The rates of base-catalyzed protium-deuterium exchange (a-thioenolization) of the exo and endo protons of thiocamphor (I), in 2: 1 dioxane-D,O at 25.0 i 0.5"C, have been determined by monitoring the uptake of deuterium mass spectrometrically. The exo and erzdo exchange rate constants, 2.20 x lo-' and 5.60 x M -I s-I , respectively, are 23.2 and 12.3 times larger than the rate constants for exo and erzdo exchange in camphor (2). Factors which may determine the rate enhancements are discussed.
NICK H E~R YWERSTIUK et PAUL ANDREWS. Can. J. Chem. 56, 2605Chem. 56, (1978. Faisant appel a la spectrornetrie de masse pour evaluer l'absorbtion du deuterium, on a determine les vitesses, catalysees par les bases, des Cchanges protium-deuterium (a-thioeno- Thione-thioenol tautomeris~n was first studied by Reyes and Silverstein in 1958 (1). Since then other workers have attempted to detail the factors which afTect the equilibrium (2, 3). However, u p to the present time, there have been no reports ; f studies of acid-or base-catalyzed a-thioenolization of thiones. In this communication, we report the rates of base-catalyzed protiurn-deuterium exchange of the exo and endo protons in thiocamphor (I) and discuss possible rate-co~ltrolling factors.We chose to study P because there is a substantial difference in exchange rates of the diastereotopic protons and the substrate does not undergo trin~er-ization, a reaction which unhindered thiones undergo very readily (4).The rate constants listed in Table 1 were obtained by the method developed in these laboratories (5). That good first-order kinetics were observed and that analytical gas chromatographic analysis showed no significant conversion of I into 2 established that sulfur-oxygen exchange, a potentially complicating process that \vould decrease the base concentration, is insignificant.Several factors may contribute to the increase in the a-deprotonationl rates of 1 relative to 2. Because 'We have established through several exploratory runs that [3-thioenolization (hon~othioenolization) of 3,3,6,6-tetramethylbicyclo[2.2.1]heptan-2,5-dithione occurs more readily than a-enolization (homoenolization) of 3,3,6,6-tetramethylbicyclo[2.2.1]heptan-2,5-dione (7). oxygen is more electronegative than sulfur and the carbonyl group would be expected to be more electron withdrawing than the thiocarbonyl group, base-catalyzed hydrogen isotope exchange of ketones should be more facile than exchange of thiones. From a survey of carbonyl and thiocarbonyl compounds (8), it is clear that the dipole moments of the thiocarbonyl compounds are close to or greater than the dipole moments of the carbonyl compounds. The