A DFT investigation of the mechanism of ester hydrogenation by an octahedral iron-amino hydride catalyst

Abstract

Ester hydrogenation is an important reaction in chemistry that is currently done using stoichiometric environmentally unfriendly hydrogenation reagents such as lithium aluminum hydride. The search for more green methods based on hydrogen gas resulted in the discovery of several classes of homogeneous catalysts for this reaction. However, these catalysts are still not efficient enough for practical applications, and there is still limited understanding of the mechanism. In this work, density functional theory (DFT) is used to investigate three fundamentally different mechanisms leading to C-OMe bond cleavage in the reaction between methyl benzoate and the octahedral hydrogenation catalyst trans-[Fe(H)2(iPr₄PNP)(CO)] (where PNP has an aliphatic diethyl-amine backbone). First we calculate a conventional Noyori type mechanism following two stages: (i) stepwise bifunctional hydrogenation of the carbonyl group of the ester into separated hemiacetal and an unsaturated amido complex, and (ii) C-OMe cleavage by another bifunctional reaction between the hemiacetal and the amido complex to give the octahedral trans-[Fe(H)(OMe)(iPr₄PNP)(CO)] and benzaldehyde. The combined two stages are equivalent to an H-OR metathesis for hydride-alkoxide exchange between an acyl group and a metal center. Second we consider two variations on a hemiacetaloxide ion-pair formation and 1,1-slippage mechanism to the same H-OR metathesis, one following direct -H-OR slippage of the hemiacetaloxide, and another following a sequence of -H-O- and -O--OR slippages that mediate, respectively, carbonyl group (ester) insertion and (aldehyde) de-insertion. When computed in a polarizable solvent continuum representing THF as solvent the two routes to H-OR metathesis is found to be competitive. However, when a methanol continuum is applied in the calculations the slippage route is greatly stabilized over hemiacetal formation. Finally, a distinct bifunctional mechanism for C-OR cleavage of the hemiacetal leading to a metal c