Reaction of 4′-nitrobenzenesulfenanilide (NBSA) with Lewis acids. A study of its application in sulfenocyclization of alkenes and alkynes

Abstract

Phenylsulfenocyclization of a number of alkenes and alkynes possessing internal hydroxyl, carboxyl or vinyl functionality has been investigated with NBSA. Thioetherification of 4-penten-1-ol 4 as well as thiolactonization of 4-penten-1-oic acid 7 and, to a modest extent, 4-pentyn-1-oic acid 8 can be successfully achieved with NBSA in the presence of boron trifluoride. On the other hand, under analogous conditions 3-buten-1-ol 3 and 3-buten-1-oic acid 6 fail to undergo thiocyclization and give instead oxa- and/or aza-sulfenylation 1,2-adducts. Similar failure is observed with 4-pentyn-1-ol 5, in which case diphenyl disulfide and 4-nitroaniline are the exclusive products. Hexa-1,5-diene 9, to some extent, affords a cyclized arylaminosulfide, i.e. 23, ascribable to formal loss of a methylene unit from the initial (phenylthiomethyl)- cyclopentyl cation 24. The reaction products are discussed in terms of intermediate thiiranium and thiirenium ions whose decomposition mode is strictly dependent upon their structural features. Novel evidence is also presented that aluminium chloride and bromide can promote reaction of NBSA with alkenes and alkynes to afford chloro- and bromo-sulfenylation adducts in varying yields.