Sulfonium ion

Similar to oxonium ions, our studies of sulfonium ions also showed protosolvolytic activation in superacids to give sulfur superelectrophiles. The parent sulfonium ion (HjS ), for example, gives H4S (diprotonated hydrogen sulfide) in superacids. 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

Tetramethyl-l,2-oxathietane (138) was prepared by diazotization of 139, which was prepared from the aziridine (140) (86JA3811).Tlie reaction presumably involves the decomposition of the sulfonium ion intermediate (141).Tire dichloromethane solution of 138 at -20°C is sufficiently stable to permit exploration of the chemical reactions. Tire oxathietane 138 undergoes a formal [[Pg.248]

In order to enable the dimethyl sulfoxide 3 to oxidize the alcohol substrate effectively, it has to be converted into an reactive agent. This is carried out by treatment with oxalyl chloride 4, hence leading to sulfonium ions 5 or 6 as the active species ... 

Sulfonium ions are themselves useful alkylating agents because a nucleophile can attack one of the groups bonded to the positively charged sulfur, displacing a neutral sulfide as leaving group. We saw an example in Section l 1.6... 

Thiols, the sulfur analogs of alcohols, are usually prepared by Sjv 2 reaction of an alkyl halide with thiourea. Mild oxidation of a thiol yields a disulfide, and mild reduction of a disulfide gives back the thiol. Sulfides, the sulfur analogs of ethers, are prepared by an Sk2 reaction between a thiolate anion and a primary or secondary alkyl halide. Sulfides are much more nucleophilic than ethers and can be oxidized to sulfoxides and to sulfones. Sulfides can also be alkylated by reaction with a primary alkyl halide to yield sulfonium ions. 

Prolonation of thiepins leads to //-thiepinium ions, for which a homoaromatic character has been discussed.24 The 5//-1 -benzothiepinium ion can be regarded as a benzohomothiopyrylium ion, whereas the 3//-l-benzothiepinium ion is postulated as having a localized sulfonium ion structure.25... 

C. C. Price and S. Oae, Sulfur Bonding, Ronald Press, New York, 1962. (After a quarter of a century this is still a valuable source of information on matters related to bonding in sulfoxides, sulfones, and sulfonium ions.)... 

Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA) differs from the other promoters in that its cation is a radical, and as such produces radical cationic sulfonium ions as glycosylating species from thioglycosides.85 The use of this promoter arose from earlier work on the electrochemical generation of 5-glycosyl radical cations as glycosylating species. 

Interestingly, the sulfanes H2S are both proton acceptors and donors. In the first case sulfonium ions H3S are formed, in the second case hydrogen polysulfide anions HS are the result. While the latter have never been isolated in salts, several salts with sulfonium cations derived from the sulfanes with n = 1, 2, and 4 have been published. However, none of these salts has been structurally characterized by a diffraction technique. Therefore, the structures of the HsSn cations and HS anions are known from theoretical calculations only. 

In an extensive ab initio MO study the structures, energies and vibrational spectra of the sulfonium ions H3S with n=l-4 were studied at the MP2/6-311(2df,2pd) level of theory [70]. It was confirmed that HsS is of Csv symmetry with dsH= 134.6 pm and bond angles of 94.2° This cation had previously been isolated as a component of the salt [H3S][SbF6] [71] and had been observed spectroscopically [72]. The experimental gas phase re geometry is dsH= 135.02 pm and 0 hsh=94.189° [72] which agrees well with the ab initio calculated values by Botschwina et ah dsH=135.0 pm, aHsn=94.2° [73]. 

Cyclic sulfonium ions were observed in hydrolysis... 

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

They proposed that [l,4]-sulfanyl participation gave a [2.2.1] bicyclic sulfonium salt intermediate 10a. This could be captured by the chloride anion to give either 19 (by path a) or 20 (by path b) as outlined in Scheme 3. It should be noted that the usual [l,2]-sulfanyl participation, via spirocyclic sulfonium ion 21, would only give the unrearranged chloride 19. 

Pinto and co-workers have synthesized various thioniabicyclo[4.3.0]nonanes via a protocol that involves intramolecular displacement of a leaving group on a pendant acyclic chain by cyclic thioethers. A sulfonium ion analogue 54 of swainsonine <2006CAR1685> and an analogue 55 of epi-swainsonine <2006JOC1262>, which differs from swainsonine in the stereochemistry at C-3, were synthesized using this approach. 

The numerous straightforward examples of internal displacement reactions leading to isolable cyclic products will not be discussed here, but only, for the most part, those ionization reactions in which a cyclic intermediate or transition state is deduced from the rearranged structure of the product. A well-known example is mustard gas and other alkyl chlorides with sulfur on the /3-carbon atom. Although mustard gas is a primary and saturated alkyl chloride, its behavior is like that of a typical tertiary alkyl chloride. It reacts so fast by a first order ionization that the rate of the usual second order displacement reaction of primary alkyl halides is not measureable. Only the ultimate product, not the rate, is determined by the added reagent.228 Since the effect of the sulfur is too large to be explicable in terms of a carbon sulfur dipole or similar explanation, a cyclic sulfonium ion has been proposed as an... 

The sulfur accelerates the ionization by direct participation as an internal displacing reagent. The reagent Y may attack the sulfonium ion at either of the ring carbon atoms, causing rearrangement in suitable derivatives 229 CHS... 

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