Towards a complete account of the mechanism of hydrogen isotope exchange of diastereotopic protons of carbon acids. II. Acid- and base-catalyzed enolization of bicyclo[2.2.1]heptan-2-ones. Evidence for exchange by inversion

Abstract

NICK HENRY WERSTIUK and SUJIT BANERJEE. Can. J. Chem. 63, 534 (1985). A study of acid-and base-catalyzed hydrogen isotope exchange of bicyclo[2.2. Ilheptan-2-one ( l a ) and its 3-deuteriated analogs has been carried out. We find that the k,.,,,/k,.,,,,, ratio (658 ? 66) for deuteroxide-catalyzed H + D exchange of I n at C-3 is 7.2 * 1.5 times greater than the k,.,,,/k,.,,,,,, ratio (91 * 9) for hydroxide-catalyzed D + H exchange of lb. For acid-catalyzed exchange in CH,COOD(H)-D(H)Cl the rate ratios are 156 * 20 and 29 2 2 for H + D and D + H exchange, respectively. Equations which relate the observed selectivity k,..,,,/k,.,,,,,, (kI;,,,/ksl,,,,) to the intrinsic selectivity and the primary, secondary, and solvent KlEs are developed. The differences between the rate ratios for H + D and D + H exchange are interpreted on the basis of a significant contribution of an inversion pathway to exchange of the slow proton (deuteron). The significance of our study -it relates to the mechanism of hydrogen isotope cxchange of diastereotopic protons (deuterons, tritons) of any carbon acid -is discussed.