In Situ Surface-Enhanced Raman Spectroscopic Studies and Electrochemical Reduction of α-Ketoesters and Self Condensation Products at Platinum Surfaces

Abstract

The adsorption and hydrogenation of ketopantolactone (KPL) and ethyl pyruvate (EP) on Pt electrodes, measured by surface-enhanced Raman spectroscopy, are compared. In addition, pure samples of self-condensation products of ethyl pyruvate including a dimer (in both linear and cyclized forms) and a lactone have been synthesized. These dimeric and aldol condensation intermediates have previously been proposed by ourselves as playing a crucial role in explaining the significant rate enhancement observed during enantioselective hydrogenation of α-ketoesters at supported platinum catalysts. The adsorption of the linear dimer at platinum leads to cyclization. At hydrogen evolving potentials in dimer-free aqueous sulfuric acid, the dimer may be hydrogenatively desorbed readily from the electrode surface. KPL is found to accumulate at the electrode surface under hydrogen evolving conditions. From comparison with the behavior of EP under similar hydrogenating conditions, it is deduced that self-condensed dimers do not form when EP is being either hydrogenated using gas phase hydrogen or electrocatalytically hydrogenated in aqueous sulfuric acid. In addition, unlike EP, KPL does not form a long-lived half-hydrogenated state surface intermediate, a significant contributory factor in rate enhancement observed when EP is hydrogenated at supported platinum catalysts. Hence, the role of cinchona alkaloids in establishing rate acceleration cannot be ascribed simply to a destabilization of self-condensation products of ethyl pyruvate under reaction conditions. Models to explain the relative degrees of rate acceleration in KPL and EP are discussed.