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 430 h⁻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.