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- W2952950462 abstract "Proton NMR spectroscopy and x-ray crystallographic studies demonstrate the predominance of the axial conformer of 2-(diphenylphosphinoyl)-1,3-dithiane (1). Chemical equilibration of anancomeric models (3 ⇌ 4) allows quantitative determination of the conformational free energy in 1, 1.0 kcal/mol, which corresponds to an anomeric effect of 2.64–3.74 kcal/mol, depending on the method used for estimating it. This value indicates that the anomeric effect operative in 1 may be among the largest yet measured. Five distinct rationalizations are considered to account for such a strong S-C-P(O) anomeric effect: (a) dipole-dipole interactions, (b) electrostatic, attractive interaction between the phosphoryl oxygen and the syn-axial hydrogens in 1-axial, (c) delocalization of the lone pair on the endocyclic heteroatom into the antiperiplanar (axial) adjacent polar C-P bond, (d) through-space 3p-3d electron donation from sulfur to phosphorus, and (e) repulsive interaction between the lone pairs on sulfur and on the phosphoryl oxygen. All these hypotheses have some experimental and/or the-oretical support, but evidence contrary to expectation can be found also to argue against each one of them. The conformational preference of the diphenylthiophosphinoyl group in the 1,3-dithian-2-yl ring was determined by nuclear magnetic resonance (NMR) analysis and by chemical equilibration of diastereomeric models. The slight predominance of axial 2-(diphenylthiophosphinoyl)-1,3-dithiane (13-ax) over 13-eq reflects nonetheless the influence of a strong S-C-P anomeric interaction, worth ca. 2.3 kcal/mol. Spectroscopic evidence for the predominance of the equatorial conformers in 2-(diphenylphosphinoyl)-1,3-oxathiane (19) and 2-(diphenylphosphinoyl)-1,3-dioxane (22) was confirmed by the study of derivatives containing counterpoise substitutents, or by chemical equilibration on anancomeric models. ΔD°27°C [P(O)Ph2] = -3.23 kcal/mol was determined for the dioxane (chloroform solution), and ΔG°C55°C [P(O)Ph2] = -1.42 kcal/mol for the oxathiane (ethanolic solution). Nevertheless, evaluation of the different steric requirements in 1, 13 and 22 reveals that the magnitude of the O-C-P(O) and S-C-P(O) anomeric effects in the heterocycles is, in fact, quite similar, close to kcal/mol." @default.
- W2952950462 created "2019-06-27" @default.
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- W2952950462 date "1990-05-01" @default.
- W2952950462 modified "2023-09-23" @default.
- W2952950462 title "Second-row anomeric interactions: The involvement of phosphorus" @default.
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- W2952950462 doi "https://doi.org/10.1002/hc.520010313" @default.
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