Sulfonium ion, formation

Because sulfide is used in large excess compared with the acid (r = [S]o/ [HOTflo > 1) and the equilibrium constant for sulfonium ion formation is large (fifeq > 1), integration of Eq. (81) simiplifies to Eq. (82). 

Deactivation of growing carbenium ions by reaction with sulfides is evidently very fast. Sulfonium ion formation is exothermic (AH = -40 kJ/mol) and exoentropic (AS = -74 J/mol-K) [271]. High equilibrium constants (Keq = 104 moI-1L) for sulfonium ion formation were calculated from the apparent rate constants of propagation and the rate constants of carbocationic growth. Dynamic NMR experiments of model systems with tetrahydrothiophene indicate that the bimolecular deactivation rate constant is kdeacl 106 mol-1-Lsec-1 at 0° C (AH = 20 kJ/mol, AS = -37 J-mol-K), and that activation is faster than bimolecular exchange (k act foe) [67]. 

The equilibrium constants for sulfonium ion formation were calculated from the apparent rate constants of propagation and the rate constants of carbocationic propagation [257], using Eq. (31) obtained by the combination of Eq. (30) with Eqs. (28) and (29). 

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

The formation of the dication involves protonation of one of the sulfur atoms forming the sulfonium ion and subsequent elimination of H2S. 

The proposed mechanism of the oxidative cleavage of S-protecting groups by the chlorosilane/sulfoxide procedure is outlined in Scheme 8. 95 The first reaction is considered to be formation of the sulfonium cation 9 from diphenyl sulfoxide (7) and the oxygenophilic silyl compound 8. The formation of a sulfonium ion of this type is known and has been utilized for the reduction of sulfoxides. 97 Subsequent electrophilic attack of 9 on the sulfur atom of the S-protected cysteine residue leads to the formation of intermediate 10, whereby the nature of the silyl chloride employed should be the main factor that influences the electrophilicity of 9. The postulated intermediate 10 may then act as the electrophile and react with another S-protected cysteine residue to generate the disulfide 11 and the inert byproduct diphenyl sulfide (12). This final step is analogous to the reaction of a sulfenyl iodide as discussed in Section 6.1.1.2.1. 

The transannular formation of sulfonium ion intermediates is not limited to six-membered rings. A similar contraction of a benzo[6]thiepin derivative occurred when (288) was refluxed in dry dioxane, giving a quantitative yield of (289) (74CC626). 

Sulfonium ion 93 has been prepared by treatment of the starting disulfur compound with NO+PF6 and the transformation was interpreted with the involvement of a dicationic species252 [Eq. (4.71)]. Treatment of the corresponding sulfoxide with Tf20 resulted in the formation of a cyclic product ion253 [Eq. (4.72)]. 

Irrespective of whether the initially formed sulfonium ion B or the subsequently formed sulfonium ion D reacts with the alcohol, the alcohol is taken up by such a sulfonium ion with formation of sulfuranes A (first case) or C (second case). Any of these sulfuranes would yield the sulfonium salt E after dissociation. Once this sulfonium salt has formed, five equivalents ofNEtj are added to the reaction mixture, which then is allowed to warm up from -60 to -45°C. Under these conditions, the sulfonium salt E is deprotonated to give the sulfonium ylide F. This ylide undergoes a /3-elimination via a cyclic transition state to form the desired carbonyl compound and dimethyl sulfide as a side-product. 

Displacement of the sulfonium ion by a hydroxy group of acceptor 83 in an SN2-like fashion resulted in the formation of the a-glycoside 84. This strategy was applied to the synthesis of trisaccharide 86 containing both a and p linkages in a one-pot fashion in 52% yield. 

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

C-NMR spectra were in accord with the formation of only one silicon-containing species having only one type of isopropyl group. According to the authors, the isomeric sulfonium ion structure and the dimer structure can be eliminated with a high degree of confidence. (4) Treatment of the product with diisobutylaluminum hydride led only to the recovered silane precursor. 

Three-membered cyclic sulfonium ions, representing [3 carbocations, are often encountered in participation reactions. We have seen this already in the way mustard gas works, but almost any arrangement of a sulfide with a leaving group on the (3 carbon atom leads to participation and the formation of a three-membered ring. The product is formed by migration of the PhS group from one carbon atom to another (Chapter 37). 

A similar effect was observed for trityl derivatives. Initiation of vinyl ether polymerization with trityl salts is very slow and often incomplete [257]. This precludes preparation of well-defined polymers with predetermined molecular weights and narrow MWDs. However, polymerization of vinyl ethers initiated by trityl salts in the presence of tetrahydrothiophene leads to controlled polymers [135]. The equilibrium constant for the formation of sulfonium ions is much smaller for trityl salts than for the growing species (K, < Kp), which increases the ratio of the apparent initiation to the propagation rate constants a thousand times (Scheme 14) ... 

These systems are of little synthetic value, however, because extensive chain transfer to polymer leads to cyclization and formation of branched sulfonium ions even for mono- and disubstituted thiiranes [156]. 

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