Hydrogenation of Unsaturated Fatty Acid Methyl Esters by Dicobalt Octacarbonyl

Abstract

Although the olefin hydrogenation by dicobalt octacarbonyl is now well known as the side-reaction of oxo-reaction so far as the olefin has any conjugated structure, the investigation devoted only to hydrogenation has not yet been reported.In this present investigation conducted at 120190°C in 2530 atm. of hydrogen with unsaturated fatty acid methyl esters, it was found that the unconjugated double bond as well as conjugated can also be reduced more rapidly to monoene than the case of iron pentacarbonyl and monoene component remained entirely unchanged. But catalyst decomposition by ester group was also faster and all the cobalt carbonyl added was converted to cobalt soap.To elucidate these results the following reaction procedure was assumed; Co-ordinately unsaturated hydride of HCo(CO)3 is formed in the first place and its hydrogen-cobalt bond adds to double bond and makes carbon-cobalt bond, simultaneously its unfilled d-orbital catches the π-electrons of adjacent double bond and makes π-bonded complex, which has tautomeric structure of π-allyl-complex, then the scission of the cobalt-carbon bond is accomplished by the homolytically activated hydrogen already in the orbital through a displacement of π-electrons, and with consequent regeneration of HCo(CO)3. The unconjugated will be hydrogenated through conjugation process caused by addition-elimination of HCo (CO)3. Catalyst decomposition would be explained by nucleophilic attack of oxygen of ester group on the unfilled orbital.