Carboxylate-Functionalized Mesoionic Carbene Precursors: Decarboxylation, Ruthenium Bonding, and Catalytic Activity in Hydrogen Transfer Reactions

Metalation of a carboxylate-functionalized
pyridyl-triazolium salt
containing a N1-bound pyridyl substituent either by direct means with
[RuCl2(cymene)]2 or via a transmetalation procedure
involving Ag2O and [RuCl2(cymene)]2 induces rapid decarboxylation without concomitant metalation. Subsequent
metalation of the formed C4- and C5-unsubstituted triazolium salt
is selective and occurs at the C4 position, i.e. remote from the pyridyl
substituent, when the reaction is under kinetic control and at the
C5 position adjacent to the pyridyl group, when the reaction is performed
under thermodynamic control. Preservation of the carboxylate functional
group in the complex is achieved when the corresponding ester-functionalized
pyridyl-triazolium salt is metalated first and then subjected to ester
hydrolysis. The formed complex contains a N,C-bidentate chelating
pyridyl-triazolylidene ligand with a pendant carboxylate unit that
is not coordinating to the metal center. These new triazolylidene
ruthenium complexes show modest catalytic activity in alcohol oxidation
and better performance in the transfer hydrogenation of ketones. The
data suggest that the presence of a pendant carboxylic acid or ester
group is beneficial for enhancing the activity of the catalyst.