Theoretical Evidence for the Relevance of n(S) → σ*(C-P), σ(C-S) → σ*(C-P), and n(F) → σ*(C-X) (X = H, C, O, S) Stereoelectronic Interactions

Three decades after the discovery of a strong S-C-Panomeric effect in 2-diphenylphosphinoy1-1,3-dithiane (1), a suitable interpretation was pendimi: nevertheless, very recent OFT geometry optimization of 1-ax and L-eq did reproduce the S-C-P anomeric effect in 1, worth 5.45 kcal/mol (in chloroform solvent). Furthermore, NBO computational analysis suggests the involvement of n(X) -> sigma*(C-P(O)Ph-2) stereoelectronic interactions that stabilize the axial conformer. Along similar lines, theoretical calculations on r-1,c-3,c-5trifl uorocyclohexane (2), r-2,c-4,c-6-trifluoro-1,3,5-trioxane (3) and r-2,c-4,c-6-trifluoro-1,3,5-trithiane (4) confirm the relevance of n(F) -> sigma*(C-X)(gen), hyperconjugative interactions, where X = H, C, O, S. Thus, contrary to generally accepted concepts, fluorine is a good lone pair electron donor towards geminal sigma bonds.