Benzimidazol-2-ylidene ruthenium complexes for C-N bond formation through alcohol dehydrogenation.

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.