Sulfur-carbon-phosphorus anomeric interactions. 4. Conformational analysis of 2-(diphenylphosphinoyl)-1,3-dithiane

Theoretical Examination of the S–C–P Anomeric Effect

“…Indeed, according to B3LYP/6-31G(d) and B3LYP/6-311+G(d,p) level of theory calculations, in solvent ethanol at 294 K the conformer with the diphenylphosphinoyl group in the axial position (1-ax in Scheme 2) is lower in energy, ∆G° +1.36 kcal/mol and +1.30 kcal/mol, respectively. These values are very close to the experimentally observed ∆G°2 94 K = +0.99 kcal/mol in ethanol 6,7. Nevertheless, at the B3LYP/6-311+G(d,p) the calculated ∆G°2 98 K = +2.90 kcal/mol in chloroform is significantly larger than the experimentally obtained value.…”

Applications of the Quantum Theory of Atoms in Molecules in Organic Chemistry – Charge Distribution, Conformational Analysis and Molecular Interactions

“…X-ray diffraction data of the axial conformer do not, however, show the bond shortening and lengthening patterns required by this model -the CaS bond should be shorter and the CaP bond longer in the axial conformer than in the equatorial conformer, but this cannot be observed (Fig. 14.5a) [48]. An alternative explanation proposed a through-space electronic interaction between the S and P atoms.…”

“…In addition, introduction of a third S atom at position 5 of 1,3-dithiane, 3, should increase the acidity of the CaH group, with a concomitant increase of r b ðrÞ. The experimental conformational preference of 1,3,5 trithiane is 1.43 kcal mol À1 [48], a larger value than that for 1. Something similar happens when the amount of oxidation of the sulfur atoms is increased, as shown for molecule 4.…”

Second‐row anomeric interactions: The involvement of phosphorus

“…A most interesting observation suggests, however, some form of electron transfer to the axial phosphinoyl group. Indeed, the aromatic ring I3C chemical shifts for the ortho and para carbons in the axial isomers 1 and 3 appear at significantly higher fields than those in equatorial 4 [3] (Table 3). By contrast, the signal for the meta carbons in 1, 3, and 4 are essentially constant.…”

“…Thus, if dipole/dipole interactions were dominant in the conformational equilibria of 1 (and 3 e 4), it would be expected that the contribution of the equatorial form should increase with increasing dielectric constant of the medium [lo]. In fact, the results did not show the expected solvent effect (Table 1 [3]); rather, they gave an erratic trend, which hints at the simultaneous participation of several effects.…”