Sulphonium salts elimination from

A carbanion mechanism is also favoured for elimination from the sulphonium salt (44) , as it undergoes facile elimination compared to (45) and (46) which possess alpha substituents lacking a heteroatom with low-lying (/-orbitals . ... 

In all solvents, the p value for the ammonium salts is greater than for the sulphonium salts, a result in agreement with the isotope studies (Section 2.2.1) which show that elimination from the former proceeds through a more carbanion-like E2 transition state. 

For alpha alkyl substitution, the rate of elimination from ammonium and sulphonium salts increases sharply along the series ethyl < isopropyl < r-butyl when ethoxide in ethanol is the base - . Product studies reflect a similar accelerating effect as isopropyl and r-butyl groups are more easily eliminated from R R fCHslzi OH than ethyl . These alpha substituents aid C -X heterolysis and shift the transition state away from the carbanion-like extreme towards one possessing more double-bond character. 

The present available evidence seems to exclude steric control as determined by the size of the halogen leaving groups, but this does not exonerate the more bulky substituents. It may be difficult to formulate precisely the regions dominated by steric control. For instance, in the elimination from the series of sulphonium salts (121) ,... 

PPV (62a) has been prepared by a number of different methods which were studied in detail by Horhold and Opfermann [390]. It can be synthesized by bifunctional carbonyl olefination of terephthalaldehyde according to Wittig s reaction and from />-xylylene-bis(diethyl phosphonate), as well as by dehydrochlorination of/p-xylylene dichloride with sodium hydride in A jiV-dimethylformamide and with potassium amide in liquid ammonia. The most popular route to PPV used today is the precursor route, first described by Wessling and Zimmermann [391-394] and Kanabe and Okawara [395], starting from the monomers p-xylylene-bis(di-methylsulphonium tetrafluoroborate) [395] or chloride (Scheme 11.17) [391-394]. The latter is polymerized to yield a water soluble sulphonium salt polyelectrolyte (63d) which is then purified by dialysis [396]. The precursor polymer is converted to PPV (63e) by the thermal elimination of dimethyl sulphide and HCl. The method was later developed by Horhold et al. [397], Lenz and co-workers [398,399], Murase et al. [400] and Bradley [401], One of the major improvements within the last years has been the use of tetrahydrothio-phene instead of dimethyl sulphide in the synthesis of the precursor polymer [402]. The use of the cyclic leaving group facilitates the elimination when the precursor polymers is heated at 230-300°C and leads... 

Ylides of Sulphur, Selenium, Tellurium, and Related Structures 343 (trans cis = 90 10) in 54% yield, but a major by-product was (65), which undoubtedly resulted from proton transfer in the intermediate to re-form a new substituted ylide (66), which added to another molecule of methyl acrylate. A similar proton migration occurred in the reaction of diethylsul-phoniumcyanomethylide with vinyltetramethylenesulphonium bromide, but in this instance a second addition did not occur and the elimination took place directly to afford a sulphonium salt (67). Even though stabilized... 

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