Bis-N-heterocyclic carbene aminopincer ligands enable high activity in Ru-catalyzed ester hydrogenation

Bis-N-heterocyclic carbene (NHC) aminopincer ligands were for the first time successfully applied in catalytic hydrogenation of esters. We have isolated and characterized a well-defined catalyst precursor as a dimeric [Ru2(L)2Cl3]PF6 complex and studied its reactivity and catalytic performance. Remarkable initial activities up to 283 000 h-1 were achieved in hydrogenation of ethyl hexanoate at only 12.5 ppm Ru loading. A wide range of aliphatic and aromatic esters can be converted with this catalyst to corresponding alcohols in near quan-titative yields. The described synthetic protocol makes use of air stable reagents available in multigram quantities rendering the bis-NHC ligands an attractive alternative to the conventional phosphine-based systems.