Piano-stool benzothiazol-2-ylidene Ru(II) complexes for effective transfer hydrogenation of carbonyls

Abstract

Benzothiazolium bromides (NSHC center dot HBr) (1a-j) with a variety of alkyl chain or benzyl substituents on N atom were prepared. The synthesis of ruthenium(II) (NSHC) complexes, (2a-j) could be achieved by in situ deprotonation of benzothiazolium salts with Ag2O and [RuCl2(p-cymene)](2). They were characterized by H-1, C-13, F-19 NMR, IR spectroscopy and elemental analysis. The molecular structures of 2d, 2e and 2g were ascertained by single-crystal X-ray diffraction studies. The catalytic properties of complexes, (2aej), with electron-donating or -withdrawing groups were investigated in transfer hydrogenation (TH) of ketones, and aldehydes with good to excellent yields. The presence of the CH2(CH2)(16)CH3 or CH2C6F5 on the N atom of the benzothiazol-2-ylidene complexes (2g, 2j) showed the highest activity for TH reaction. The catalytic properties of the N-alkyl substituted ruthenium(II) (NSHC) complexes(2h-j) may be interpreted by micelle effects. (C) 2016 Elsevier B.V. All rights reserved.