Sulfonium salts solvolysis

We are currently applying further kinetic tests to the SCE to determine the range of its applicability to kinetic phenomena. One such test, for example, is to determine the sensitivity of reaction rate of sulfonium salt solvolysis to solvent electrophilicity (i.e. its a value). Because the leaving group is neutral in this case, such a reaction would be expected to have a very weak dependence on electrophilicity. 

Kevill and co-workers first address the much-debated issue of nucleophilic involvement in solvolysis of tert-butyl derivatives. Interestingly, the tert-butyl sulfonium salt shows more rate variation with solvent changes than does the 1-adamantyl salt. In particular, the tert-butyl salt shows a rate increase in aqueous TFEs (where both Y and N increase) that is not found for 1-adamantyl. Because a variation in Y cannot explain the result, Kevill argues that the tert-butyl derivative is receiving nucleophilic solvent assistance. On the basis of the available evidence, Harris et al. (Chapter 17) propose that tert-butyl chloride is inaccurately indicated by some probes to receive nucleophilic solvent assistance because the model system (1-adamantyl chloride) has a different susceptibility to solvent electrophilicity. Kevill and coworkers disagree with this proposal, noting that essentially the same tert-butyl to 1-adamantyl rate ratio is found for the chlorides and the sulfonium salts if solvent electrophilicity were important in one case but not the other, then the rate ratio should vary. 

Repeating the experiment with lb, but quenching the reaction by addition of diethyl ether as soon as the effervescence had subsided, afforded the tetrathiafulva-lenium salt 2b this compound was then subjected to solvolysis in undried deutero-acetone and afforded the corresponding alcohol 3b, consistent with its intermediacy in the reaction. The basic mechanism of the reaction can thus be represented by Scheme 2. Aryl radicals are formed following electron transfer to the diazonium cation and subsequent loss of dinitrogen. Rapid cyclization is followed by formation of the sulfonium salt 2b, and a facile solvolysis occurs to afford the alcohol 3b. Since the tertiary alcohol 3c was formed from substrate Ic, a similar pathway may have been followed, but the direct oxidation of the tertiary radical by electron transfer to a diazonium cation cannot yet be ruled out. The resistance of the primary salt to solvolysis is a classic hallmark of an Sn 1 reaction. (A refinement for the mechanism of the solvolysis step will be presented in Section 2.7.3.1 of this review, backed by very recent results). 

It is possible to chemically tune the optical properties of PPV-type polymers by the copolymerization of different sulfonium salts. This was used to advantage to increase the electro- and photoluminescence efficiency of PPV by the incorporation of up to 20% dimethoxyphen-ylene units within the polymer (Fig. 13,5) [113]. The sulfonium groups adjacent to a methoxyphenylene ring rapidly undergo solvolysis and become substituted with a... 

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