Abstract
A low temperature hydrogen borrowing approach to generate secondary amines using benzimidazole-based N-heterocyclic carbene (BNHC) ruthenium complexes is reported. A series of the piano-stool complexes of the type [(η(6)-p-cymene)(BNHC)RuCl(2)] (1a-g) were synthesized via one-pot reaction of the NHC salt precursor, Ag(2)O, and [RuCl(2)(p-cymene)](2) and characterized using conventional spectroscopic techniques. The geometry of two precursors, [(η(6)-p-cymene)((Me4Bn)Me(2)BNHC(CH2OxMe))RuCl(2)] (1f) and [(η(6)-p-cymene)((Me5Bn)Me(2)BNHC(CH2OxMe))RuCl(2)] (1g), was studied by single crystal X-ray diffraction. These catalysts were found to dehydrogenate alcohols efficiently at temperatures as low as 50 °C to allow Schiff-base condensation and subsequent imine hydrogenation to afford secondary amines. Notably, this ruthenium-based procedure enables the N-alkylation of aromatic and heteroaromatic primary amines with a wide range of primary alcohols in excellent yields of up to 98%. The present methodology is green and water is liberated as the sole byproduct.