Cobalt-Catalyzed Reductive Deoxygenation of Aldehydes, Ketones, Alcohols, and Ethers to Alkanes

The development of simple and general catalytic methods using abundant base metal catalysts for the deoxygenation of organic compounds for the valorization of biomass and chemical synthesis remains a formidable challenge. Cobalt pincer-catalyzed reductive deoxygenation of aldehydes, ketones, alcohols, and ethers to their corresponding alkanes is reported. Biomass-derived compounds were deoxygenated to their corresponding methylarenes. This catalytic system employs diethylsilane as a reductant and requires a substoichiometric amount of base. Investigations revealed the initial deprotonation of the amine arm on the catalyst, and catalysis begins with Si–H activation of silane facilitated by amine-amide metal–ligand cooperation. In situ-formed Co–H species carry out the hydrosilylation of carbonyl compounds, dehydrosilylation of alcohols, and hydrosilanolysis of ethers, resulting in common arylmethylsilyl or alkylsilyl ether intermediates. Further, the reaction of diethylsilane with arylmethylsilyl or alkylsilyl ether leads to deoxygenation and the formation of siloxane oligomers. Further, a DFT study reveals closely lying singlet–triplet electronic states of different intermediates facilitating the mechanism on different spin surfaces, verified through the located minimum energy crossing points (MECPs). In the case of alcohol, another low energy pathway is located where the amide arm of pincer, instead of hydride, can abstract the hydroxy proton and enable metal–ligand cooperation.