The preparation of soluble mixed-ligand rhodium hydrogenation catalysts by reduction of dioxygenrhodium complexes

Abstract

The reduction of the dioxygen species [{RhCl(O₂)(PPh₃)₂}₂] with ethanol in the presence of 1 equivalent of a phosphine ligand, L, other than PPh₃, gives a catalytically active mixture having a PPh₃ : L ratio of 3 : 1. When the reduction is carried out under hydrogen, this mixture is composed of two species, [RhClH₂(PPh₃)₂] and [RhClH₂(PPh₃)L]. A comparison is made of the rate and stereochemical data obtained using these mixtures for the hydrogenation of 1-methylene-4-t-butylcyclohexane with data obtained using the [RhClL₂] complexes as catalysts. This comparison and others have shown that in the mixed-ligand species, [RhClH₂(PPh₃)L], the ligand L is present exclusively at the site trans to the position of olefin complexation prior to hydrogen transfer.