Ligand-free reductive deoxygenation of benzylic carbonyl compounds using [Co₂(CO)₈] and syngas

Deoxygenation is an important pathway for adding value to biomass-derived feedstocks in both the commodity and fine chemical sectors, and benzylic aldehydes and ketones serve as useful model compounds for these substrates. In this work, we demonstrate that the reductive deoxygenation of benzylic aldehydes and ketones can be achieved using [Co₂(CO)₈] and syngas (H₂/CO) in alcoholic media, in the absence of additional ligands or additives. The simplicity of this system contrasts with other methodologies, which often require bases and/or stoichiometric reagents, as well as expensive second- and third-row transition metal complexes. Notably, isopropanol emerges as the most suitable solvent and represents an environmentally friendly option. The protocol is effective for a range of benzylic aldehydes and ketones bearing different electron-donating groups on the aromatic ring, as well as for indole derivatives containing an aldehyde functionality at the β-position.