Iron-Catalyzed Homogeneous Hydrogenation of Alkenes under Mild Conditions by a Stepwise, Bifunctional Mechanism

Hydrogenation of alkenes containing polarized CC double bonds has been achieved with iron-based homogeneous catalysts bearing a bis­(phosphino)­amine pincer ligand. Under standard catalytic conditions (5 mol % of (PNHPiPr)­Fe­(H)2(CO) (PNHPiPr = NH­(CH2CH2PiPr2)2), 23 °C, 1 atm of H2), styrene derivatives containing electron-withdrawing para substituents reacted much more quickly than both the parent styrene and substituted styrenes with an electron-donating group. Selective hydrogenation of CC double bonds occurs in the presence of other reducible functionalities such as −CO2Me, −CN, and N-heterocycles. For the α,β-unsaturated ketone benzalacetone, both CC and CO bonds have been reduced in the final product, but NMR analysis at the initial stage of catalysis demonstrates that the CO bond is reduced much more rapidly than the CC bond. Although Hanson and co-workers have proposed a nonbifunctional alkene hydrogenation mechanism for related nickel and cobalt catalysts, the iron system described here operates via a stepwise metal–ligand cooperative pathway of Fe–H hydride transfer, resulting in an ionic intermediate, followed by N–H proton transfer from the pincer ligand to form the hydrogenated product. Experimental and computational studies indicate that the polarization of the CC bond is imperative for hydrogenation with this iron catalyst.