REARRANGEMENTS OF CARBOCATION SULFINATE ION-PAIRS

The chirality of alkyl p-toluenesulfinates and of O-18-labeled p-tolue nesulfinate ions was utilized to study the stereoselectivity of ion-pa ir recombinations. The diastereomers of 2-norpinyl (13), 2-norbornyl ( 16), 2-methyl-2-norbornyl (25), and exo-4-protoadamantyl (34) p-toluen esulfinates were rearranged in formamide or trifluoroacetic acid (TFA) . Solvolysis competed to a minor extent. Predominant return of the car bocations 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 ex ception of 25, 1,2 shifts of the p-toluenesulfinate anion proceed fast er than oxygen exchange. The migration origin of the carbocation retur ns preferentially to the oxygen atom of ArSO2- from which the migratio n terminus departed. Conversely, the sulfinate anion discriminates bet ween 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 approximate to 4-protoadamantyl > 2-norbornyl > 2-methyl-2-norbornyl, i.e., with i ncreasing stability of the carbocation. The rearrangements of 13 and 3 4 proved to be more selective in TFA at 0 degrees C than in formamide at 120-130 degrees C. The p-toluenesulfinates 13 and 34 were compared with the analogous tosylates and 3,5-dinitrobenzoates. More oxygen scr ambling was observed with less nucleophilic anions (tosylate much grea ter than p-toluenesulfinate > 3,5-dinitrobenzoate). Oxygen scrambling is also enhanced if the anion migrates over a longer distance (2-norpi nyl --> exo-2-norbornyl vs. 2-norpinyl --> endo-2-norbornyl).