Intermediacy of o-sulfonylium arenide ylides in the reactions of arenesulfonyl derivatives with strong bases: Insight into the puzzling rearrangement of N-arylarenesulfonamides into 2-aminodiaryl sulfones

Article abstract of DOI:10.1021/jo950727j

Benzenesulfonyl derivatives, PhSO₂E, such as benzenesulfonyl fluoride, phenyl benzenesulfonate, and N-methyl-N-phenylbenzenesulfonamide, have been found to undergo ortho-lithiation with lithium 2,2,6,6-tetramethylpiperidide (LTMP) in THF at -78 °C. The resulting lithio derivatives undergo 1,3-elimination of LiE to form the transient o-sulfonylium benzenide. The latter ylide, which may be stabilized by its carbenoid character and its complexation with LiE, can be trapped by benzophenone acting as a 1,3-dipolarophile to form the adduct, 3,3-diphenyl-1,2-benzoxathiole 1,1-dioxide. The alternative formation of the latter cyclic sulfonate by reaction of the initially formed o-lithio derivative with benzophenone and by subsequent cyclization was shown not to occur. If not trapped with benzophenone, such an ylide decomposes partly into SO₂ and benzyne and then undergoes self-condensation to yield poly(phenylene-o,o′-biphenylene sulfones). o-Sulfonylium benzenide also can be captured by lithium alkoxides, either present adventitously or intentionally added, to generate the corresponding alkyl benzenesulfonate. However, such alkoxides, unaided by LTMP, are themselves unable to extract an ortho-proton from benzenesulfonyl fluoride or from N-methyl-N-phenylbenzenesulfonamide to yield the corresponding o-lithio derivative. With benzenesulfonyl fluoride the lithium alkoxide can also form the sulfonate ester by direct substitution at the sulfonyl group. The Closson-Hellwinkel rearrangement of N-methyl-N-phenylbenzenesulfonamide into o-(methylamino)diphenyl sulfone by RLi or LTMP is reinterpreted as proceeding by way of the 2-lithio- or 2,6-dilithiobenzenesulfonyl derivative which eliminates lithium N-methylanilide to form the o-sulfonylium benzenide or its 6-lithio derivative. Attack of the latter ylide, acting as an electrophile, upon the ortho-position of the presumably complexed LiNMePh then consummates the rearrangement.