Hydrogenation of dienes with cationic rhodium complexes

Abstract

Abstract Methyl cis ‐9, cis ‐12‐octadecadienoate (methyl linoleate; c 9, c 12), its t 10, t 12 and t 10, c 12 isomers and methyl cis ‐9‐octadecenoate (methyl oleate; c 9) were hydrogenated with rhodium complexes, the active species of which consisted of [RhL 2 ] + and [RhL 2 H 2 ] + with ligands L=P(C 2 H 5 ) 2 C 6 H 5 (catalyst A) P(i‐C 4 H 9 ) 3 (catalyst B) and P(CH 3 ) 3 (catalyst C). Using these catalysts the influence of steric effects on the reaction mechanism of hydrogenation of dienes was studied. The reactions were carried out in 2‐propanol at atmospheric hydrogen pressure and ambient temperature. During hydrogenation of c 9 on catalysts A and B, geometrical isomerization mainly occurred, whereas on catalyst C some positional isomerization also took place. C 9, c 12 was almost exclusively hydrogenated via conjugated intermediates on catalyst A. On catalyst C, one of the double bonds was hydrogenated directly, in most cases. In the absence of hydrogen, catalysts A and B conjugated c 9, c 12 very fast. The conjugation activity of catalyst C was much lower. Catalyst C showed a high 1,5‐shift activity for the conjugated cis, trans and trans, cis intermediates during hydrogenation, in contrast to catalysts A and B, which showed a poor activity in this respect. T 10, t 12 was hydrogenated almost exclusively via 1,4‐addition of hydrogen to the cisoid conformation, whereas only a slight preference was found in this mechanism over 1,2‐addition for the hydrogenation of t 10, c 12. On the sterically unhindered catalysts A and C the trans double bond in t 10, c 12 was preferentially hydrogenated whereas on catalyst B, with its bulky ligands, the cis double bond was reduced faster than the trans double bond.