product was the enamine Most hydroformylation catalysts show low isomerization activity in the presence Nr2 CHO of strong o-donor ligands such as amines This constitutes the first report of olefin HNR isomerization/ hydroformylation in the presence of amines. R H/CO hydroformylation Isomere CHO catalyst hydroform Thermodynamic equilibrium mixture linear amines are valuable contains less than <5% of terminal olefin dicating that hydroformylation of the terminal olefin must occur at significantly higher rates than the process with internal olefins and with high n-selectivity
M.C. White, Chem 153 Hydroformylation -206- Week of October 29, 2002 One-Pot Hydroaminomethylation Rh(I) (BF4 - ) R R N P P F3C CF3 CF3 CF3 CF3 CF3 F3C CF3 + + Ligand 0.1 mol% 0.4 mol% H2:CO (50 atm:10 atm), piperdine (1 eq), tol/THF, 120oC, 24h R= Me, 97% yield n:iso (90:10) Subtle ligand effects observed. Comparable ligand with p-CF3 substituted aryls gave only 11% yield of the amine products. The major side product was the enamine. Non-cyclic 1o and 2o amines also resulted in high yields of linear hydroaminomethylated products. Beller Science 2002 (297) 1676. R R R P R R P Rh(I) H CO R R CHO R CHO R CHO R NR2 R NR2 R NR2 R NR2 R NR2 R R P R R P Rh(I) H CO H2 H2 H2 R NR2 possible monohydride catalyst olefin isomerization H2/CO H2/CO hydroformylation hydroformylation HNR2 HNR2 HNR2 non-catalyzed non-catalyzed non-catalyzed Most hydroformylation catalysts show low isomerization activity in the presence of strong σ-donor ligands such as amines. This constitutes the first report of olefin isomerization/ hydroformylation in the presence of amines. possible monohydride catalyst linear amines are valuable chemical feedstocks. Thermodynamic equilibrium mixture contains less than <5% of terminal olefin, indicating that hydroformylation of the terminal olefin must occur at significantly higher rates than the process with internal olefins and with high n-selectivity
M C. White, Chem 153 Hydroformylation-207 Week of october 29. 2002 Hydroformylation of alkynes Acetylenes have lower reactivities than alkenes towards hydroformylation. This is attributed largely to formation of stable metal-acetylene complexes that resist hydroformylation under typical conditions. CO (OC)aCq Co(CO)3 =(OC)4 Co-Co(CO)4 (OC)C Co(CO)3 Under forcing conditions of high CO pressure and high temperatures, terminal acetylenes are converted to fully internal alkyne h, 3P CHO H3C CO(0.01mol%)H(0.05mo1%) H/Co(1: 1, 100 atm), 80.C, 24 hrs, 20%conversion 32% CHO H,/CO H hydroformylation of cis-2-butene ldehyde with the same configuration. The isolation of (E)-2-methylbutenal is taken to be the strongest evidence that the fully saturated aldehyde arrises via hydrogenation of an initially formed unsaturated aldehyde PPh Ph3P-RI co(0.01 mol%) H (0.05 mol%) CHO 0(1:1,100atm,.80°C,24hrs,78% CaH ully saturated aldehydes arise via hydroformylation of highly reactive alkene intermediates. However. when I-octene was hydroformylated under these conditions, (R)-2-methyloctanal was Salomon TL 1974(49)4285 formed as the major enantiomer
M.C. White, Chem 153 Hydroformylation -207- Week of October 29, 2002 Hydroformylation of alkynes Acetylenes have lower reactivities than alkenes towards hydroformylation. This is attributed largely to formation of stable metal-acetylene complexes that resist hydroformylation under typical conditions. (OC)3Co Co(CO)3 C C O O (OC)4Co Co(CO)4 R R' + 2 CO (OC)3Co Co(CO)3 R R' Takahashi Bull. Chem. Soc. Jpn. 1988 (61) 4353. Under forcing conditions of high CO pressure and high temperatures, terminal acetylenes are converted to fully saturated aldehydes. H3C CH3 Rh(I) PPh3 PPh3 H CO Ph3P O O PPh2 PPh2 H H + (0.01 mol%) (0.05 mol%) H2/CO (1:1, 100 atm), 80oC, 24 hrs, 20% conversion 32% CHO CHO (S) + 68% CHO H2/CO H2 hydroformylation of cis-2-butene gave an aldehyde with the same configuration. The isolation of (E)-2-methylbutenal is taken to be the strongest evidence that the fully saturated aldehyde arrises via hydrogenation of an initially formed unsaturated aldehyde intermediate. internal alkynes terminal alkynes C6H13 H Rh(I) PPh3 PPh3 H CO Ph3P O O PPh2 PPh2 H H + (0.01 mol%) (0.05 mol%) H2/CO (1:1, 100 atm), 80oC, 24 hrs, 78% conversion C6H13 CHO (S) 27% C6H13 73% CHO + no 1-octenal or 2-methyloctenal was observed, suggesting that the fully saturated aldehydes arrise via hydroformylation of highly reactive alkene intermediates. However, when 1-octene was hydroformylated under these conditions, (R)-2-methyloctanal was Salomon TL 1974 (49) 4285. formed as the major enantiomer
M.C. White Chem 153 Hydroformylation -208- Week of october 29. 2002 Hydroformylation of internal alkynes C3H7 C3H7 C3H te.after 6 hrs reaction C3HA PdCl2(PCy3)2(2mol%), NEt3(60 mol%) H,/CO(1: 1, 35 atm), benzene 150C, Ih went to 84% conversion o% cony 16 o, CHo 2,<1% CHo generating 83%1 and <1% PdCl(PCy 3)2-Co2(COk(2mol %) Co metal is thought to promot -C3h NEt(60 mol%). H/CO(1: 1. 35 atm_ 3H7C归大 the co insertion into the pd-c benzene 150 c 1h bond of the vinylpall 100% conv 1,959CHO 2,2%CHo Intermediate to form Hidai JACS 1997(119)6448. For a Rh system w/bulky ligand see: Buchwald ACIEE 1995 (34) H2 HC ( Cy)3P P(Cy)3( Cy)3Pu P(Cy) P(Cyl P(y)3 (Cy)3p, ICo(CO)4 (OC)C Pd(H the product (OC)C (OC)C monometallic Pd catalyst and that of the heterobimetallic catalytic sy similar metathesis the chemistry is thought to proceed on the Pd center Hidai J. Chem. Soc. dalton PCy3 The Co,( COlg accelerates the Trans. 1995 3489. Reaction of rate of the reaction. It's role PdPh(PMe3(OTf) may be to deliver co to the (CyP (Cy)3R Co( CO)4l- results in facile CO insertion into the P-aryl bond to (OC) accelerating Co insertion. OC)C give(PMe3)(PhCO)PdCo(CO)4 R
M.C. White, Chem 153 Hydroformylation -208- Week of October 29, 2002 Hydroformylation of internal alkynes C3H7 C3H7 C3H7 C3H7 CHO C3H7 C3H7 CHO C3H7 C3H7 C3H7 C3H7 CHO C3H7 C3H7 CHO PdCl2(PCy3)2 (2mol%), NEt3 (60 mol%) H2/CO (1:1, 35 atm), benzene 150oC, 1h 20% conv. 1, 16 % 2, <1 % PdCl2(PCy3)2- Co2(CO)8 (2mol%) NEt3 (60 mol%), H2/CO (1:1, 35 atm) benzene 150oC, 1h 100% conv. 1, 95 % 2, 2 % note: after 6 hrs, reaction went to 84% conversion generating 83% 1 and <1 % of 2. Co metal is thought to promote the CO insertion into the Pd-C bond of the vinylpalladium intermediate to form an acylpalladium species. + + A possible mechanism: Cl Pd(II) (Cy)3P Cl P(Cy)3 Cl Pd(II) (Cy)3P H P(Cy)3 H2 HCl heterolytic cleavage Cl Pd(II) (Cy)3P H R R P(Cy)3 Cl Pd(II) (Cy)3P R R (OC)4Co Pd(II) (Cy)3P (OC)4Co Pd(II) (Cy)3P R R O R R (OC)4Co Pd(II) (Cy)3P H (OC)3Co Pd(II) (Cy)3P R R C O (OC)4Co Pd(II) (Cy)3P H R R migratory insertion [Co(CO)4]- H2 P(Cy)3 P(Cy)3 PCy3 CO σ-bond metathesis Because the product selectivity obtained for the monometallic Pd catalyst and that of the heterobimetallic catalytic system are similar, the chemistry is thought to proceed on the Pd center. The Co2(CO)8 accelerates the rate of the reaction. It's role may be to deliver CO to the Pd center, thereby accelerating CO insertion. Hidai J. Chem. Soc. Dalton Trans. 1995 3489. Reaction of PdPh(PMe3)3(OTf) with [Co(CO)4]- results in facile CO insertion into the P-aryl bond to give (PMe3)2(PhCO)PdCo(CO)4. Hidai JACS 1997 (119) 6448. For a Rh system w/bulky ligand see: Buchwald ACIEE 1995 (34) 1760
M.C. White, Chem 153 Silylformylation -209 Week of octo ber 29. 2002 terminal alkane Silylformylation of alkynes terminal sp C is selectively silylated Chaco)12(1 mol%) Formal product of MepPhSiH(I eq) SiMezph OHC H terminal alkynes with NEt(I eg), Co(29 atm opposite regioselectivity rises from isomerization under th arbon lation conditions CHr=Me RhgCo)2( mol%o ). C Em Other substituents tested: Ph, CO,R. OHC SiMe?Ph NEty(I eq), CO(29 atm OHC H sp c bearing the h,100C 85% yield ZE(70:30) When R3Si-D was used 且 Rhy(co)1 metal co clusters decompose to lower Bm时 alkenal deuterated at the R SiH CO complexes under CO pressure formyl carbon was produced >98% When deuterated alkyne was used, alkenal CO (CO武 RheSis3 as selectively (CO)3 SiR3)Rh (CO)3RhHSiR3 Rssi RasA (CO)aHh ( CO)4Rh OM1997(16)4327
M.C. White, Chem 153 Silylformylation -209- Week of October 29, 2002 Silylformylation of alkynes Me H Me H OHC SiMe2Ph CsF EtOH Me H OHC H C3H7 Me C3H7 Me OHC SiMe2Ph CsF EtOH C3H7 Me OHC H Rh4(CO)12 (1 mol%) Me2PhSiH (1 eq) NEt3 (1 eq), CO (29 atm) 2h, 100oC terminal sp C is selectively silylated 99% yield Z:E (80:20) Z-isomer is the kinetic product. E-isomer arises from isomerization under the carbonylation conditions. terminal alkynes: internal alkynes: Formal product of hydroformylation of terminal alkynes with opposite regioselectivity. Rh4(CO)12 (1 mol%) Me2PhSiH (1 eq) NEt3 (1 eq), CO (29 atm) 2h, 100oC 85% yield Z:E (70:30) sp C bearing the bulkier substituent is selectively formylated tri-substituted olefin Other substituents tested: Ph, CO2R. Matsuda JACS 1989 (111) 2332. Matsuda OM 1997 (16) 4327. Rh4(CO)12 metal CO clusters are known to decompose to lower nuclearity metal R CO complexes under CO pressure 3SiH (CO)4Rh(I) SiR3 (CO)3Rh(I ) SiR3 R' (CO)4Rh(I) R' R3Si (CO)3Rh(I) O R' R3Si (CO)3(SiR3)Rh(III) O R' R3Si H R' H OHC SiR3 CO CO R3SiH oxidative addition migratory insertion migratory insertion When R3Si-D was used, alkenal deuterated at the formyl carbon was produced >98%. When deuterated alkyne was used, alkenal deuterated at the vinyl carbon was selectively isolated (>94%)
M.C. White Chem 153 Silylformylation-210 Week of october 29, 2002 Silylformylation of epoxides cO AOSiMe2Ph note: I-methylpyrazole is essential to HSIMe2 Ph(1.2 eq), Co (50 atm) ring-opening silylformylation. w/out it n=0.1 cyclopentanol silyl ether is the main 0,1 Coco)s was also product observed. Although NEt, was tried; however yields ffective at promoting the rxn with nIng. no cIs cyclohexene, l-methylpyrazole was lower(51% based product observed. 1.82% uniquely effective at promoting silane ing-opening sily formylation over epoxide which was wide range of substrates(other amines used in 3-fold excess) CHo tried: pyridine, pyrrole, DBU failed).It I mol% that I-methylpyrazole OSiMe? Ph promotes CO incorporation, however HSIMe2Ph(1.2 eq), CO (50 atm no discussion of it's mechanism of OSiMe?Ph action is presented(perhaps it's act N 40 mol% as a ligand to the rh) CHO Murai Joc1993(58)4187 60% yield linear: branched(77: 23) Proposed mechanism CO CIRhFSiR3 exact structure HSiR3 not known AOSiMe,Ph Rh h OSiR3 epoxide ring tion CO Muria Inlet 1996414 While C
M.C. White, Chem 153 Silylformylation -210- Week of October 29, 2002 Silylformylation of epoxides Murai JOC 1993 (58) 4187. OC Rh OC Cl Cl Rh CO CO Cl[Rh] SiR3 H HSiR3 exact structure not known O O SiR3 + [RhI II] OSiR3 RhIII OSiR3 H H O RhIII H Cl Cl Cl CO [RhI]Cl HSiR3 OSiMe2Ph CHO nucleophilic epoxide ring opening stereospecific migratory insertion Proposed mechanism: O n= 0,1 OC Rh OC Cl Cl Rh CO CO 1 mol% HSiMe2Ph (1.2 eq), CO (50 atm) N N 40 mol% n= 0,1 OSiMe2Ph O H Stereospecific epoxide ring opening: no cis product observed. n = 0, 72% 1, 82% O OC Rh OC Cl Cl Rh CO CO 1 mol% HSiMe2Ph (1.2 eq), CO (50 atm) N N 40 mol% CHO OSiMe2Ph + OSiMe2Ph CHO 60% yield linear: branched (77:23) note: 1-methylpyrazole is essential to ring-opening silylformylation. w/out it cyclopentanol silyl ether is the main product observed. Although NEt3 was effective at promoting the rxn with cyclohexene, 1-methylpyrazole was uniquely effective at promoting ring-opening silylformylation over a wide range of substrates (other amines tried: pyridine, pyrrole, DBU failed).It is suggested that 1-methylpyrazole promotes CO incorporation, however no discussion of it's mechanism of action is presented (perhaps it's acting as a ligand to the Rh). Co2(CO)8 was also tried; however yields were significantly lower (51% based on silane, 17% based on epoxide which was used in 3-fold excess). Muria Synlett 1996 414
M C. White Chem 153 Silylformylation-211 Week of octo ber 29. 2002 Silylformylation" of alkenes Desired sily/formylation product CO, R]i-H acetate aldol equivalent Observed product: silyl enol ether of formylated alkene near alkenes OSiEt,Me Similar product distributions were obtained RhCI(PPh3)3(1.3 mol%) with CO(CO) Ru(CO)12 however the CHe CHH HSiEt2Me(1 eq) Meet,so overall yields were lower(57% and 40% espectively) Et3 N (3 mol%), CO(x atm) linear: Z: 56% E. 23% branched. Z: 18% E: 13% nzene. 140 C 20h Murai ACIEE 1977(16)881 cv OSiEt,Me OSiEr Me product appears if rxn carried out at higher HSiEt Me(l eq) temperatures or with a high concentration of CO(X atm) benzene. 140%C 20h Murai ACIEE 1977(16)174
M.C. White, Chem 153 Silylformylation -211- Week of October 29, 2002 “Silylformylation” of alkenes Observed product: silyl enol ether of formylated alkene C4H9 RhCl(PPh3)3 (1.3 mol%) HSiEt2Me (1 eq) Et3N (3 mol%), CO (x atm), benzene, 140oC, 20h C4H9 OSiEt2Me 88% linear:Z: 56%; E: 23% + C4H9 MeEt2SiO branched:Z: 18%; E: 13% linear alkenes cyclic alkenes Similar product ditributions were obtained with CO2(CO)8, Ru3(CO)12; however the overall yields were lower (57% and 40% respectively). Murai ACIEE 1977 (16) 881. Co2(CO)8 (0.7 mol%) HSiEt2Me (1 eq) CO (X atm) benzene, 140oC, 20h OSiEt2Me 89% OSiEt2Me product appears if rxn carried out at higher temperatures or with a high concentration of catalyst Murai ACIEE 1977 (16) 174. Desired silylformylation product: R catalyst* CO, R3Si-H R O H SiR3 R O H OH * Tamao * oxidation acetate aldol equivalent polyacetate polyol
M. C. White Chem 153 Silylformylation-212 Week of octo ber 29. 2002 Mechanism OSiEt, Me CofCO) (0.7 mol%) DSiEt2Me (1 eg) D 91% incorporation of the benzene,40°C.20h Proposed mechanism: (OC)cAcO(CO)3 (OC) Co-Co(CO)4 (OC)4Cp…9o(CO)4 DCo(CO)4t RySiCo(CO)4 a-bond metathesis It catalytic cycle SIeTe DCo(CO)4 D-SiEt Me co(X atm)HCo(Co) 4+ OSiR3 HCO(CO)4 Co(cO) Co(CO)4 R3Sr-Co(CO)4 Co(cO)4 Co(co)3 Muria ACIEE 1979(18)837
M.C.White Chem 153 Silylformylation -212- Week of October 29, 2002 Mechanism OSiEt2Me D Co2(CO)8 (0.7 mol%) DSiEt2Me (1 eq) CO (X atm) benzene, 140oC, 20h 91% incorporation of the deuterium in the vinylic position Proposed mechanism: (OC)3Co Co(CO)3 C C O O (OC)4Co Co(CO)4 D SiR3 (OC)4Co Co(CO)4 σ-bond metathesis D-SiEt2Me DCo(CO)4 + R3SiCo(CO)4 catalyst activation: 1st catalytic cycle: DCo(CO)4 D-SiEt2Me HCo(CO)4 OSiEt2Me D D + CO (X atm) + catalytic cycle: HCo(I )(CO)4 H Co(CO)4 O Co(CO)3 O Co(III)SiR3(CO)3 D O D R3Si Co(CO)4 Co(CO)4 SiR3 O D OSiR3 Co(CO)4 D H OSiEt2Me D + DSiR3 Muria ACIEE 1979 (18) 837
M.C. White Chem 153 Silylformylation-213- Week of october 29, 2002 Intramolecular alkene sily formylation -SiPh2 Alkyl 1000 psi CO, allyl, 64%, 4: FPr.79%.6:1 in this system, hydrosilylation -CH, CHOTBS cH, CH otBs. 60%.4: 1 Interestingly, when phenyl homoallylic alcohol are used as substrates, phenyl substituted silanes lead to hydrosilylation whereas diisopropyl silanes result in silylformylated products cO I mol% 1000 pSi CO R groups on Si in conjunction w/R'may dictate which olefin binding mode is preferable. Binding perpendicular to Rh-Si bond results in poor orbital H alignment with Rh-Si but good orbital reductive Rh( CO R入mR时 asertion of H that results in hydrosilylated product. oxidative R2S R R Rh(n H H CO R2Si silane migratory H leighton JACS 1997(119)12416
M.C. White, Chem 153 Silylformylation -213- Week of October 29, 2002 Intramolecular alkene silylformylation Alkyl O Si Ph Ph H Alkyl O SiPh2 O H O Rh O (I)CO CO alkyl = Me allyl i-Pr -CH2CH2OTBS alkyl = Me, 67%, 4.5:1 (cis: trans) allyl, 64%, 4:1 i-Pr, 79%, 6:1 -CH2CH2OTBS, 60%, 4:1 1 mol% 1000 psi CO, benzene, 60o C i-Pr O Si i-Pr i-Pr H O Rh(I) O CO CO 1 mol% i-Pr CH3 O SiPh2 1000 psi CO, benzene, 60o C Silylformylation reactivity depends on the nature of the silicon substituents. If isopropyl is replaced for Ph in this system, hydrosilylation results. Interestingly, when phenyl homoallylic alcohols are used as substrates, phenyl substituted silanes lead to hydrosilylation whereas diisopropyl silanes result in silylformylated products. R groups on Si in conjunction w/R' may dictate which olefin binding mode is preferable. Binding perpendicular to Rh-Si bond results in poor orbital alignment with Rh-Si but good orbital alignment w/Rh-H. This may lead to preferential migratory insertion of H that results in hydrosilylated product. Leighton JACS 1997 (119) 12416. Proposed mechanism: R' O Si R R H O Rh O (I) CO CO O Rh(Iii) O H CO R2Si O R' O Rh(Iii) O H CO CO O R2Si R' O Rh(Iii) O H CO O R2Si R' O R' O SiR2 O H silane migratory insertion oxidative addition CO migratory insertion reductive elimination O Rh(Iii) O H CO R2Si O R' or i-Pr CH3 O SiPh2