Effective catalytic oxidation of alcohols and alkenes with monomeric versus dimeric manganese(II) catalysts and t-BuOOH

Abstract

Two new Mn(II) complexes, [Mn(C6H5COO)(H2O)(phen)(2)](ClO4)(CH3OH) (1) and [Mn-2(-C6H5COO)(2)(bipy)(4)]2(ClO4) (2) (phen = 1,10-phenanthroline; bipy = 2,2-bipyridine), were synthesized and characterized using UV-visible and infrared spectroscopies and single-crystal X-ray diffraction analyses. Complexes 1 and 2 have six-coordinate octahedral geometry around the Mn(II) centre. Complex 1 is a monomer and consists of a deprotonated monodentate benzoate ligand together with two neutral bidentate amine ligands (phen) and a water molecule. Complex 2 has a dinuclear structure in which two Mn(II) ions share two carboxylate groups, adopting a two-atom bridging mode, and two chelated bipy ligands. Both complexes catalyse the oxidation of alcohols and alkenes in a homogeneous catalytic system consisting of the Mn(II) complex and tert-butyl hydroperoxide (TBHP) in acetonitrile. The system yields good to quantitative conversions of various alkenes and alcohols, such as styrene, ethylbenzene and cyclohexene to their corresponding ketones, and primary alcohols and 1-octanol, 1-heptanol, cyclohexanol, benzyl alcohols and cinnamyl alcohol to their corresponding aldehydes and carboxylic acids. Complexes 1 and 2 exhibit very high activity in the oxidation of cyclohexene to cyclohexanone (ca 80% selectivity) as the main product (ca 94% conversion in 1 h) and of cinnamyl alcohol to cinnamaldehyde (ca 64% selectivity) as the main product (ca 100% conversion in 0.5 h) with TBHP at 70 degrees C in acetonitrile. In addition, optimum reaction conditions were also determined for benzyl alcohol with complexes 1 and 2 and TBHP. Copyright (c) 2016 John Wiley & Sons, Ltd.