Sulfenyl halides, reactions

MO calculations for the gas phase indicate that sulfurane intermediate (17) is more stable than the ion (18) by about 380 kJ moP, which suggests that sulfuranes may be important in the reaction of sulfenyl halides with alkenes in non-polar solvents (77JCS(P2)1019). 

Three substituted 5-phenyl unsymmetrical disulfides have been prepared, i, ii, and iii —compounds i and ii by reaction of a thiol with a sulfenyl halide, compound iii from a thiol and an aiyl thiosulfonate (ArS02SAr). The disulfides are cleaved by reduction (NaBH4) or by treatment with excess thiol (HSCH2CH2OH). 

In earlier days it was fairly common to suggest that sulfenium ions, RS+, were involved as intermediates in a number of these substitutions, particularly those in which sulfenyl halides RSX reacted with very weak nucleophiles, or those where electrophilic catalysis of the substitution was observed (Parker and Kharasch, 1959). However, it has since become evident (Owsley and Helmkamp, 1967 Helmkamp et al., 1968 Capozzi et al., 1975) that sulfenium ions are almost impossible to generate as intermediates. For example, Capozzi et al., (1975) showed that although treatment of a sulfenyl chloride RSC1 with the powerful Lewis acid antimony pentafluoride led to the complete conversion of the sulfenyl chloride to a cation, what was formed was, not the sulfenium ion RS+, but rather the cation [59] in reaction (172). These results, and others... 

In agreement with the mechanism reported in equation 90, the reaction generally follows a second-order rate law (equation 92), first order in the sulfenyl halide and in the alkene, respectively. 

Simple 1,2-additions to this compound have been observed123131132 also in other sulfenylation reactions, and in other electrophilic additions involving strongly bridged intermediates. Although these results have been interpreted as evidence that additions of sulfenyl halides to symmetrical alkenes do not involve open carbenium ions before the product-determining step, the different behavior observed in the case of 49 suggests123 that close proximity is necessary to have transannular participation of 7r-bonds, at least in additions of sulfenyl derivatives and of some other electrophiles carried out in the presence of efficient nucleophiles. 

Chlorination of thiols serves for the preparation of temperature-sensitive sulfenyl halides, since the main reaction products are easily separated from the side products by fractional condensation, e.g.,... 

Disulfane Starting materials Thiocarbonyl Sulfenyl compound halide Reaction time (hr) Yield (%)... 

Electrophilic substitutions on thiophene, like those on benzene, can be carried out only in the presence of catalysts. In reactions with sulfenyl halides, SnCl4 proved to be particularly suitable in the case of the less reactive sulfenyl halides, Grignard reactions lead to the desired products ... 

With CFsSCl and in the presence of pyridine as HCl acceptor, a reaction takes place to give 5-trifluoromethylmercaptouracil. Under identical reaction conditions the sulfenyl halides CFnCl3 SCl ( = 0, 1) afford dinitrogen-substituted products ... 

The yield decreases with increasing degree of fluorination of the reactant sulfenyl halides. In the reaction with CF3SCI, no appreciable quantity of the desired product could be isolated. Although in the mass spectrum of the reaction mixture a peak was observed corresponding to the molecular ion M = 212, the position of the substituent could not be unequivocally determined. 

Mechanistic studies have been most thorough with the sulfenyl halides.63 The reactions show moderate sensitivity to alkene structure, with electron-releasing groups on the alkene accelerating the reaction. The addition can occur in either the Markownikoff or anti-Markownikoff sense.64 The variation in regioselectivity can be understood by... 

The reaction of 2//-1,2,3-diazaphospholes with bromine involves a sequence of 1,2-addi-tion/elimination equilibria, R = Me, Ph (Scheme 22) <85CBi62l). 4-Bromo-1,2,3-diazaphospholes become accessible this way. Similarly, 4-phenylthio- and 4-methylthio-1,2,3-diazaphospholes result from reactions with sulfenyl halides <848591, 85CB1621). Iodine chloride reacts to give both the 4-chloro- and the 4-iododiazaphosphole <848591). Chlorination and bromination of diazaphospholes at C-4 has been achieved also with SO2CI2 and BrCN or NBS, respectively <848591). 

Alkali acetylides react very easily with disulfides, R SSR, thiocyanates, R SC=N, and thiosulfonates, R SS02R The reactions can be carried out in liquid ammonia as well as in organic solvents and generally give excellent yields of the acetylenic sulfides, RChCSR [2,105]. Although sulfenyl halides seem suitable reagents for the introduction of alkylthio- or arylthio groups, they are seldom used for this reaction, because of their sensitivity and the... 

One Hundred Years of Sulfenic Acid Chemistry II, Substitution and Cyclization Reactions of Sulfenyl Halides E. Kuehle, Synthesis, 1971, 617-638. 

A concerted elimination-cyclization mechansim, involving a sulfenyl halide in a 1,3-butadiene-1-thio system, is the most probable mechanism for the formation of benzo[6 Jthiophenes from cinnamic acids or 4-aryl-2-butanones by treatment with thionyl chloride. The reactions shown in Scheme 5 have been carefully worked out, and the intermediates isolated (75JOC3037). The unique aspect of this synthesis is the reduction of the sulfinyl chloride (a) by thionyl chloride to form the sulfenyl chloride (b). The intermediate (b) was isolated and converted in pyridine to the 3-chlorobenzo[6]thiophene-2-carbonyl chloride in 36% yield (73TL125). The reaction is probably initiated by a sulfenyl ion attack on the aromatic ring, since it is promoted by electron-releasing groups para to the site of ring closure. For example, when X in (36) was N02, a 23% yield of (37), a mixture of 5-and 7-nitro derivatives, was obtained, but when X in (36) was OMe, a 54% yield of (37) was obtained, contaminated with some 3,4-dichloro-5-methoxybenzo[6]thiophene-2-carboxylic acid. 

Addition of a selenenyl halide to an alkyne takes a course analogous to that of the sulfenyl halide, as seen in the reaction of l,4-dichlorobut-2-yne with PhSeCl or PhSeBr (Scheme 19).28-62 Selenenyl halides also undergo facile addition to allenes with exclusive attack by selenium occurring at the central allenic carbon.63... 

A recent paper276 has shown that some halocyclizations involving sulfur functionalities other than thiols (e.g. thiazolidinones) may involve initial reaction at sulfur to generate a sulfenyl halide which cy-clizes via attack of the electrophilic sulfur. This type of reaction would be an alternative to the nucleophilic heterocyclization mechanism previously proposed for a synthesis of a biotin intermediate from a vinylthiazolidine. P 277... 

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