Catalysis of Organic..

248Homogeneous Hydrogenationthe presence of NEt 3 , proceeds according to the following scheme (in the casereported below we consider a Z geometric isomer as starting material):RCH 2CH 2CH 2CH 2OHH 2H 2RH3CCH 24H2C1CH 2OHisomRHCCH 2CCH 2OHHisomisomRCH 2CH 2CH 2COHThree main reaction routes are operating. In addition to the fully hydrogenatedproduct, double bond and geometric isomerization derived compounds were alsodetected. Indeed, the double bond migration process from 2-3 carbons to the 2-1 (seethe above scheme), involving, in this case, the carbon atom bearing the -OH group,affords the aldehydes through a vinyl alcohol intermediate. No hydrogenolysisproducts were observed and only a small amount of compounds, where the doublebond moves from the 2-3 to the 3-4 carbons, was detected.The selectivity and the yield of the process depend mainly on three differentfactors: i) the catalytic species in solution; ii) the geometric isomer structure of theorganic substrate; iii) the chain length of the reacting compound. Taking into accountsome of our previous results (1), showing the formation of an 1:1 electrolyte byconductometric measurements on the same rhodium(I) coordinated system, used inthis paper, and including also an interesting observation by Schrock and Osborn (2),the following equilibria may be considered in our system:RhCl(PPh 3 ) 3 + H 2 + solv H 2 RhCl(PPh 3 ) 2 (solv) + PPh 3H 2 RhCl(PPh 3 ) 2 (solv) + NEt 3 + PPh 3RhH(PPh 3 ) 3 + NEt 3 H + + Cl - + solvConsequently, in the absence of NEt 3 , the main catalytic species should beH 2 RhCl(PPh 3 ) 2 (solv), whereas, in the presence of NEt 3 , RhH(PPh 3 ) 3 should beformed. We are aware that such a conclusion is somewhat speculative; however, itseems the most likely if we look at all the experimental data reported on the subject.Table 1 reports experimental results concerning both activity and product distribution,determined in different conditions. Since the Rh(I) mono hydride complex is acatalytic species, it is reformed in every step of the reaction and its concentrationremains constant. Therefore, rate data are calculated by the ratio of slopes of plots oforganic substrate concentration, divided by the Rh(I) concentration, versus reactiontime. The slope of these curves is obtained at about 70 % conversion of the substrate.