Phenylsulfenocyclixation of a number ofalkenes and aIkyaea posses&g intemal hydroxyl, carboxyl or vinyl functionality has been investigated with NBSA. Thioezherification of4-pemeo-1-01 4 as welI as tbiolactcnixadcn of4-pentea-l-ok acid 7 and, IO a modest extent, 4-pentyn-l-oic acid 8 canbesucceaPfuUyachievedwithNBSAiatbepresenceofboroatrifluoride.Ontbeotherhand under analogous conditions 3-buten-l-013 and 3-buten-l-oic acid 6 fail to undergo thiocyclixation and give instead oxa-and/or aza-sulfeayhtioa l&&ducts. Sii faihe is observed with Qpentynl-015, in which case dipbenyl dhdfide and 4-nitroaaihe an2 the exclusive produds. Hexa-l&dicne 9. to some extent. affords a cyclixed arylammosulfMe, i.e. 23, asaibable to formal loss of a mcthylene unit from the initial @henyltbiometl~yl)-cyclopentyl catioa 24. lk &on produas are discussed io terms of intermediate thiiraaium and thiirenium ions whose decompasitlon mode is strictly dependent up their structural features. Novel evideace is also presented that aluminilrm chlorideandbromidecanpnwotereactiooofNBSAwitballreoesandalkynestoaffordchloro_and bromo-sulfenylation adduas ia varying yields. In a number of previous papers we have shown that boron trifluoride transforms 4'nitrobenzenesulfenauilide (NBSA), a readily available and quite stable compound, into a reactive electrophilic species capable of transferring the phenylthio moiety to various nucleophiles, including simple alkynes and alkenest Recent spectral and chemical evidence suggests that an anilide radical cation (NBSA.+), occurring through mono-electron transfer from NBSA to the Lewis acid, is the likely sulfenylating agent.2 The BF3promoted reaction of NBSA with alkenes and alkynes results in the intermediacy of thiimni~ and thiiteni~ ions, respectively. lhiimnium ions, in non nucleophilic solvents, can be efficiently trapped by the tetraborate counterion (ArNHBFj-) to give ~(4-nitrophenylamino)alkyl sulfidesth, whereas in acetonitrile and benz.onitrile they preferentially react with the poorly nucleophilic solvent to evenmaUy afford &amidino and/or bamidoakyl sulfIdea ted lhiimnium ions generally failed to undergo counterion attack at the ring carbons, but reacted with smaller (though weaker) nucleophiles such as acetonitrile.tf acetic acidth and a fluoroborate fluorine atomti to give the corresponding vinyl sul6de.s with trans-stereospecificity and high tegioselectivity (Markovnikov orientation). In all the investigated NBSA additions to alkenes and alkynes varying amounts of diphenyl disultIde 1 (and aniline 2) were found. Their formation was initiaUy claimed to result from selfreaction of NBSA. More recent studies have uncovered that 1-phenylthiimnium ions can also lead to the compound 1 (and the suitable atkyne) through a still unknown route, whose feasibility is strongly enhanced by a decrease in alkyl and phenyl substitution on the ring carbons. ti