Conformational isomerism of a-ketoesters.: A FTIR and ab initio study

The conformational behaviour of several alpha-ketoesters was investigated using solution FTIR in combination with ab initio calculations. The alpha-ketoesters show marked differences in the O=C-C=O torsional potential energy surface depending on the substituent at the alpha-keto group. In general the torsional potential is characterised by broad minima corresponding to s-cis and s-trans conformations and low interconversion barriers. The s-trans conformation is more stable but the fraction of s-cis is considerable at room temperature and increases with solvent polarity due to the higher dipole moment of the latter. Hydrogen bonding with alcoholic solvents also leads to a stabilisation of the s-cis conformer. The interaction of ethyl pyruvate with R3N+-H is much stronger when ethyl pyruvate adopts an s-cis conformation due to strong ion-dipole interaction. This type of interaction between ethyl pyruvate and protonated cinchonidine is considered to be crucial for the enantio-differentiation in the heterogeneous enantioselective hydrogenation of ethyl pyruvate over cinchonidine modified platinum in acidic media.