Rearrangements of carbocation sulfinate ion pairs

Article abstract of DOI:10.1002/jlac.199719971005

The chirality of alkyl p-toluenesulfinates and of ¹⁸O-labeled p-toluenesulfinate ions was utilized to study the stereoselectivity of ion-pair recombinations. The diastereomers of 2-norpinyl (13), 2-norbornyl (16), 2-methyl-2-norbornyl (25), and exo-4-protoadamantyl (34) p-toluenesulfinates were rearranged in formamide or trifluoroacetic acid (TFA). Solvolysis competed to a minor extent. Predominant return of the carbocations to the oxygen atoms of ArSO₂^- was observed if the isomeric p-toluenesulfinates persisted (kinetic control). On repeated ionization (thermodynamic control), sulfones were eventually formed. With the exception of 25, 1,2 shifts of the p-toluenesulfinate anion proceed faster than oxygen exchange. The migration origin of the carbocation returns preferentially to the oxygen atom of ArSO₂^- from which the migration terminus departed. Conversely, the sulfinate anion discriminates between positions 1 and 2 of the symmetrical, bridged 2-norbornyl cation in favor of the carbon atom from which it departed. The selectivity of ion-pair recombination decreases in the order 2-norpinyl = 4-protoadamantyl > 2-norbornyl > 2-methyl-2-norbornyl, i.e., with increasing stability of the carbocation. The rearrangements of 13 and 34 proved to be more selective in TFA at 0°C than in formamide at 120-130°C. The p-toluenesulfinates 13 and 34 were compared with the analogous tosylates and 3,5-dinitrobenzoates. More oxygen scrambling was observed with less nucleophilic anions (tosylate ? p-toluenesulfinate > 3,5-dinitrobenzoate). Oxygen scrambling is also enhanced if the anion migrates over a longer distance (2-norpinyl → exo-2-norbornyl vs. 2-norpinyl → endo-2-norbornyl). Wiley-VCH Verlag GmbH, 1997.