Conformational analysis of 2-phosphoryl and 2-thiophosphoryl 1,3-heteroanes by molecular mechanics calculations. A question of the origin of the anomeric effect

Conformational analysis of six-membered, sulfur-containing saturated heterocycles

“…Of course, the predominance of 13-ax over 13-eq may be due not to a stabilizing effect in the axial isomer but rather to a destabilizing interaction in the equatorial isomer. In fact, Mikolajczyk et al 27 have proposed a repulsive interaction between the lone pairs on sulfur and on phosphoryl oxygen in the equatorial conformation. While experimental support for this hypothesis is lacking, it should be pointed out that such an effect would be a particular example of the repulsive gauche effect.4,5 nance of the axial conformer (eq 3).…”

Theoretical Examination of the S–C–P Anomeric Effect

“…8 Among various subsequent studies aiming to explain the nature of the "anomeric effect in the S-C-P molecular segment", 9,10 in 1986 it was found that the axial preference of the diphenylphosphinoyl group in 1 (Scheme 1) vanishes in trifluoroacetic acid as solvent. This finding could be in line with neutralization of the electrostatic, attractive interaction between the phosphoryl oxygen and the axial hydrogens H(4,6ax) in 1-ax upon protonation of the phosphoryl oxygen.…”