Disulphides, reactions with alkynes

Compounds of sulphur(II) undergo the expected oxidations with DIB thiols to disulphides and sulphides to sulphoxides. Thiols form initially unstable iodine(III) intermediates which react with alkynes (Section 3.7). With some sulphides, DIB proved to be uniquely efficient, for example in the oxidation of thioxanthone [132]. The best conditions for high yields involved the use of acetic acid as solvent and catalytic amounts of sulphuric acid no heating was required and the reaction was completed in about 5 minutes [133]. 

Benati L, Montevecchi PC, Spagnolo P. Free-radical reactions of benzenethiol and diphenyl disulphide with alkynes. Chemical reactivity of intermediate 2-(phenylthio) vinyl radicals. J. Chem. Soc., Perkin Trans 11991 2103-2109. 

Under photochemical conditions several 1,4-dipoles reacted with alkenes, alkynes, phenyl isothiocyanate and carbon disulphide to afford, after iodobenzene expulsion, cyclic adducts in moderate to low yields. This drawback is offset by the fact that it is difficult to obtain some of these compounds through alternative methods. Table 10.6 are listed selected examples of cyclic compounds formed by this methodology. Formally, these reactions can be considered as 1,3-dipolar cycloadditions, in which the 1,3-dipole comes from the iodonium precursor after elimination of iodobenzene actually, they arise most probably through 6-membered intermediate iodanes. 

A different source of arenesulphenyl cations has been reported by Montevecchi and coworkers . They showed that the BFs-promoted reaction of 4 -nitrobenzenesulphena-nilide (35) with aryl-substituted alkynes in poorly nucleophilic solvents such as chlorobenzene generally led to bissulphides (36) and sulphimides (37) in addition to diphenyl disulphide and 4-nitroaniline. In acetonitrile as solvent, products of capture of the thiirenium ion by the solvent were also observed (equation 22). The thiirenium ion from phenylacetylene reacted even with the poorly nucleophilic solvent chlorobenzene to ( )-PhSCH=CPhC6H4Cl. With alkyl-substituted alkynes in chlorobenzene as solvent, a small amount of ( )-2-fluorovinyl sulphide PhSC(R )=CF(R ) was also detected. The yield of this sulphide could be increased when the reaction was performed in the presence of tetrabutylammonium tetrafluoroborate. Terminal alkynes gave the corresponding 2-fluorovinyl sulphides in 35-55% and internal alkynes in 65-87% yield. The procedure was unsuccessful for di-tcrt-butylacetylene and gave low yields for arylacetylenes. In acetic acid as solvent the thiirenium ion was captured as ( )-PhSC(R )=CR (OAc). 

The reaction of the triazole (323) with carbon disulphide does not yield the expected triazolopyrimidine (325) the triazolothiazine (324) is formed instead (Scheme 125). Derivatives of pyrrolo[3,2- f]pyrimidine (327) are obtained, in 54—64% yield, from pyrimidotriazines (326), the initial step being one of cycloaddition of the alkyne to the nitrone function (Scheme 126)/ ... 

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