M.C. White, Chem 153 Hydrogenation-145- Week of october 15. 2002 wilkinson: directing group effects Phb, 0.04mol% note: when pd/c was used a mixture of cis and trans K+B. 0.04 mol% Meo H2(6.8 atm, 100psi ) benzene, 50C CIs Isomer( exclusIv reaction without the alkoxide is OK attibuted to the steric hinderance of the PPh tri-substituted double bond, which renders it less able to coordinate to the Rh. The protonated alcohol is not a strong enough nucleophile to associatively displace the anionic chloride ligand Base-assisted formation of the alkoxide results in effective displacement of the chloride ligand and trisubstituted thus directs olef leo Thompson JACS 1974(96)6232 Jardine Prog. Inorg. Chem. 1981(28)63
M.C. White, Chem 153 Hydrogenation -145- Week of October 15, 2002 Wilkinson: directing group effects MeO OH trisubstituted Ph3P Rh(I) Ph3P PPh3 Cl 0.04 mol% H2 (6.8 atm, 100psi), benzene, 50oC no reaction Ph3P Rh(I) Ph3P PPh3 Cl 0.04 mol% H2 (6.8 atm, 100psi), benzene, 50oC 68% K+ BMeO OH H MeO O-K+ trisubstituted H O Rh H PPh3 H PPh3 cis isomer (exclusive) note: when Pd/C was used a mixture of cis and trans isomers resulted Thompson JACS 1974 (96) 6232. Jardine Prog. Inorg. Chem. 1981 (28) 63. The slow reaction without the alkoxide is attibuted to the steric hinderance of the tri-substituted double bond, which renders it less able to coordinate to the Rh. The protonated alcohol is not a strong enough nucleophile to associatively displace the anionic chloride ligand. Base-assisted formation of the alkoxide results in effective displacement of the chloride ligand and thus directs olefin complexation from the same face
M C. White, Chem 153 Hydrogenation -146 Week of october 15. 2002 Schrock-Osborn /Crabtree: Cationic catalysts y3 (PF6) H cis-oxidative ClS-migratory (PF6) PC (PF6) solvated active catalyst Crabtree Acc Chem Res 1979(12)331 Turnover Frequency(ToF) Wilkinsons catalyst ordinatively"unsat benzene/EtOH 25C atonic hydrogenation catalysts Schrock-Osborn catalyst (PF6) weakly coordinating solvents PPh provides the olefin substrate with relatively free access to the metal,s reactive site. These cationic catalysts are also P Crabtree's catalyst (PF6) TOF =mol reduced substrate/mol catalyst/h
M.C. White, Chem 153 Hydrogenation -146- Week of October 15, 2002 Schrock- Osborn /Crabtree: Cationic catalysts Diene ligated cationic catalysts mode of activation: Ir(I) PCy3 N (PF6-) Ir(III) PCy3 H (PF6-) H N Ir(III) PCy3 H (PF6-) N + + + S Ir(I) PCy3 N (PF6-) + repeat S Ir(I) S PCy3 N (PF6-) + diene ligated catalyst precursor solvated active catalyst H2 cis-oxidative addition cis-migratory insertion cis-reductive elimination Crabtree Acc Chem Res 1979 (12) 331. Ph3P Rh(I) Ph3P PPh3 Cl Ir(I) N PCy3 (PF6-) Rh(I) PPh3 PPh3 (PF6-) + + Turnover Frequency (TOF) TOF = mol reduced substrate/mol catalyst/h CH2Cl2, 25oC CH2Cl2, 25oC benzene/EtOH, 25oC Schrock-Osborn catalyst Crabtree's catalyst Wilkinson's catalyst 650 4000 6400 700 10 4500 13 ---- 3800 ---- ---- 4000 "Coordinatively" unsaturated cationic hydrogenation catalysts are the most active homogeneous hydrogenation catalysts developed thus far. Use of weakly coordinating solvents provides the olefin substrate with relatively free access to the metal's reactive site. These cationic catalysts are also remarkably selective