M.C. White Chem 153 Hydrogenation-141- Week of october 15, 2002 Wilkinson: substrate selectivity .N PPh3 rate of hydrogenation of PhaP competition unsaturated substrate I mol% unsaturated rate of hydrogenation of substrate H2(1 atms ), benzene,rt aturated 1-octene substrate saturated substrate competition figure 14.7 HO→ Unsaturated substrates containing functionality are hydrogenated more rapidly than their unfunctionalized counterparts. The effect is suggested to result from polar functional group assisted olefin coordination to the catalyst. Terminal alkynes are hydrogenated more rapidly than terminal alkenes. This selectivity may be enhanced by use of acidic alcohol co-solvents (e.g. in benzene also 1-heptyne, 1-octyne Terminal alkenes between C6-C12 are hydrogenated at the same rate. The same is observed for terminal alkynes. An C+H9 ncrease in carbon chain length does not appear to affect also. l-decene. 1-dodecene olefin/catalyst interaction Conjugated dienes are reduced slower than isolated alkenes 0.75 Internal and branched alkenes(alkynes) are hydrogenated slower than terminal alkenes(alkynes). These differences are C3h rationalized in terms of steric effects on olefin interaction 0.54 with the catalyst and have been used to effect selective C3h alkene hydrogenations in polyene compounds 0.17 C3h Candlin Faraday Discuss. Chem. Soc. 1968(46)60M.C. White, Chem 153 Hydrogenation -141- Week of October 15, 2002 Ph3P Rh(I) Ph3P PPh3 Cl 1 mol% H2 (1 atms.), benzene, rt + unsaturated substrate + saturated substrate competition figure = rate of hydrogenation of unsaturated substrate rate of hydrogenation of 1-octene Wilkinson: substrate selectivity Candlin Faraday Discuss. Chem. Soc. 1968 (46) 60. HO HO EtO C3H7 , C4H9 C2H5 C2H5 C2H5 also 1-heptyne, 1-octyne also, 1-decene, 1-dodecene C3H7 C3H7 C3H7 C3H7 NC unsaturated substrate competition figure 14.7 9.1 3.4 1.8 1.7 2.6 0.92 0.75 0.71 0.69 0.54 0.17 1.0 cyclohexene 1,3-cyclooctadiene Terminal alkenes between C6-C12 are hydrogenated at the same rate. The same is observed for terminal alkynes. An increase in carbon chain length does not appear to affect olefin/catalyst interaction. Internal and branched alkenes (alkynes) are hydrogenated slower than terminal alkenes (alkynes). These differences are rationalized in terms of steric effects on olefin interaction with the catalyst and have been used to effect selective alkene hydrogenations in polyene compounds. Unsaturated substrates containing functionality are hydrogenated more rapidly than their unfunctionalized counterparts. The effect is suggested to result from polar functional group assisted olefin coordination to the catalyst. Terminal alkynes are hydrogenated more rapidly than terminal alkenes. This selectivity may be enhanced by use of acidic alcohol co-solvents (e.g. in benzene/ 2,2,2-trifluoroethanol, 1-hexyne: 1-octene (12:1). Conjugated dienes are reduced slower than isolated alkenes