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 [(η⁶-p-cymene)(BNHC)RuCl₂] (1a-g) were synthesized via one-pot reaction of the NHC salt precursor, Ag₂O, and [RuCl₂(p-cymene)]₂ and characterized using conventional spectroscopic techniques. The geometry of two precursors, [(η⁶-p-cymene)(^Me4BnMe₂BNHC^CH2OxMe)RuCl₂] (1f) and [(η⁶-p-cymene)(^Me5BnMe₂BNHC^CH2OxMe)RuCl₂] (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.