Selective catalytic oxidation using hydrogen/oxygen mixtures with solutions of iridium and rhodium complexes

Abstract

An iridium(III) hydride complex has been used in solution under a hydrogen/oxygen atmosphere at ambient temperatures to catalyze slowly the selective oxygenation of cyclooctene to cyclooctanone. The H2 acts as a two-equivalent reducing agent in accepting the second oxygen atom of the O2 molecule to generate water. The overall reaction shows similarities to catalytic oxygenation reactions performed by a class of P450 enzymes. Spectroscopic evidence for the iridium system indicates an oxygen atom transfer to the olefin via a hydroperoxide intermediate (IrOOH), formed by insertion of O2 into the metal hydride bond; the hydride is thought to be regenerated, together with formation of water, by hydrogenolysis of the resulting iridiumhydroxo species. Catalytic hydrogenation of the monoene as well as hydrogenolysis of oxygen to water are observed as competing reactions. Some related rhodium systems do not give selective oxidation products.