Reactions sulfuranes

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). 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

Fluoride ion attacks the sulfur atom in 2,3-diphenylthiirene 1,1-dioxide to give ck-1,2-diphenylethylenesulfonyl fluoride (23%) and diphenylacetylene (35%). Bromide or iodide ion does not react (80JOC2604). Treatment of S-alkylthiirenium salts with chloride ion gives products of carbon attack, but the possibility of sulfur attack followed by addition of the sulfenyl chloride so produced to the alkyne has not been excluded (79MI50600). In fact the methanesulfenyl chloride formed from l-methyl-2,3-di- -butylthiirenium tetrafluoroborate has been trapped by reaction with 2-butyne. A sulfurane intermediate may be indicated by NMR experiments in liquid sulfur dioxide. 

The great reactivity of the sulfurane prepared by this procedure toward active hydrogen compounds, coupled with an indefinite shelf life in the absence of moisture, makes this compound a useful reagent for dehydrations,amide cleavage reactions, epoxide formation, sulfilimine syntheses, and certain oxidations and coupling reactions. 

Synthesis and stereochemical study of reactions of optically pure cyclic sulfuranes, S(rV)-derivatives and their Se(IV)- and Te(IV)-analogs 99YGK587. 

Compared to a bromonium ion, the C-S bonds are stronger and the TS for nucleophilic addition is reached later. This is especially true for the sulfurane structures. Steric interactions that influence access by the nucleophile are a more important factor in determining the direction of addition. For reactions involving phenylsulfenyl chloride or methylsulfenyl chloride, the intermediate is a fairly stable species and ease of approach by the nucleophile is the major factor in determining the direction of ring opening. In these cases, the product has the anti-Markovnikov orientation.62... 

Stereochemical features of 130 favor formation of a sulfurane dication 131 by treatment with concentrated sulfuric acid.132,133 The sulfuranyl dication 133 is postulated to be involved in the 1,5-oxygen migration observed in the reaction of monosulfoxide 132 with trifluoroacetic acid (Scheme 50).82... 

The 1 1 reaction of 76 or 77 (R = Me, R = H) with bromine resulted in oxidation and the formation of a carbene complex of SBr2 (79) or SeBr2 (80), respectively (55, 58-62). The X-ray crystal structure of 79 has been reported and showed that this sulfurane possesses an almost collinear axial three-center geometry with elongated S-Br... 

The first indication of the chirality of sulfuranes was provided by the X-ray analysis of spirosulfurane 176 (192). This work clearly demonstrated the presence of enantiomeric pairs of 176 in the crystal lattice. In 1975, the optically active chlorosulfurane 177, the first example of an optically active tetracoordinate sulfurane, was synthesized by Martin and Balthazor (194,195) by the route indicated in Scheme 15. Reaction of (-)-(5)-menthyl benzenesulfinate 178 with the protected Grignard reagent 179 gave the corresponding sulfoxide alcohol (-)-(5>180 which was cyclized to the chlorosul-... 

In addition to the foregoing explanation of retention of configuration, two possibilities may be taken into account. The first is that apical entry is followed by basal departure, and vice versa. Second, the sulfurane intermediate formed may have the structure of a square basal pyramid. In both cases nucleophilic substitution reactions may occur with retention of configuration without ligand reorganization. It appears that the apicophilicity of substituents in sulfurane species... 

Finally, although sulfurane intermediates have been proposed in many cases, they have not been isolated from nucleophilic substitution reactions. However, the concept of an addition-elimination mechanism is supported by the independent syntheses of a number of stable sulfuranes these compounds have a trigonal-bipyramidal structure and in some cases the ligand reorganization was found to occur very easily (189-191). 

Direct formation of 5,5-dihydro-5-iminothianthrene by reaction with hydroxylamine mesitylsulfonate presumably involves, first, electrophilic amination at sulfur (74TL1973). Electrophilic S-bromination (see also Section III, A, 3, b) must be presumed to initiate the conversion of thianth-rene into the sulfurane 25 by reaction with bromine in the presence of the alcohol (RfOH = l,l,l,3,3,3-hexafluoro-2-phenyl-2-propanol) (77JOC3222). 

Under these circumstances there is no need to postulate unimolaiular rate determining reactions. Instead, the bimolecular reaction mechanism (c) discussed above would explain all experimental observations much better. The reaction could proceed via a sulfurane-like molecule which after pseudorotation at the central atom could dissociate to the observed products ... 

Another dialkoxydiarylsulfurane has been prepared under conditions similar to those reported here, indicating that this reaction for sulfurane formation may have wide applicability. 

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