Cationic Rhodium Diphosphane Complexes as Efficient Catalysts for the Semi-Hydrogenation of Dehydroisophytol

The selective reduction of terminal alkynes to alkenes was investigated using common cationic diphosphane rhodium complexes of the type [Rh(PP)(diolefin)]X (PP=diphosphane, X=anion). The effectiveness of the catalyst was demonstrated in the semi-hydrogenation of dehydroisophytol (DIP), an industrial produced intermediate of vitamin E. The present study highlights the high activity and good selectivity of this simple catalytic system. However, deactivation increases at higher DIP concentrations. Several strategies to circumvent the deactivation are presented. The semi-hydrogenation of dehydroisophytol (DIP) to isophytol (IP) is the key step in the production of synthetic vitamin E. In place of the usual heterogeneous catalysts, Schrock-Osborn catalysts of the type [Rh(PP)(diolefin)]+ were used in the study with great success in terms of selectivity. The limits of the system were elucidated and a deactivating side reaction was identified and characterized.image