Chiral mixed phosphorus/sulfur ligands in late transition metal-catalyzed reactions: Structure, reactivity and mechanism

A new class of mixed phosphorus/sulfur ligands was highly successful for the asymmetric hydrogenation of a variety of unsaturated substrates. Varying substitution on the sulfur and phosphorus donors and on the ligand backbone was shown to have a significant effect on the enantioselectivity. X-ray crystal structures of several catalyst precursors provided important information about the factors associated with high selectivity. Two optimum catalysts were identified and a wide variety of enamides could be hydrogenated in >90% ee using one or both of them. Details about the mechanism of action of these catalysts were obtained from the structure of the catalyst/substrate complex, which existed as a single isomer in solid state and in solution and correctly predicted the sense of induction in the product. Kinetic and thermodynamic selectivity in the oxidative addition of hydrogen to analogous iridium complexes was observed. Both formation of the catalyst/substrate complex and subsequent transformation of this intermediate were controlled by a combination of the trans influence and the stereochemistry at the sulfur donor.; The utility of the mixed phosphorus/sulfur ligand class in other late transition metal-catalyzed processes has also been demonstrated. The rhodium-catalyzed hydrosilylation of prochiral ketones was accomplished in high enantioselectivity for a wide variety of aryl and alkyl ketones. Extension of the hydrosilylation process to the rhodium-catalyzed reductive aldol reaction yielded some promising selectivities, and the formation of an unusual by product shed some light on an interesting mechanistic problem. Iridium complexes of mixed phosphorus/sulfur ligands were capable of catalyzing hydrogenation and isomerization reactions in moderate to good enantioselectivity. Finally, the utility of these ligands was extended to the copper-catalyzed conjugate addition reaction, and the structure of the precatalyst was determined.