M.C. White, Chem 153 Hydrogenation-142- Week of october 15. 2002 Wilkinson hydrogenation: classic dihydride mechanism aPPh3 a PPh PPh solution structure determined by NMr strong T-acids(e.g. ethylene)bind PPh electron rich rh center and inhibit hydrogenation +PPh3l-PPh +PPh3|-PPh: alytic cycle PPh H Ph m M Cl PPh wPPh3 H PhaP CI PPh coordinatively unsaturated complex reacts w/H2 10Tx reductive migratory PhaEn MlCL elimination PPh3 Insertion RDS Phal mediates observed by NMR or as isolated Halpern Chem. Comm. 1973 629 solids in the reaction system. Formation of Halpern J. Mol. Catal. 1976(2)65 e-products"results in a reduction in Halpern Inorg. Chim. Acta. 1981(50)11 te of hyM.C. White, Chem 153 Hydrogenation -142- Week of October 15, 2002 Wilkinson hydrogenation: classic dihydride mechanism Ph3P Rh(I) Ph3P PPh3 Cl H2 Ph3P Rh(III) H PPh3 Cl H PPh3 oxidative addition -PPh3 Ph3P Rh(I) S PPh3 Cl solution structure determined by NMR. Ph3P Rh(III) H PPh3 Cl H R +PPh3 Ph3P Rh(III) PPh3 Cl H S R migratory insertion RDS Ph3P Rh(III) H PPh3 Cl H S -PPh3 +PPh3 H2 reductive elimination oxidative addition catalytic cycle coordinatively unsaturated complex reacts w/ H2 104 x faster than Rh(Cl)PPh3 Ph3P Rh(I) PPh3 Cl strong π-acids (e.g. ethylene) bind tightly to the electron rich Rh center and inhibit hydrogenation Ph3P Rh(I) Ph3P Cl Cl Rh(I) PPh3 PPh3 Ph3P Rh(I) Ph3P Cl Cl Rh(III) H H PPh3 PPh3 H2 Intermediates observed by NMR or as isolated solids in the reaction system. Formation of these "side-products" results in a reduction in the rate of hydrogenation. Halpern Chem. Comm. 1973 629. Halpern J. Mol. Catal. 1976 (2) 65. Halpern Inorg. Chim. Acta. 1981 (50) 11