Heterogeneous Low-Valent Mn Catalysts for α-Alkylation of Ketones with Alcohols through Borrowing Hydrogen Methodology

Alcohol transformations through the borrowing hydrogen (BH) methodology have attracted attention due to its high atom economy and substrate availability. Nonprecious metal heterogeneous catalysts have recently been extensively explored; however, the difficulty in the observation of the active metal species has prevented mechanistic studies. Here, we report on supported Mn catalysts that act as reusable heterogeneous catalysts for the construction of C–C bonds by the α-alkylation of ketones with alcohols through the BH methodology. The catalyst, a Mn2+ species-MgO mixture-deposited Al2O3 support (Mn-MgO/Al2O3), exhibits catalytic performance for the reactions to give the corresponding products in 50–92% yield. The present catalyst did not require the addition of homogeneous strong bases that are typically indispensable for these reactions using the reported Mn-based heterogeneous catalysts and that require large energy consumption for separation, recycling, and waste treatment. While the addition of bases to such reaction systems has been considered to accelerate the dehydrogenation of alcohols and/or aldol condensation, MgO in Mn-MgO/Al2O3, a heterogeneous base, does not contribute to these steps. Fourier transform infrared spectroscopy (FT-IR) measurements indicated not only the incorporation of Mn hydride species, which has never been observed on heterogeneous Mn-based catalysts by the dehydrogenation of alcohols, but also enhancement of the hydrogenating capability of the Mn hydride species by co-deposited MgO on Al2O3. While such hydride species had been found to accelerate direct amination of alcohols over a metallic Ru nano particles-MgO mixture, the present study reveals that the reaction mechanism is extended to α-alkylation of ketones with alcohols over oxidized Mn, a base metal, in contact with MgO.