Mechanistic Studies of Transfer Hydrogenation Catalyzed by Ru(ll)-Half-Sandwich and Group 6 NHC complexes

Abstract

The thesis explores the mechanistic aspects of transfer hydrogenation of ketones and hydrosilylation of imines; two commonly used organic transformations catalyzed by ruthenium half-sandwich complexes of 2-oxazolidinethiones, 2-thiozolidinethiones, and orthometallated ligands. Hydrosilylation of imines catalyzed by group 6-NHC complexes is also presented. A series of NHC complexes of chromium, molybdenum, and tungsten are synthesized by the carbene transfer reaction from the corresponding Ag-NHC complexes and the metal hexacarbonyl complexes. These NHC complexes are examined for their catalytic activity in the hydrosilylation of N-benzylideneaniline. The reactions carried out in refluxing ethanol under nitrogen for 24 h give the products in reasonable yields. The effect of additives like peroxides and Me3NO, and U.V irradiation to promote the reaction is also studied. Four of the representative complexes are examined for hydrosilylation of various aldimines with different electronic and steric properties. These studies show that the homoleptic carbonyls of Cr, Mo, and W which show very low catalytic activity in the hydrosilylation reactions can be made to catalyze the hydrosilylation by substitution of a CO with an NHC ligand. The catalytic activity improves when two carbonyls are substituted with a biscarbene ligand as compared with monocarbene complexes. This trend points to the importance of the increased electron density on the metal center for catalytic activity. In summary, the thesis presents mechanistic aspects of transfer hydrogenation and hydrosilylation using a variety of metal complexes and hydrogen donors