STRUCTURAL EFFECTS ON THE ENERGETICS OF S-CIS AND S-TRANS DIENOLATES

The rates of deuterium exchange of the cis and trans gamma protons of 2-cyclopentylidenecyclopentanone were determined in 90 % CD3OD/10% D2O with NaOD concentrations varying from 0.020 M to 0.352 M. The trans p rotons were observed to exchange 2.4-fold more rapidly than the cis pr otons. Similarly, when a solution of the dienolate of 2-cyclopentylide necyclopentanone in DMSO is quenched in D2O, incorporation of deuteriu m at the trans carbon is 4-fold greater than at the cis carbon. Both t he kinetic and quenching experiments indicate that the transition stat e for the formation of the s-cis dienolate is higher in energy than th at for the formation of the s-trans dienolate by a factor of ca. 4-fol d. These results combined with an estimate of the Bronsted beta value for deprotonation give an energy difference between these dienolate io ns of ca. 1.7 kcal/mol.