Abstract
In order to reduce methyl linoleate selectively, catalytic hydrogen transfer from eleven steroids, six terpenes, and an alkaloid was investigated at 140°C. In the hydrogenation with cholesterol and D-limonene, RuH2 (PPh3)4 was the most active catalyst and RhH (PPh3)4 was the next, but the latter showed higher selectivity to monoenes and less formation of isolated trans double bonds than the former. The catalytic activity of RuCl2 (PPh3)3 was low and that of (NH4)2PdCl4, PdCl2, Pd-carbon, Pd-black, Pt-asbestos, and Pt-black was negligible. Most of the other natural products showed high hydrogen-donating ability in the presence of RuH2 (PPh3)4 and RhH (PPh3)4. In the cholesterol-RhH (PPh3) 4 system, complete selectivity to monoenes and scarce formation of trans bonds were realized with high conversion. This system in which the reactions of other substrates were also carried out showed the following characteristics : (1) the reactivity of trans-9, trans-12-octadecadienoate was comparable to that of linoleate, and that of elaidate was comparable to that of oleate; (2) the reactivity of alkali conjugated octadecadienoates was much lower than that of the nonconjugated dienoates; (3) in the reactions of the dienoates, the selectivity to monoenoates was complete but oleate and elaidate were also reduced considerably, that is, monoenes were reduced in the absence of dienes but were not reduced in the presence of dienes; (4) the ratio of cis to trans in the isolated double bonds became nearly constant in the absence of conjugated dienes and it was about 3 : 1. in the cholesterol-RuH2 (PPh3)4 system, the reactivity of the conjugated dienoates was comparable to that of the nonconjugated dienoates and the selectivity to monoenes was low. Also in this system, the ratio of cis to trans isomers became approximately constant and it was about 2 : 1.
