Highly active iridium catalysts for the hydrogenation of ketones and aldehydes

Abstract

The pressure hydrogenation capabilities of the iridium pincer complexes IrH₂Cl[(ⁱPr₂PC₂H₄)₂NH] (1) and IrH₃[(ⁱPr₂PC₂H₄)₂NH] (2) are described and compared to related results obtained previously in transfer hydrogenation. Complex 1 was shown to act as a convenient air-stable entry point to the active catalyst 2, in the presence of base and hydrogen gas. The catalysts are active in a range of solvents, including CH₂Cl₂ and CHCl₃, in contrast to related ruthenium systems. This class of iridium complexes is very effective for the direct hydrogenation of a wide range of carbonyl compounds including ketones, diketones, α,β-unsaturated ketones and aldehydes. A catalytic cycle is proposed for this system which involves an ionic heterolytic bifunctional hydrogenation mechanism.