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

Authors: ZAHID NAWAZ, NEVİN GÜRBÜZ, MUHAMMED NAVEED ZAFAR, NAMIK ÖZDEMİR, BEKİR ÇETİNKAYA, İSMAİL ÖZDEMİR

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 [( η⁶ -p-cymene)(BNHC)RuCl2 ] (1a-g) were synthesized via one-pot reaction of the NHC salt precursor, Ag2 O, and [RuCl2 (p-cymene)]2 and characterized using conventional spectroscopic techniques. The geometry of two precursors, [(η⁶ -p-cymene)(Me4BnMe2 BNHCCH2OxMe)RuCl2 ] (1f) and [(η⁶ -p-cymene)(Me5BnMe2 BNHCCH2OxMe)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.

Keywords: Benzimidazol-2-ylidenes, ruthenium complexes, amine alkylation, C-N bond formation, mild conditions

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