Iridium-Catalyzed Transfer Hydrogenation of 1,10-Phenanthrolines using Formic Acid as the Hydrogen Source

The iridium-catalyzed highly regioselective transfer hydrogenation of a variety of 2-substituted and 2,9-disubstituted 1,10-phenanthrolines under mild conditions with formic acid as the hydrogen source is described. In the presence of a catalytic amount of the iridium complex [Cp*IrCl2](2), the transfer hydrogenation proceeded smoothly in 1,4-dioxane under ligand-free conditions, exclusively leading to a range of 1,2,3,4-tetrahydro-1,10-phenanthroline products in high yields. The catalyst generated in situ from [Cp*IrCl2](2) and (R,R)-(CF3)(2)C6H3SO2-dpen [N-(2-amino-1,2-diphenylethyl)-3,5-bis(trifluoromethyl)benzenesulfonamide] could efficiently catalyze the asymmetric transfer hydrogenation of these 1,10-phenanthrolines in isopropyl alcohol (i-PrOH) to afford chiral 1,2,3,4-tetrahydro-1,10-phenanthrolines in high yields with up to >99% ee. The key to the success of the reduction is the choice of solvent and hydrogen source.