Dehydrogenation of primary alcohols to carboxylic acids in water catalyzed by PNHN-manganese(I) carbonyl complexes

Three classes of manganese(I) complex cation, [(PNHN)Mn(CO)3]Br (Mn1), [(SNHN)Mn(CO)3]Br (Mn2) and [(NRNHN)Mn(CO)3]Br (Mn3 (NMe), Mn4 (NEt), Mn5 (NiPr)), all incorporating a chelating 5,6,7,8-tetrahydro-8-quinolinamine (NHN) but distinct in their third donor atom, have been assessed as catalysts for the dehydrogenative oxidation of primary alcohols to form their corresponding carboxylic acids. Using water as solvent and NaOH as base, PNHN-containing Mn1 proved the most effective allowing both aromatic and aliphatic alcohols to be converted at 160 °C with excellent functional group tolerance (25 examples disclosed). A possible mechanism for this process, that makes use of an acceptorless dehydrogenation pathway, has been proposed and supported by targeted experimentation. Overall, these catalysts show great promise for applications in atom-economic carboxylic acid synthesis as well as in the development of organic hydride-based hydrogen storage systems.