The Mechanism of Markovnikov-Selective Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes

The homogeneous catalysis
of epoxide hydrogenolysis to give alcohols
has recently received significant attention. Catalyst systems have
been developed for the selective formation of either the Markovnikov
(branched) or anti-Markovnikov (linear) alcohol product. Thus far,
the reported catalysts exhibiting Markovnikov selectivity all feature
the potential for Noyori/Shvo-type bifunctional catalysis, with either
a RuH/NH or FeH/OH core structure. The proposed mechanisms of epoxide
ring-opening have involved cooperative C–O bond hydrogenolysis
involving the metal hydride and the acidic pendant group on the ligand,
in analogy to the well-documented mechanism of polar double-bond hydrogenation
exhibited by catalysts of this type. In this work, we present a combined
computational/experimental study of the mechanism of epoxide hydrogenolysis
catalyzed by Noyori-type PNP and PNN complexes of ruthenium. We find
that, at least for these ruthenium systems, the previously proposed
bifunctional pathway for epoxide ring-opening is energetically inaccessible;
instead, the ring-opening proceeds through opposite-side nucleophilic
attack of the ruthenium hydride on the epoxide carbon, without the
involvement of the ligand N–H group. For both catalyst systems,
the rate law and overall barrier predicted by density functional theory
(DFT) are consistent with the results from kinetic studies.