Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes

Highly Enantiomerically Enriched Secondary Alcohols via Epoxide Hydrogenolysis

In this article, we report the development of ruthenium-catalyzed hydrogenolysis of epoxides to selectively give the branched (Markovnikov) alcohol products. In contrast to previously reported catalysts, the use of Milstein's PNN-pincer-ruthenium complex at room temperature allows the conversion of enantiomerically enriched epoxides to secondary alcohols without racemization of the product. The catalyst is effective for a range of aryl epoxides, alkyl epoxides, and glycidyl ethers and is the first homogeneous system to selectively promote hydrogenolysis of glycidol to 1,2-propanediol, without loss of enantiomeric purity. A detailed mechanistic study was conducted, including experimental observations of catalyst speciation under catalytically relevant conditions, comprehensive kinetic characterization of the catalytic reaction, and computational analysis via density functional theory. Heterolytic hydrogen cleavage is mediated by the ruthenium center and exogenous alkoxide base. Epoxide ring opening occurs through an opposite-side attack of the ruthenium hydride on the less-hindered epoxide carbon, giving the branched alcohol product selectively.

Metal-Catalyst-Controlled Divergent Synthesis of γ-Butyrolactones via Intramolecular Coupling of Epoxides with Alcohols

A metal-controlled divergent protocol for the synthesis of α- and β-substituted γ-butyrolactones was developed through intramolecular coupling of epoxides with alcohols. This method provides an efficient and practicable way to afford γ-butyrolactones with good efficiency, excellent regioselectivity, and broad substrate scope.

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.

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated PIII donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (-NH-, -NR-) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.