Mild homogeneous oxidation and hydrocarboxylation of cycloalkanes catalyzed by novel dicopper(II) aminoalcohol-driven cores

Abstract

N-benzylethanolamine (Hbea) and triisopropanolamine (H3tipa) were applied as unexplored aminoalcohol N,O-building blocks for the self-assembly generation of two novel dicopper(II) compounds, [Cu2(μ-bea)2(Hbea)2](NO3)2 (1) and [Cu2(H3tipa)2(μ-pma)]·7H2O (2) {H4pma=pyromellitic acid}. These were isolated as stable and aqua-soluble microcrystalline products and were fully characterized by IR spectroscopy, ESI–MS(±), and single-crystal X-ray diffraction, the latter revealing distinct Cu2 cores containing the five-coordinate copper(II) centers with the {CuN2O3} or {CuNO4} environments. Compounds 1 and 2 were used as homogeneous catalysts for the mild oxidation of C5–C8 cycloalkanes to give the corresponding cyclic alcohols and ketones in up to 23% overall yields based on cycloalkane. The reactions proceed in aqueous acetonitrile medium at 50°C using H2O2 as an oxidant. The effects of different reaction conditions were studied, including the type and loading of catalyst, amount and kind of acid promoter, and water concentration. Despite the fact that different acids (HNO3, H2SO4, HCl, or CF3COOH) promote the oxidation of alkanes, the reaction is exceptionally fast in the presence of a catalytic amount of HCl, resulting in the TOF values of up to 430h−1. Although water typically strongly inhibits alkane oxidations due to the reduction of H2O2 concentration and lowering of the alkane solubility, in the systems comprising 1 and 2 we observed a significant growth (up to 5-fold) of an initial reaction rate in the cyclohexane oxidation on increasing the amount of H2O in the reaction mixture. The bond-, regio- and stereo-selectivity parameters were investigated in oxidation of different linear, branched, and cyclic alkane substrates. Both compounds 1 and 2 also catalyze the hydrocarboxylation of C5–C8 cycloalkanes, by CO, K2S2O8, and H2O in a water/acetonitrile medium at 60°C, to give the corresponding cycloalkanecarboxylic acids in up to 38% yields based on cycloalkanes.