Relative Stability of the Key Enamine,Oxazolidinone, and Imine Intermediates in Some Proline-catalyzed Asymmetric Reactions

Many proline-catalyzed asymmetric addition reactions with ketones as substrates were assumed to involve a key intermediate, an enamine, produced by the condensation of proline and ketone. In this paper, the key intermediate enamines derived from L-proline and cyclohexanone (or acetone) as well as the corresponding oxazolidinone and imine tautomers have been investigated by means of density functional calculations at the B3LYP/6-311 G** level. The predicted order of stability for these tautomers is oxazolidinones > enamines > imines in gas phase and oxazolidinones > imines > enamines in aprotic THF solvent. This prediction explains why enamine intermediate can not be observed experimentally. The predicted energy/enthalpy difference between the formal oxazolidinone structure and the zwitterionic imine structures is very small in THF solvent, suggesting the oxazolidinone-to-imine tautomerization can be readily induced in solvent. 13C NMR chemical shifts of the oxazolidinone and imine structures have been computed and used to explain the experimental NMR spectra observed in oxazolidinone-to-imine tautomerization induced by protic solvent.