One-pot dual catalysis for the hydrogenation of heteroarenes and arenes

A simple dinuclear monohydrido bridged ruthenium complex [{(η 6 - p -cymene)RuCl} 2 (μ-H-μ-Cl)] acts as an efficient and selective catalyst for the hydrogenation of various heteroarenes and arenes. The nature of the catalytically active species was investigated using a combination of techniques including in situ reaction monitoring, kinetic studies, quantitative poisoning experiments and electron microscopy, evidencing a dual reactivity. The results suggest that the hydrogenation of heteroarenes proceeds via molecular catalysis. In particular, monitoring the reaction progress by NMR spectroscopy indicates that [{(η 6 - p -cymene)RuCl} 2 (μ-H-μ-Cl)] is transformed into monomeric ruthenium intermediates, which upon subsequent activation of dihydrogen and hydride transfer accomplish the hydrogenation of heteroarenes under homogeneous conditions. In contrast, carbocyclic aryl motifs are hydrogenated via a heterogeneous pathway, by in situ generated ruthenium nanoparticles. Remarkably, these hydrogenation reactions can be performed using molecular hydrogen under solvent-free conditions or with 1,4-dioxane, and thus give access to a broad range of saturated heterocycles and carbocycles while generating no waste. A catalytic system resulting from a monohydrido bridged ruthenium complex hydrogenated both heteroarenes and arenes, exhibited dual catalysis and provided access to valuable saturated heterocycles and cycloalkanes.