Ketone Asymmetric Hydrogenation Catalyzed by P‑NH-P′ Pincer Iron Catalysts: An Experimental and Computational Study

Our group previously reported the development of iron carbonyl catalysts bearing chiral tridentate P–N–P′ ligands for the asymmetric hydrogenation of prochiral ketones in THF. An NMR study into the activation process identified the amine hydride alkoxide complexes Fe­(P-NH-P′)­(CO)­(H)­(OR1) with R1 = Me, tBu, or tAmyl and P-NH-P′ = PPh2CH2CH2NHCH2CH2PiPr2 or (S,S)-PPh2CHPhCHMeNHCH2CH2PCy2. These still required treatment with excess KOtBu and H2(g) to be catalytically active in THF. Both experimental methods and density functional theory (DFT) calculations were used to show that this treatment leads to the formation of a hydride amide complex Fe­(P–N–P′)­(CO)­(H), which reacts with dihydrogen to form cis and trans dihydride complexes Fe­(P-NH-P′)­(CO)­(H)2, identified by NMR spectroscopy. In the presence of KOtBu, NaOtBu, or KOtBu/2,2,2-cryptand and H2(g), these species are active for the catalytic hydrogenation of acetophenone, whereas in the absence of H2(g), inactive Fe(0) complexes are formed. Ketone hydrogenation is proposed to occur in an outer-sphere stepwise process, and this enantio-determining step has been modeled by DFT. The calculations suggest that the energy barriers for hydride attack on the ketone or dihydrogen splittingeither to the nitrogen of the amide complex in the inner coordination sphere or to the oxygen of an alkoxide group in the outer sphereare similar and that either hydride transfer or dihydrogen splitting could determine the turnover frequency depending on the nature of the ketone.