Iron-Catalyzed Homogeneous Hydrogenation of Alkenes
under Mild Conditions by a Stepwise, Bifunctional Mechanism
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Abstract
Hydrogenation of alkenes containing
polarized CC double
bonds has been achieved with iron-based homogeneous catalysts bearing
a bis(phosphino)amine pincer ligand. Under standard catalytic conditions
(5 mol % of (PNHP<sup>iPr</sup>)Fe(H)<sub>2</sub>(CO) (PNHP<sup>iPr</sup> = NH(CH<sub>2</sub>CH<sub>2</sub>P<i>i</i>Pr<sub>2</sub>)<sub>2</sub>), 23 °C, 1 atm of H<sub>2</sub>), 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 CC
double bonds occurs in the presence of other reducible functionalities
such as −CO<sub>2</sub>Me, −CN, and N-heterocycles.
For the α,β-unsaturated ketone benzalacetone, both CC
and CO bonds have been reduced in the final product, but NMR
analysis at the initial stage of catalysis demonstrates that the CO
bond is reduced much more rapidly than the CC 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 CC bond is imperative for hydrogenation
with this iron catalyst
