Enantioselective hydrogenation of α-ketoesters: An in Situ Surface-Enhanced Raman Spectroscopy (SERS) study

Adsorption of ethyl pyruvate (EP) and methyl pyruvate (MP) has been studied on a SERS-active platinum electrode. Gas-phase experiments under a hydrogen atmosphere show that decarbonylation to form adsorbed CO together with formation of a half-hydrogenated state of MP/EP are the dominant surface reaction pathways occurring under these conditions. Similar findings were obtained using a hydrogen-evolving platinum electrode under potentiostatic control in 0.1 M aqueous sulfuric acid containing EP or MP. DFT calculations have been used to support the assignment of vibrational bands in the SERS obtained from the half-hydrogenated state intermediate. At hydrogen evolution potentials and using concentrations of MP and EP < 0.02M, adsorbed hydrogen adatoms were detected. At higher EP concentrations, hydrogen-atom formation was inhibited by the greater surface coverages of the half-hydrogenated state of EP/MP and to a lesser extent adsorbed CO derived from decarbonylation side reactions. Addition of the chiral modifier cinchonidine (CD) to the acidic electrolyte under conditions of hydrogen evolution resulted in significantly less CO production and a reduction in the intensity of all SERS peaks associated with the half-hydrogenated state. Most importantly, however, the presence of surface hydrogen, previously absent on the unmodified surface, was always observed irrespective of the EP concentration. The significance of this result in relation to the question of the origin of rate enhancement during heterogeneously catalyzed enantioselective hydrogenation of α-ketoesters is discussed.