Catalysis of Organic..

250Homogeneous Hydrogenationmetal surface is more open to interactions with olefinic substrates while, in thepresence of a homogeneous catalyst, the crowding of ligands around a metal maymake such an interaction more difficult. In this case, the reactivity of the morehindered E compounds is drastically reduced.Table 1. Hydrogenation and isomerization of α,β-unsaturated alcohols catalyzed byRhCl(PPh 3 ) 3 ( ∼ 3 x 10 -4 M) at 303 K, using ethanol as solvent.r bSelectivityAldeyde(%) aE-isomerSubstrate [NEt 3 ](M) (min -1 )SaturatedalcoholOtherproductA2 0 1.12 81.5 18.5A2 0.03 2.72 80.8 19.2EB2 0 0.18 n. d. c n. d.EB2 0.03 1.47 79.1 19.4 1.4ZP2 0 0.54 67.8 8.2 24.0ZP2 0.03 2.49 13.8 14.4 64.1 7.7EP2 0 0.16 n.d. n.d.EP2 0.03 0.32 n.d. n.d.ZH2 0 0.31 n.d. n.d. n.d.ZH2 0.03 1.94 37.8 10.6 48.4 3.15a At 80% conversion of the unsaturated alcohol; b Rate data (see text);c n.d. = non-determined.Taking into account all these considerations, the following general mechanismmay be operating, when the hydrogenation of a Z geometric isomer is carried out inthe presence of NEt 3 :H 2Rh Cl(PPh 3) 2(solv)HRRhH(PPh 3) 3RHCCCH 2OHNEt 3k 1> k 2, k 3HPh 3PH HRhPh 3PPPh 3HCCRCH 2OHCHCHOH 2C HRhPh 3PBPPh 3PPh 3Ak 1k 2 k3H 2HOH 2CHHOC C R CH 2CH 2C HOH 2C CH 2CH 2CH 2OHRHC D E+++RhH(PPh 3) 3RhH(PPh 3) 3RhH(PPh 3) 3We infer that final products, C, D and E, stem from the same metal-carbon σ-bondedintermediate B, and their relative amounts are due to kinetic factors. Carrying out thesame reaction starting from an E geometric derivative, the E/Z isomerization is, as