Solvolytic Generation of α-Sulfonyl and α-Sulfinyl Carbocations

Article abstract of DOI:10.1021/ja00242a031

A series of mesylates of general type ArC(CH3)(SO2Ph)OMs, 14, have been prepared and studied under solvolytic conditions.The Hammett p value in methanol is -7.98.A solvent effect study on 14b, where Ar=p-CH3C6H4, gave a good correlation with Y_OTs values and an m value of 0.85.These data were interpreted in terms of the involvement of α-sulfonyl-substituted carbocations, which have a large demand for aryl group stabilization.In solvolyses of 14b, capture of the α-sulfonyl carbocation by acetic or trifluoroacetic acid gave simple substitution products which could be isolated.In alcohol solvents subsequent unimolecular loss of benzenesulfinate ion from the initially formed α-alkoxy sulfone led ultimately to the formation of ketones or ketals.Analogous loss of benzenesulfinate from the primary product is also seen on prolonged reaction of 14b in carboxylic acid solvents.Solvolyses of a series of α-bromo sulfoxides of general structure ArC(CH3)(SOPh)Br, 35, also proceed via the intermediacy of carbocations.The demand for aryl stabilization in these α-sulfinyl cations is also quite large as evidenced by the p value of -7.18 in trifluoroethanol.The trifluoroethanolysis rate of 35b, where Ar=p-CH3C6H4, is 1.4x10⁵ times slower at 25 deg C than that of the α-H analogue ArCH(CH3)Br, 29.This, along with the large negative p value, implies that the interaction of the sulfur nonbonding electrons with the adjacent cationic center is of minimal importance in the α-sulfinyl carbocation derived from 35b.These data conflict with previous suggestion that α-sulfinyl cations are greatly stabilized by the sulfur nonbonding electrons.Suggestions are offered to explain this apparent conflict.The relative reactivities of a series of bromides containing electron-withdrawing groups have been determined in the common solvent trifluoroethanol in order to evaluate the effect of these groups on the rate of carbocation generation.The relative reactivity order for sovolyses of substrates of general type p-CH3C6H4C(CH3)BrE, where E is an electronegative group, is COPh>PO(OEt)2>CN>SOPh>CF3>SO2Ph.