Retention of configuration in ligand coupling reactions in σ-sulphuranes

In accord with the ligand coupling concept, an axial and an equatorial ligand are considered to be extruded from the valence-shell-expanded heteroatom in a concerted fashion which thus affords the coupling product. Therefore, the coupling ligands should retain the original configuration. Indeed, the ligand coupling product, p-(1 -Phenylethyl)phenylsulphonylbenzene (3) formed in the reaction of 1 -phenylethyl benzene-p-sulphonylphenyl sulphoxide (2) with ethylmagnesium bromide is found to retain completely the 1 -phenylethyl group configuration, as in our earlier example of the coupling reaction of optically active 1 -phenylethyl 2-pyridyl sulphoxide with Grignard reagents and that of allylic and vinylic sulphoxides with Grignard reagents. The absolute configuration of (+)-(2) and (+)-(3) are determined unequivocally to be SsRc and Rc, respectively, by X-ray methods.