CONFORMATIONAL-ANALYSIS OF 2-(DIPHENYLPHOSPHINOYL)-TETRAHYDROTHIOPYRAN - LACK OF ADDITIVITY OF THE S-C-P ANOMERIC EFFECT

The conformational preference of the diphenylphosphinoyl group at the alpha position in the thiane ring was estimated by low temperature NMR , Delta G(173K)degrees = +0.46 +/- 0.11 kcal/mol, and at 50 degrees C, via chemical equilibration of anancomeric models, Delta G(323K)degree s = -0.26 +/- 0.10 kcal/mol. These thermodynamic data allowed the esti mation of the enthalpic and entropic contributions to the axial revers ible arrow equatorial conformational equilibrium of 2-(diphenylphosphi noyl)thiane, Delta H degrees = +1.29 +/- 0.12 kcal/mol, and Delta S de grees = +/- 4.8 +/- 0.7 cal/Kmol. Comparison of the enthalpic S-C-P(O) anomeric effect in the thiane and 1,3-dithiane systems, 2.68 versus 3 .39 kcal/mol, respectively, leads to the conclusion that no additivity of the S-C-P(O) anomeric effect takes place, (C) 1998 Elsevier Scienc e Ltd. All rights reserved.