Regioselective Dehydrogenation of the Secondary Amine Complexes into Imine Complexes under Visible-Light Irradiation

The dehydrogenative oxidation of amine into imine is an important transformation; however, the regioselective dehydrogenation in the secondary amine is still a challenge. Here, the development of a mild and regioselective oxidation of amines into imines is described under visible-light irradiation in the presence of oxygen. The photoreaction of Δ-[Ir­(L)2(l-AA)] (where L is 2-phenylquinoline (pq), 2-phenylpyridine (ppy), or 2-phenylisoquinoline (piq); AA is secondary amino acid) diastereomer is highly selective in the presence of O2 in an EtOH solution, affording the corresponding α-imine product Δ-[Ir­(L)2(AA-2Hα)] in an excellent yield, while the photoreaction of Λ-[Ir­(pq)2(l-AA)] diastereomer is competitive in the interligand C–N cross-coupling and dehydrogenative oxidation reactions depending on the reaction temperature. More interestingly, the dehydrogenative oxidation reactions are regioselective in Λ-[Ir­(pq)2(l-AA)] diastereomer, affording α′-imine as a main product at low temperature. The influence of cyclometalated ligands, diastereomeric substrates, reaction temperature, solvents, and base as well as the structure of the secondary amines on the competitive pathways are systematically investigated. The syntheses of the α- and α′-imine regioisomers, which are useful substrates for further reactivity, are also developed.