HOMOGENEOUS CATALYSIS BY OSMIUM COMPLEXES - A REVIEW

Homogeneous catalysis by osmium complexes is more promising than hitherto realized. Most of the reactions studied have concentrated on simple model substrates, and therefore a demonstration of the utility of these catalysts for reactions of more sophisticated organic molecules is needed; highly selective reduction of terminal and internal carbon-carbon triple bonds in presence of other unsaturations within the same molecule should become an immediate goal. In view of the excellent results obtained in the asymmetric dihydroxylation of alkenes, other highly enantioselective processes, such as hydrogenation, oligomerization, hydrosilylation and hydroformylations may be envisaged with osmium complexes; catalytic activation of C-H, C-S, C-N, N-H, and OH bonds also seem reasonable and important targets. As a consequence of the high thermal and oxidative stability observed for osmium complexes, their syntheses, manipulation, and recycling may prove much simpler than for analogous, less robust catalysts derived from other metals, and these advantages may counter the higher cost involved in the case of osmium. Furthermore, this enhanced stability will certainly be convenient for kinetic and mechanistic studies which could lead to a deep understanding of the catalytic chemistry.