Hydrogenation of Quinaldine and Benzylic Aldehydes both Separately and Combined in a Tandem Hydrogenation-Reductive Alkylation of Quinaldine by Aldehydes with Iridium Benzoquinoline Catalysts

A series of cyclometalated benzoquinoline complexes of Ir(III) catalyze the hydrogenation of the heterocyclic ring of quinolines under mild conditions. Our best catalyst is active in a significantly wider range of solvents than our previous systems. In the presence of a suitable base, the Ir(III) species is also able to hydrogenate the C=O bonds of aldehydes. When quinolines and aldehydes are present together, the Ir(III) complex catalyzes a tandem reaction in which the quinoline is first hydrogenated to a tetrahydroquinoline that is subsequently reductively alkylated by the aldehyde. The reductive alkylation competes with the hydrogenation of the aldehyde to the alcohol, and therefore good yields of the alkylated tetrahydroquinoline require the presence of excess aldehyde.