Regioselective catalytic hydrogenation of nitrogen rings of fused heteroaromatic compounds with an iridium-triphenylphosphine complex

Abstract

Catalytic hydrogenation of fused heteroaromatic compounds, acridine, quinoline and quinaldine, selectively occurs at the nitrogen containing rings to give 9,10-dihydroacridine, 1,2,3,4-tetrahydroquinoline and 1,2,3,4-tetrahydroquinaldine in the homogeneous solution prepared from the reaction of [Ir(COD)(PPh₃)(PhCN)]ClO₄ (COD=1,5-cyclooctadiene) with H₂ (5 atm) at 50°C, while isoquinoline and indole are not hydrogenated under the same experimental conditions. In the presence of the rhodium analog [Rh(COD)(PPh₃)(PhCN)]ClO₄ however, both isoquinoline and indole are also hydrogenated at the nitrogen containing rings, which is understood in terms of heterogeneous catalysis by metallic rhodium powders produced in the reduction of the rhodium compound under the same experimental conditions (50°C, 5 atm of H₂).