Sulphonium ions—

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

The reductive cleavage of sulphonium salts in aprotic solvents leads to the generation of radical and then carbanions in a further electron transfer step. Protonation of the carbanion by extraneous water leaves a hydroxide ion. Basic species formed in this way can abstract a proton from sulphonium ion to give the ylid, which is not reducible. A good example is the reduction of 9 in dimethylsulphox-ide, which consumes only one Faraday and follows the course shown [58]. 

In a second type of side reaction, the carbanion formed can be alkylated by reaction with the sulphonium ion to yield unexpected products [59], This reaction sequence is followed for the reduction of ethoxysulphonium salts 10. Cleavage of the sulphur-oxygen bond, which is the weakest bond in the species, leads to an alkoxyl radical. This is reduced further to the alkoxide, which attacks an ethoxysulphonium ion to form diethyl ether [60],... 

Even more stable salts than those already mentioned can be prepared from oxonium, R30+, sulphonium, R3S+, and ammonium, R4N+ ions. Not surprisingly, however, these are in general too stable to initiate vinyl polymerisations though Et30+BF4 has been reported to polymerise alkyl vinyl ethers (96). Both oxonium and strained sulphonium ion salts are very efficient initiators of ring opening polymerisations as we shall see later (Section IV). 

Polymerisations are characterised by a fast initiation process generating strained cyclic sulphonium ion active centres, which then propagate, before terminating by formation of stable linear tertiary sulphonium ions by reaction with sulphur residues on the polymer backbone, e.g. 

Values of a were computed assuming the dissociation constant Kd, for the polymeric ion pair to be the same as that for some model sulphonium ion salts (see Table 1, Section III.B), and hence a plot of kpappBrent versus a allowed estimation of kp and kp separately. The results obtained are shown in Table 10. 

However a slow re-initiation of polymerisation, giving rise to a second slower stage, occurs on formation of a three membered cyclic sulphonium ion salt by intramolecular nucleophilic attack of the penultimate sulphur in the dormant polymer ... 

Another claim for the syw-addition (this time for Br+ and OH) across a double bond has been made212. This unusual outcome has been observed for bromolactonization of 121 employing a mixture of MesSiBr, Me2SO and an amine as the source of Br+. The mechanism is believed to involve the sulphonium ion 122 as an intermediate arising by... 

These internal salts are stabilized by hydration. Removal of the water of crystallization causes polymerization. The nucleophilic phenolic anion attacks the a carbon of the sulphonium ion, generating a linear chain... 

The A -unsaturated-5a-thiol (350) reacted with bromine or chlorine to give the 3j8-halogeno-2a,5a-epithio-derivative (353). ° It is suggested that a 5a-sulphenyl halide e.g. (351)] is first formed, and then attacks the olefinic bond to give the sulphonium ion (352), which suffers nucleophilic attack upon C(3) by halide ion. Lead tetra-acetate similarly afforded the 3j8-acetoxy-2a,5a-epithio-compound. 

Ionic additions which transform cyclopropenes into cyclopropanes are well known. The electrophilic addition of sulphenyl halides proceeds by a two-step mechanism to give trans-disubstituted products but with little regioselectivity. Cyclic sulphonium ion intermediates are probably involved as illustrated for 1-methylcyclopropene. Electrophilic... 

Unlike THE this system has an inherent termination reaction since the polymer yields are always less than 100%. When triethyl sulphonium tetrafluoroborate, Et3S" BF4, was found to be inactive as an initiator it was concluded that termination involved reaction of a backbone sulphur atom with an active centre, to form a stable tertiary sulphonium ion, viz. 

As mentioned already when a cyclic tertiary sulphonium ion salt [136] was used as an initiator then almost identical results were obtained. However, it was observed that the value of kp fell slightly as the initial catalyst concentration was increased. In fact careful analysis of the data for initiation by Et3 0 BF4 indicates a similar trend. This variation of fep is an indication that more than one ionic species contributes to propagation, and Drijvers and Goethals [130] have analysed in this way data from polymerizations using ethyltetramethylene sulphonium tetrafluoro-borate. 

Most recently a rather similar study has been carried out on the polymerization of propene sulphide [137]. This system, however, is kinetically more complicated because of the formation of a relatively stable twelve membered ring sulphonium ion, which can apparently slowly re-initiate a second stage polymerization. A comparative study of various substituted thietans has been reported and confirms that the rates and the equilibria of reactions between sulphides and sulphonium salts depend upon the difference in basicity of the reacting and product sulphides. 

It has long been known that acids will initiate a thiirane polymerization reaction. By analogy to the oxygen compounds it was reasonably suggested that propagation occurs via a cyclic sulphonium ion. The relatively greater ease of formation of a sulphonium ion compared to an... 

The cationic polymerization of thietane is generally accepted to proceed in the same manner as that of the oxetanes. The most detailed study was carried out by Goethals et al. [42] on thietane and methyl thietanes in CH2CI2 with Et3 0 BF4 as initiator. The initiation reaction was immediate and quantitative [68]. For these monomers propagation also occurred by nucleophilic attack of the monomer on the a-carbon of the cyclic sulphonium ion. For example, the propagation reaction for... 

Termination occurred when a sulphur atom of an existing polymer molecule reacted with the growing cyclic sulphonium ion to give a non-strained, branched sulphonium ion which was not capable of reinitiating polymerization, viz. 

Before leaving Et3 0 BF4 initiated polymerizations of THE one other study needs to be mentioned. In polymerizations of thietanes above we saw that free sulphonium ions did not propagate very much more rapidly than ion pairs (Section 3.2.1). Sangster and Worsfold [88] have found that in THE polymerizations with BE4 counter-ions in CH2CI2 the free ion rate is only a factor of 7 greater than that of the ion pair rate. 

C.J. Murray and W.P. Jencks, Proton Abstraction from Dimethyl(2-substituted-9-fluorenyl)sulphonium Ions. Evidence for Changes in Transition State Structure, J. Am. Chem. Soc., 1990,112, 1880. 

Organochlorine compounds with monosulphides form simple sulphonium ions, but disulphides offer the possibility of more reactive species which can increase the lubricating properties of the system. Recently, the synergistic behaviour of chlorinated paraffin in dibenzyl disulphide has been explained in terms of the negative ion-radical action mechanism [85, 91]. The reaction of these compounds with electrons of low energy, 1-4- eV, emitted during the friction process, reactions of thermal decomposition and reactions of ions with radicals are presented as the following reaction sequence ... 

An ingenious method of measuring kp was evolved by Goethals 17) for the polymerisation of thietans by Et30+BF4. These polymerisations stop at incomplete conversion, because the sulphur atom in the polymer is more basic than that in the monomer, and therefore the reaction of a growing chain (II) with polymer yields a stable, non-propagating tertiary sulphonium ion (III), so that this reaction is kinetically a termination (suicidal polymerisation) ... 

Several different reactions available to the oxy-sulphonium ion explain the variety of possible products. 

In reactions that resemble the pinacol rearrangement, thiopyranoside sulphonate esters undergo ring contraction and expansion reactions via the intermediacy of sulphonium ions, e.g. equations 37 and 38102. Similarly, cyclic sulphonates are able to trap oxime anions at the carbon atom (equation 39)103. Such carbon alkylation of oxime ions is rare, and by suitable choice of substrate both ring-contraction and ring-expansion reactions are possible103. 

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