Ester Hydrogenation Catalyzed by CNN-Pincer Complexes of Ruthenium

Ruthenium
complexes supported by two new CNN-pincer ligands were
synthesized. Both were tested as catalysts for the hydrogenation of
esters under mild conditions (105 °C, 6 bar H2). A
striking dependence on ligand structure was observed, as a dimethylamino-substituted
ligand gave a nearly inactive catalyst, while a diethylamino-substituted
variant gave up to 980 catalytic turnovers for the hydrogenation of
benzyl benzoate. This system catalyzes the hydrogenation of various
substrates including ethyl, benzyl, and hexyl esters, but is surprisingly
unreactive toward methyl esters. Experiments demonstrate that base-catalyzed
transesterification is rapid under the reaction conditions and that
methyl esters are effectively hydrogenated when benzyl alcohol is
added to the reaction mixture. The reverse reaction, dehydrogenation
of primary alcohols to give esters, was tested as well; up to 920
catalytic turnovers were observed for the dehydrogenation of 1-hexanol
to hexyl hexanoate.