M.C. White. Chem 153 Overview -282- Week of november l1. 2002 Functionalization of terminal olefins via H migratory insertion /reductive elimination sequence logemann LM-H t Hh Hydrosilation L\H H-SiR3 RYSiR, Hydrocyanation Ln\ H t HCN R Hydroformylation LnH +H-H+
M.C. White, Chem 153 Overview -282- Week of November 11, 2002 Functionalization of terminal olefins via H migratory insertion /reductive elimination sequence R LnM H R R MLn H H H MI R H H R H H H SiR3 LnM H R R MLn H SiR3 SiR3 MI R SiR3 H R H CN LnM H R R MLn H CN CN MI R CN H R H LnM H R R MLn H MI R H O R O R O R O R + Hydrogenation RE Hydrosilylation + RE Hydrocyanation + RE Hydroacylation + RE masked Hydroformylation R H H C O LnM H R R MLn H CO CO MI R H RE O MLn H MI R H O + + H
M.C. White, Chem 153 Overview -283- Week of november 11. 2002 Wilkinson's Catalyst H2(I atm), benzene, rt OA ofl Hydroacylation: OA ofRC(O)-H Phap HRH PhaB,, H Rapid Pr3/C( occurs with W under a CO atmosphere(Rh has a igh affinity for CO). COs H2CO(1:1,100atm) strong T-acids and may serve to NEt3 disfavor OA of H, to generate
M.C. White, Chem 153 Overview -283- Week of November 11, 2002 Wilkinson’s Catalyst Hydrogenations: OA of H2 Ph3P Rh(I) Ph3P PPh3 Cl R Ph3P Rh(III) H PPh3 Cl H R R H H Hydrosilylations: OA of R3Si-H Ph3P Rh(I) Ph3P PPh3 Cl R Ph3P Rh(III) H PPh3 Cl SiR3 R R H cat. SiR3 R3Si-H, benzene, rt cat. H2 (1 atm), benzene, rt quantitative Hydroformylation Ph3P Rh(I) Ph3P PPh3 Cl R Rh(I) PPh3 CO H CO R H cat. H2:CO (1:1, 100 atm) NEt3 O H R Hydroacylation: OA of RC("O")-H Ph3P Rh(I) Ph3P PPh3 Cl R cat. Rh(III) H N N Ph Cl Ph3P R N N H Ph N N R Ph Rapid PR3/CO ligand exchange occurs with Wilkinson's catalyst under a CO atmosphere (Rh has a high affinity for CO). CO's are strong π-acids and may serve to disfavor OA of H2 to generate a Rh(III) dihydride
M.C. White, Chem 153 Overview -284- Week of november 11. 2002 Hydrogenation via o-bond metatheses: Rulll) catalysts Monohydride identified as active catalyst: Formation of Ru(iv) dihydride is prohibitively high in cat RuCl(PPh3)3 NEt3 (I eql, benzene Ph3PRy(ay"PPh3 H2(1 eq) +¨CI"HNE Et3 H2(l atm), benzene: ethanol, rt H2cO HcO NCOCH3 (R-1(0.5-1mo% OAe EtOH: CH,CI,(5: 1 ), H,(4 atm), 23 OCH3 H3 Possible monmohydride hydrogenation mechanisms(Hydrogenations, pg. 156) LmMX々H,耳 H、H a-bond metathesis no oxidation state C-bond metathesis B LmH_LmMH—LmM LM ozonolysis
M.C. White, Chem 153 Overview -284- Week of November 11, 2002 Hydrogenation via σ-bond metatheses: Ru(II) catalysts Possible monohydride hydrogenation mechanisms (Hydrogenations, pg. 156). L mMn X H2 LnMn X H H L mMn H σ-bond metathesis no oxidation state change R L mMn L m Mn H H H R R σ-bond metathesis L mMn HX H H L mMn H2 base-promoted heterolytic cleavage B L mMn H HB L mMn H R L mMn R HB L mMn H R L mMn protonolysis Ph3P Ru(II) PPh3 PPh3 H Cl RuCl2(PPh3)3 H2 (1 eq) NEt3 (1 eq), benzene + -Cl +HNEt3 cat. H2 (1 atm), benzene:ethanol, rt quantitative R R RuCl2(PPh3)2 Monohydride identified as active catalyst: Formation of Ru(IV) dihydride is prohibitively high in energy. Wilkinson's Ru catalyst Noyori's Ru catalyst NCOCH3 H3CO AcO OAc OCH3 P Ph2 Ru(II) Ph2 P O O O O NCOCH3 H3CO AcO OAc OCH3 (0.5-1 mol%) EtOH:CH2Cl2 (5:1), H2 (4 atm), 23o C 92% yield 95% ee (R)-1
M.C. White, Chem 153 Overview -285- Week of november 11. 2002 Rate of migratory insertion into olefin M-H>>M-C (MeO)3h MeOP (MeO)P △G=223 kcal/mol The difference of 10.3 kcal/mol in the free energy of activation of ethyl rs hydride migratory insertion into ethylene in these systems corresponds to a rate ratio of hHwdkEty of 10-10. The effect is thought to be kinetic in nature Brookhart JACS 1985(107)1443 Rationalization: Better overlap is possible because of non-directionality of the H orbital oM-H o* M-H X o+ M-C Trigonal-bipyramidal Note: at the ts of olefin insertion, both oM-H to TC=c back-donation and ic=c to o* M-H donation are necessary for bond exchange Morokuma JACS 1988(110)3417 Morokuma OM 1997(16)1065
M.C. White, Chem 153 Overview -285- Week of November 11, 2002 Rate of migratory insertion into olefin: M-H>>M-C RhIII (MeO)3P H + RhIII (MeO)3P H + ‡ RhIII (MeO)3P + ∆ G‡ = 12.0 kcal/mol RhIII (MeO)3P + RhIII (MeO)3P + ‡ RhIII (MeO)3P + ∆ G‡ = 22.3 kcal/mol The difference of 10.3 kcal/mol in the free energy of activation of ethyl vs hydride migratory insertion into ethylene in these systems corresponds to a rate ratio of kH MI/kEt MI of 107-108. The effect is thought to be kinetic in nature. Brookhart JACS 1985 (107) 1443. Rationalization: Better overlap is possible because of non-directionality of the H orbital. π* σ M-H π σ∗ M-H π* σ M-C π σ∗ M-C Trigonal-bipyramidal structure Note: at the TS of olefin insertion, both σM-H to π*C=C back-donation and πC=C to σ* M-H donation are necessary for bond exchange. Morokuma JACS 1988 (110) 3417. Morokuma OM 1997 (16) 1065
M.C. White, Chem 153 Overview -286- Week of november 11. 2002 Regioselectivity of hydrometallation Aliphatic terminal olefins: For aliphatic terminal olef re is both a strong thermodynamic preference to form the sterically less hindered M-C bond. Structure& Bonding, pg. 32: As seen for C-H o bonding, there is a general trend towards weaker M-C a bonds with increased substitution LIi- DIr-c: 58 kcal/mol Dr-c: 52 kcal/mol DIr-C: 48 kcal/mol If an equilibration mechanism exists, M-alkyls will isomerize to the least sterically hindered 1o product. For more examples of this see: Hydroformylation, pg 205 and 206 drosilylation,Pg. 183 i olefin isomerization to terminal olefin leads to 1 metal alkyl that undergo h(SirUM Ln(Sir3)R R3Sr-RhLn 少 RhCI(PPhi)z RaSi-H CAHe R3SI CAHl Conjugated terminal olefins: For conjugated olefins, hydride insertion results in formation of the 20 M-C which can be stabilised as a delocalised r -intermediate Hydroform on, pg H2CO(1:1) Rh(Co)L i branched: linear(11: 0) Ligand Effects? Hayashi's observed hydrosilylation regioselectivities with aliphatic terminal olefins cannot be rationalized using our models(Hydrosilylation, pg. 189) 0.1 mol% C!H MOP (0.002 mol%) CHHg OMe iCl regioselectivity(87: 13) MOP
M.C. White, Chem 153 Overview -286- Week of November 11, 2002 Regioselectivity of hydrometallation Aliphatic terminal olefins: For aliphatic terminal olefins, there is both a strong thermodynamic preference to form the sterically less hindered M-C bond. LnIr(III) C4H13 LnIr(III) LnIr(III) Structure & Bonding, pg. 32: As seen for C-H σ bonding, there is a general trend towards weaker M-C σ bonds with increased substitution. DIr-C: 58 kcal/mol DIr-C: 52 kcal/mol DIr-C: 48 kcal/mol Ln(SiR3)Rh H Rh(SiR3)Ln H Ln(SiR3)Rh H C4H9 R3Si RhLn R3Si C4H13 If an equilibration mechanism exists, M-alkyls will isomerize to the least sterically hindered 1o product. For more examples of this see: Hydroformylation, pg.205 and 206. RhCl(PPh3)3 R3Si-H olefin isomerization to terminal olefin leads to 1o Hydrosilylation, pg. 183 metal alkyl that undergoes RE to product. Conjugated terminal olefins: For conjugated olefins, hydride insertion results in formation of the 2o M-C which can be stabilized as a delocalized η3-intermediate. RhH(CO)(PPh3) H2:CO (1:1) Rh(CO)Ln Rh(CO)Ln O H branched: linear (11:0) Hydroformylation, pg. 192, 194, 195; Hydrocyanation, pg. 232. Ligand Effects? Hayashi's observed hydrosilylation regioselectivities with aliphatic terminal olefins cannot be rationalized using our models (Hydrosilylation, pg. 189). C4H9 OMe PPh2 Pd Cl Pd Cl HSiCl3 (1.2 eq) 0.1 mol% + MOP (0.002 mol%) C4H9 SiCl3 regioselectivity (87:13) MOP
M.C. White, Chem 153 Overview -287- Week of november 11. 2002 Unsolved problem: intermolecular sily formylation of terminal olefins Desired sily/formylation product. H H polyacetate polyol Observed silylformylation product SieTe CofCO)8(0.7 mol% C9 HSiEt2Me(l eq), co CoHo benzene.140C20h mj. product linear Route of the problem? One hypothesis is that hydrometalation of terminal olefins with Co-H occurs at a faster rate than silylmetalation with Co-SiR,. The overall result is the hydroformylated aldehyde product which can further react with Co-SiR3(recall Si is oxophilic) to give the silylenol ether. See Silylformylation, pg. 211 (OC)4C…Co(CO)4 HCo(CO)4\ R3SiCo(CO)4 Silylenol ether formation HCo(CO)4 regenerates the Co-H pecies DSik3 H Co(CO)4 R3Si-Co(CO)4 o(CO)4 Co(CO)3 CoIr CO)3
M.C. White, Chem 153 Overview -287- Week of November 11, 2002 Unsolved problem: intermolecular silylformylation of terminal olefins Co2(CO)8 (0.7 mol%) HSiEt2Me (1 eq), CO benzene, 140oC, 20h C4H9 C4H9 OSiEt2Me mj. product linear Desired silylformylation product: R catalyst* CO, R3Si-H R O H SiR3 R O H OH * Tamao * oxidation acetate aldol equivalent polyacetate polyol Observed silylformylation product: (OC)4Co Co(CO)4 H SiR3 σ-bond metathesis HCo(CO)4 + R3SiCo(CO)4 HCo(I)(CO)4 R H Co(CO)4 R O Co(CO)3 R O Co(III)SiR3(CO)3 H R O H R3Si Co(CO)4 Co(CO)4 SiR3 R O H R OSiR3 Co(CO)4 H H R OSiEt2Me H + HSiR3 R Route of the problem? One hypothesis is that hydrometalation of terminal olefins with Co-H occurs at a faster rate than silylmetalation with Co-SiR3. The overall result is the hydroformylated aldehyde product which can further react with Co-SiR3 (recall Si is oxophilic) to give the silylenol ether. See Silylformylation, pg. 211. Silylenol ether formation regenerates the Co-H species
M.C. White, Chem 153 Overview -288 Week of november 11. 2002 Unsolved problem: direct hydroacylation of terminal olefins Desired hydroacylation product direct route to di-substituted ketones 0 from unactivated precursors Observed intermolecularly: decarbonylation Phah PPh PPh3 tetrIc CoHo saturated solution Route of the problem? One hypothesis is as follows: The catalyst must have three"open"and adjacent coordination sites for the acyl hydride, and olefin moieties to effect successful hydroacylation. Upon oxidative addition of the aldehyde to give the hydridoacyl metal species, intramolecular binding of the acyl alkyl substituent to the syn open site competes effectively with intermolecular olefin binding nd results in decarbonylation Ph3.. PPh3 R l3B. Ph3po desired cycle Phal CrRhsmCO Ph3P
M.C. White, Chem 153 Overview -288- Week of November 11, 2002 Unsolved problem: direct hydroacylation of terminal olefins R O H + R' catalyst R O R' Desired hydroacylation product direct route to di-substituted ketones from unactivated precursors C4H9 O H Rh Cl PPh3 Ph3P PPh3 + Observed intermolecularly: decarbonylation saturated solution CHCl3 Rh Cl PPh3 Ph3P CO + C5H12 stoichiometric Rh Ph3P Ph3P H Cl O R Rh Cl PPh3 Ph3P PPh3 Rh Cl PPh3 Ph3P Rh Ph3P Ph3P H Cl R O R O H Rh Ph3P Ph3P Cl O R R' R' R' R' Rh Ph3P Ph3P Cl R O R' desired cycle R' Rh Ph3P Ph3P H Cl CO R Rh Cl PP h3 Ph3P CO H R Route of the problem? One hypothesis is as follows: The catalyst must have three "open" and adjacent coordination sites for the acyl, hydride, and olefin moieties to effect successful hydroacylation. Upon oxidative addition of the aldehyde to give the hydridoacyl metal species, intramolecular binding of the acyl alkyl substituent to the syn open site competes effectively with intermolecular olefin binding and results in decarbonylation
M.C. White, Chem 153 Overview -289- Week of november 11. 2002 Functionalization of terminal alkynes via migratory insertion /reductive elimination sequence? H H t h-H H H LMH R Silylformylation +H--SiR3+ LnM-SiR3 MIL,M SiR3 MI Ln(SiR3)M R LnyF H
M.C. White, Chem 153 Overview -289- Week of November 11, 2002 Functionalization of terminal alkynes via migratory insertion /reductive elimination sequence? LnM H R MLn H H H MI R H H + H H Hydrogenation RE R R LnM H R R LnM H MI R H RE + H H H H H Silylformylation H SiR3 C O R + + LnM SiR3 R CO LnM SiR3 CO R SiR3 R O Ln(SiR3)M H MI MI RE SiR3 R O H Hydroacylation H LnM H MI O R' O R' + RE masked R R MLn O R' R H R H R' O
M.C. White, Chem 153 Overview -290- Week of november 11. 2002 Unsolved problem. hydroformylation of terminal alkynes CO H2 (Er-a P-unsaturated Route of the problem: acetylenes form stable metallocyclopropane/propene complexes that resist hydroformylation under mild conditions 2 C0 (OC)Co-Co(CO)4 (OC)C Co(CO)3 Under forcing conditions of high CO/H2 pressure and high temperatures, terminal acetylenes are converted to fully saturated linear aldehydes. Ph2P一 H2CO(1:1,100atm),80PC, CHO CHO 24 hrs. 73 conversion possible intermediate (never observed)
M.C. White, Chem 153 Overview -290- Week of November 11, 2002 Unsolved problem:hydroformylation of terminal alkynes Desired hydroformylation product: catalyst* R CO, H2 R O H (E)- α,β-unsaturated aldehydes Route of the problem: acetylenes form stable metallocyclopropane/propene complexes that resist hydroformylation under mild conditions. (OC)4Co Co(CO)4 R R' + 2 CO (OC)3Co Co(CO)3 R R' R H Rh(I) PPh3 PPh3 H CO Ph3P + (0.01 mol%) H2/CO (1:1, 100 atm), 80oC, 24 hrs, 73 % conversion L R CHO possible intermediate (never observed) R CHO Under forcing conditions of high CO/H2 pressure and high temperatures, terminal acetylenes are converted to fully saturated linear aldehydes
M.C. White, Chem 153 Overview -291 Week of november 11. 2002 Silylformylation of alkynes terminal alkynes terminal sp c is selectively silylated NEt(I eq), CO (29 atm) OHC SiMe,P 2h,100°C zE(8020) Z-isomer is the kinetic product E-isomer Rh(coh (CO)3RhHH (CO)ARh--SiR (CO)3SiR3)Rh (CO)4 There is no mechanism to regenerate rh-H once it has been consumed in a cycle een available for elimination
M.C. White, Chem 153 Overview -291- Week of November 11, 2002 Silylformylation of alkynes Me H Me H OHC SiMe2P 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: Rh4(CO)12 R3SiH (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 (CO) + 4Rh(I) H (CO)3Rh(I) H R' (CO)4Rh(I) R' H (CO)3Rh(I) O R' H (CO)3(SiR3)Rh(III) O R' H H (CO)4Rh(I) SiR3 O R' H H R3Si (CO)4Rh O R' H H (CO)4Rh R3Si There is no mechanism to regenerate Rh-H once it has been consumed in a cycle. Unlike the silylformylation of alkenes, the product of aldehyde silylmetalation has no β-hydrogen available for elimination
