A modular design of metal catalysts for the transfer hydrogenation of aromatic ketones

The ability of transition metal catalysts to add or remove hydrogen from organic substrates by transfer hydrogenation is a valuable synthetic tool. Towards a series of novel metal complexes with a P―NH ligand, [Ph 2 PNHCH 2 ―C 4 H 3 O] derived from furfurylamine were synthesized. Reaction of [Ph 2 PNHCH 2 ―C 4 H 3 O] 1 with [Ru(η 6 ‐ p ‐cymene)(μ‐Cl)Cl] 2 , [Ru(η 6 ‐benzene)(μ‐Cl)Cl] 2 , [Rh(μ‐Cl)(cod)] 2 and [Ir(η 5 ‐C 5 Me 5 )(μ‐Cl)Cl] 2 gave a range of new monodentate complexes [Ru(Ph 2 PNHCH 2 ―C 4 H 3 O)(η 6 ‐ p ‐cymene)Cl 2 ] 2 , [Ru(Ph 2 PNHCH 2 ―C 4 H 3 O)(η 6 ‐benzene)Cl 2 ] 3 , [Rh(Ph 2 PNHCH 2 ‐C 4 H 3 O)(cod)Cl] 4 , and [Ir(Ph 2 PNHCH 2 ‐C 4 H 3 0)(η 5 ‐C 5 Me 5 )Cl 2 ] 5 , respectively. All new complexes were fully characterized by analytical and spectroscopic methods. 31 P‐{ 1 H} NMR, distortionless enhancement by polarization transfer (DEPT) or 1 H‐ 13 C heteronuclear correlation (HETCOR) experiments were used to confirm the spectral assignments. Following activation by KOH, compounds 1 , 2 , 3 , 4 catalyzed the transfer hydrogenation of acetophenone derivatives to 1‐phenylethanol derivatives in the presence of iso‐PrOH as the hydrogen source. Notably [Ru(Ph 2 PNHCH 2 ‐C 4 H 3 O)(η 6 ‐benzene)Cl 2 ] 3 acts as an excellent catalyst, giving the corresponding alcohols in 98–99% yield in 20 min at 82°C (time of flight ≤ 297 h −1 ) for the transfer hydrogenation reaction in comparison to analogous rhodium or iridium complexes. Copyright © 2012 John Wiley & Sons, Ltd.