Epoxide Hydrogenation Catalyzed by Ruthenium PNN and PNP Pincer Complexes

The metal-catalyzed hydrogenation of epoxides to give alcohols has advanced rapidly in the past several years, with some catalysts selectively giving linear (anti-Markovnikov) products and other catalysts providing branched (Markovnikov) products. The currently known branched-selective catalyst systems require catalyst loadings of 1% or higher and typically require a strong base additive. We report herein that PNN- and PNP-ruthenium-pincer complexes containing N-H functional groups are highly active for branched-selective hydrogenation of epoxides. When isopropyl alcohol is used as the solvent, excellent yields of the branched alcohol products are obtained without strongly basic additives, using catalyst loadings as low as 0.03%. Epoxides with a directly attached secondary carbon give very high (>99:1) selectivity for the branched products. Aryl-substituted epoxides give branched:linear ratios ranging from 2.7 to 19.0. For aryl epoxides, a PNP-Ru catalyst showed a greater preference for the branched product than a PNN-Ru catalyst, and substrates with electron-rich aryl substituents showed a lower preference for the branched product.