Studies on the effect of N-heterocyclic carbene ligands in the Ru-catalyzed hydroesterification reaction

Abstract

A new protocol of the ruthenium-catalyzed hydroesterification reaction has been developed by employing N-heterocyclic carbenes as ligands. No external CO source, such as CO atmosphere or carbonyl ligand, was required to form carbonylated products. The reaction condition was milder than compared with the previous work using ruthenium-carbonyl cluster as a catalyst. The different regioselectivity was observed between alkyl olefins and vinyl arenes. Hydroesterification of para-substituted styrene derivatives gave the opposite regioselectivity for substrates bearing electron-withdrawing substituents. A Hammett analysis produced a positive linear correlation for regioselectivity. We expected that the regioselectivity would be improved by modifying NHC ligands with their unique electronic structure, however, it ended up resulting in hardly any enhancement.