Direct synthesis of benzimidazoles by Pd(II) N^N^S-pincer type complexes via acceptorless dehydrogenative coupling of alcohols with diamines

Abstract

A sustainable synthetic method for the construction of substituted benzimidazoles via acceptorless dehydrogenative coupling of primary alcohols by palladium catalysts is reported. Three new non-classical symmetric Pd(II) N^N^S pincer complexes (1–3) were synthesized and characterized by spectral (FT-IR, UV-vis, NMR and ESI-MS) and analytical techniques. Moreover, the distorted square planar geometry of the synthesized complexes 1 and 2 was confirmed by single-crystal X-ray diffraction study. A wide range of benzimidazole derivatives (32 examples) up to 94% isolated yield has been derived via acceptorless dehydrogenative coupling of primary alcohols with derivatives of o-phenylenediamines. The present protocol operates smoothly with only 1 mol% catalyst loading. Furthermore, the mechanistic investigation with the aid of control experiments revealed that the reaction proceeded through in situ generation of aldehyde intermediate and generates hydrogen and water as the only byproducts.