Introduction and History

Abstract

Iridium had shown no special advantage in heterogeneous catalysis and this situation seemed to carry over into the early work in homogeneous catalysis. In particular, the failure of IrCl(PPh₃)₃, the Ir analog of Wilkinson’s catalyst, to give catalytic hydrogenation strongly influenced thinking at the time. The developments that changed the situation were Shrock and Osborn’s work on [(cod)RhL₂]⁺ complexes that showed how lowering the L:M ratio from 3 to 2 gave improved catalysts, followed by our own finding that [(cod)IrL₂]⁺ and especially [(cod)IrLL′]⁺ complexes are even more highly active, particularly for highly substituted alkenes. This performance is absent in the classical coordinating solvents then standard in the field but only appears in weakly coordinating solvents like dichloromethane. A special property of the Ir complexes is a pronounced directing effect caused by catalyst binding to substrate functionality, leading to very high diastereoselectivity of the reduction. Asymmetric hydrogenations consistently gave lower ee values for iridium over rhodium, however, so academic work still strongly emphasized Rh. The exceedingly rapid rates of hydrogenation with Ir attracted industrial attention, because rate is such an important factor in that case, and Novartis’ catalytic asymmetric route to the pesticide (S)-Metolachlor employs iridium, even though the ee value is only ca 80%. Leading applications of these and other Ir catalysts up to about 1,990 are also discussed. Work by Pfaltz, Burgess, and Andersson showed how efficient asymmetric Ir catalysts could be made by judicious choice of ligand. Alkane dehydrogenation by reverse hydrogenation also proved possible with Ir catalysts.