S-C-P ANOMERIC INTERACTIONS .9. EFFECT OF THE COORDINATION AT PHOSPHORUS IN THE CONFORMATIONAL EQUILIBRIA OF 2-P-SUBSTITUTED-1,3-DITHIANES

The conformational energies (A values) of the (diphenylphosphinyl)borane and diphenylphosphinyl groups were determined by multinuclear (H-1, C-13, P-31) NMR analysis of mobile (cis-4-phenylcyclohexyl)diphenylphosphine-borane, as well as the corresponding phosphine, and conformationally fixed models. The equatorial preferences observed are -DELTA-G-degrees(P(BH3)Ph2) = 3.3 kcal/mol and -DELTA-G-degrees(PPh2) = 1.8 kcal/mol. The conformational preference of these groups in the 1,3-dithian-2-yl ring were also determined by NMR analysis: -0.1 and -0.3 kcal/mol, respectively. The slight predominance of the equatorial isomers reflects nonetheless the influence of substantial S-C-P(BH3) and S-C-P: anomeric interactions, worth 1.8 and 1.0 kcal/mol, respectively. Evaluation of these values, together with previous data obtained for S-C-P(O) and S-C-P(S) systems, supports the participation of endo and exo hyperconjugative interactions, although the participation of 3p-3d electron donation among the sulfur and phosphorus atoms could also account for the results. Alternative rationalizations that have been considered to account for the strong S-C-P(O) anomeric effect appear now to play a minor role in the conformational equilibria of 2-P-substituted 1,3-dithianes.