Sustainable Access to (E)-2-(2-benzylidenehydrazinyl)-4-phenylthiazoles by Ruthenium(II) Catalyst via Dehydrogenative Coupling of Alcohols

Abstract

Medicinally important (E)-2-(2-benzylidenehydrazinyl)-4-phenylthiazoles have been derived via acceptorless dehydrogenative coupling (ADC) of readily available alcohols using new ruthenium(II) catalyst. The arene ruthenium(II)-N^O chelating complex has been synthesised and structurally characterized with the help of elemental analysis, IR, NMR and single crystal XRD techniques. A wide range of 2,4- disubstituted thiazole derivatives (15 examples) has been constructed from dehydrogenative coupling of alcohols with thiosemicarbazide followed by cyclization with 2-bromoacetophenone under mild reaction condition. The reactions proceed well using low catalyst loading (1 mol%) with the maximum of 92% yield and produce eco-friendly H2O and H2 gas as the only by-products. Various control experiments and mechanistic studies revealed that the reaction proceeds via alcohol dehydrogenation. In addition, a large-scale synthesis of one of the derivatives proves the highlight of the catalytic efficiency of the present methodology.