Selective homogeneous hydrogenation of triunsaturated fats catalyzed by tricarbonyl chromium complexes

Abstract

Abstract The need for a selective catalyst to hydrogenate linolenate in soybean oil has prompted our continuing study of various model triunsaturated fats. Hydrogenation of methyl β ‐eleostearate (methyl trans,trans,trans ‐9,11,13‐octadecatrienoate) with Cr(CO) 3 complexes yielded diene products expected from 1,4‐addition ( trans ‐9, cis ‐12‐ and cis ‐10, trans ‐13‐octadecadienoates). With α ‐eleostearate ( cis,trans,trans ‐9,11,13‐octadecatrienoate), stereoselective 1,4‐reduction of the trans,trans ‐diene portion yielded linoleate ( cis,cis ‐9,12‐octadecadienoate). However, cis,trans ‐1,4‐dienes were also formed from the apparent isomerization of α ‐ to β ‐eleostearate. Hydrogenation of methyl linolenate (methyl cis,cis,cis ‐9,12,15‐octadecatrienoate) produced a mixture of isomeric dienes and monoenes attributed to conjugation occurring as an intermediate step. The hydrogenation of α ‐eleostearin in tung oil was more stereoselective in forming the cis,cis ‐diene than the corresponding methyl ester. Hydrogenation of linseed oil yielded a mixture of dienes and monoenes containing 7% trans unsaturation. We have suggested how the mechanism of stereoselective hydrogenation with Cr(CO) 3 catalysts can be applied to the problem of selective hydrogenation of linolenate in soybean oil.