Sulfurane intermediates

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). 

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. 

Mislow and his co-workers (261) suggested, however, that epimeriza-tion of menthyl sulfinates occurs via either a sulfurane intermediate 221 or a sulfonium salt 222. 

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). 

Between 236 and 237, the two other sulfurane intermediates that may be responsible for inversion at sulfur, 237 appears to be the more probable candidate for two reasons. The first is that the ethoxy and hydroxy groups occupy the proper apical positions in accord with their Idgh apicophilicities, and the lone electron pair is placed equatorially, consistent with all the available structural data on sulfuranes. Second, the four-membered ring in a stable cyclic difluorosulfurane 238 was found by Denney et al. (287) to occupy a diequatorial position. 

The displacement of both alkoxy groups at sulfur in 239 with predominant inversion of configuration can be explained by the simultaneous formation of two different intermediates (or transition states) 242 and 243, which undergo decomposition before pseudorotation. In this context, it is interesting to note that the formation and decomposition of only one such sulfurane intermediate would give two sulfinate products with inverted and retained configuration at sulfur. 

Inspection of the results in Table 2 shows that the predominant retention of configuration was observed in the cases of isopropanol, cyclopentanol, and cyclohexanol. These preliminary data may be rationalized in terms of the paraUel formation of two sulfurane intermediates (254 and 255), which are responsible for inversion and retention at sulfur, respectively. The relative stabilities of these inter-... 

It is difficult to explain the results of enantioselective alkylation of 2-methoxycarbonyl-l-oxoindane by a simple SN2 mechanism the yield of the 2-ethyl product is greater than that of the 2-methyl product, whereas the reverse was expected, and, in addition, the configurations of the two products are opposite. Thus, a mechanism has been proposed that involves either a sulfurane intermediate or a tight ion pair10. 

Ab initio calculations have been carried out on the gas-phase acid-catalysed hydrolysis reactions of sulfinamide (319) using the 3-21G sets.290 The first step in the acid-catalysed hydrolysis of A-methylmethanesulfinamide (319 R1 = R2 = Me) is O-protonation and this form is then transformed by addition of water to the sulfurane intermediate (320). Intramolecular proton transfer from O to N follows and then slow N—S bond cleavage to give products.290 Studies with (319 R1 = Me, R2 =aryl) also... 

Desaffdrizadon of episii Ues. Desulfurization of cis- and rrunr-2-butene episulfides by n-butyllithium proceeds with complete stereospeciiicity to give cis- and trans-1-butene, respectively. Trost suggests that the reaction may proceed through a sulfurane intermediate (a). 

In hypervalent sulfur compounds like (37) and (38), the apical bonds formed by the /r-orbitals are longer and more polar than the equatorial bonds. The special stability of many polycoordinated sulfur compounds is still concluded to be associated with the added orbital interaction with the energetically accessible 3J-orbitals. The discovery of sulfuranes has provided valuable insight into many nucleophilic substitution reactions on polycoordinate sulfur atoms and into ligand-coupling reactions which occur via a-sulfurane intermediates. 

Many ligand-coupling reactions occurring via s-sulfurane intermediates involve treatment of aryl or heteroaryl sulfoxides with Grignard reagents (Scheme 14). The intramolecular nature of the ligand coupling reaction is demonstrated by the absence of cross-over products thus, the formation of 2-benzylpyridine (43) (Scheme 14a) may proceed as shown in Scheme 15. 

However, for the majority of substitution reactions at the sulfenyl sulfur atom, most of the available data favour the operation of the bimolecular SN2-type reaction mechanism involving the s-sulfurane intermediate (60) (see chapter 3, p. 35), as depicted in Scheme 36. 

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