Air‐stable Ruthenium(II)‐NNN Pincer Complexes for the Efficient Coupling of Aromatic Diamines and Alcohols to 1H‐benzo[d]imidazoles with the Liberation of H2

Abstract

AbstractTwo new phosphine‐free RuII‐NNN pincer complexes ([RuCl(L1)(CH3CN)2]Cl (1) and [RuCl(L2)(CH3CN)2]Cl (2) [L1=2,6‐bis(1H‐imidazole‐2‐yl)pyridine, L2=2,6‐bis(1‐hexyl‐1H‐imidazole‐2‐yl)pyridine] were synthesized to catalyze the condensation of benzyl alcohol and benzene‐1,2‐diamine homogeneously to 2‐pheny‐1H‐benzo[d]imidazole and H2. The reactivity in the order of 1>2 is lower than that of the phosphine‐containing RuII‐NNN pincer complex [RuCl2(L1)(PPh3)3] (3), and thus a homogeneous system that contained 1, 1 equivalent of 1,2‐bis(diphenylphosphanyl)ethane, and 10 equivalents of NaBPh4 was developed to improve the catalytic efficiency for the condensation of primary alcohols and benzene‐1,2‐diamine (or its derivatives) to 2‐substituted 1H‐benzo[d]imidazoles in excellent yields (up to 97 %) and turnover numbers (388). This system can be used to realize the facile one‐step synthesis of 1H‐benzo[d]imidazole derivatives from alcohols without the use of an oxidant and/or a stoichiometric amount of inorganic base that is usually necessary in homogeneous systems reported previously.