Rearrangements of Carbocation Sulfinate Ion Pairs

Abstract The chirality of alkyl p ‐toluenesulfinates and of 18 O‐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 ArSO 2 − 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 ArSO 2 − 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).