Copper-diphosphine complex catalysts for N-formylation of amines under 1 atm of carbon dioxide with polymethylhydrosiloxane

Abstract

N-formylation of a wide range of amines proceeded using copper-diphosphine complexes as homogeneous catalysts with polymethylhydrosiloxane (PMHS) under 1 atm of CO₂. In the reaction of piperidine, for example, the turnover number (TON) reached 11 700 in 23 h with 90% yield of the formylated product. This TON value is much higher than those of the reported catalysts for the formylation of amines under 1 atm of CO₂ with hydrosilanes. The Cu complexes with phosphines having ortho-phenylene structures acted as good ligands for the formylation, as compared to a bidentate ligand connected with a propyl chain and a monodentate ligand. Among these diphosphines, ligands with alkyl functionalities, such as isopropyl and cyclohexyl groups, produced better results than the phenyl group. Not only cyclic secondary amines, but also linear secondary amines and aromatic and aliphatic primary amines were found to be reactive substrates. In the case of 2,2,6,6-tetramethylpiperidin-4-amine, the formylation proceeded regioselectively. A catalytic reaction pathway was proposed from a separate experiment using [Me₂NCO₂][Me₂NH₂].