Arsonium ylides basicity

The pKa values of a series of conjugated acids of the arsonium ylides have been determined by potentiometric titration (73). A decrease in the basicity of the ylides with an increase in substituent electronegativity was observed. The effect of the substituents agrees well with the Hammett equation. Arsonium ylides are more basic (200-230 times) than the corresponding phosphonium compounds. 

Many arsonium ylides dissolve in acids to form salts, from which they can be re-obtained by treatment with a base, as in the salt method for their preparation. The basicity of the ylides indicates the relative stabilities of the ylides and their salts and in so doing gives some guide to the stability of the ylides. Thus stable ylides are less readily protonated than are reactive ylides and require weaker bases for their generation from salts. 

In studies of ylides having different heteronium atoms, e.g. on methoxycarbonylylides", benzoylylides fluorenylides" , tetraphenylcyclopentadienylides " and triphenyl-cyclopentadienylides , the arsonium ylides were uniformly more basic than their phos-phonium or sulphonium analogues. 

The reactions of a series of arsonium ylides with p-nitrobenzaldehyde have been shown to be first order for each reagent and there is a general tendency for the more basic ylides to be the more reactive" ". The correlation is not, however, complete, since factors other than basicity, e.g. steric, and interactions between ylidic substituents and the arsenic atom, also affect the reactivity " , but as a generalization it is largely valid and also must be a significant factor in the greater reactivity of arsonium compared to phosphonium and sulphonium ylides. A fair correlation has also been noted between the chemical shift of the signal from the methine proton in a series of ylides and their rates of reaction with p-nitrobenzaldehyde" ". ... 

In Wittig-type reactions with aldehydes and ketones, arsonium ylides have been shown to give either epoxides or alkenes or mixtures thereof (Scheme 3.88) [140]. However, the semi-stabihzed yHde 457 can be directed onto either pathway by tuning the basicity of the solvent [141]. In pure TH F, the epoxide 459 was formed, whereas in THF/HMPA mixtures the conjugated diene 461 was obtained. This complete switch was observed for a variety of aldehydes and ketones. Rationalization lies in the assumption of zwitterionic intermediates 458 and 460, which react via different conformations. Thus, the anti-conformer 458 is reactive in unipolar solvents, presumably via aggregate formation to give the epoxide, whereas in the presence of HMPA the monomeric species 460 is formed, which undergoes syn-elimination. 

In parallel with this is the ease with which the ylides are proton ated to form heteronium salts. Thus again taking the ylides (XII) as examples, the sulphonium and selenonium ylides do not form salts readily, the phosphonium, arsonium and stibonium ylides are readily protonated with increasing basicity Ptelluronium ylides are destroyed by mineral acids. [149,156]. Protonation may take place at different sites in the molecule depending partly on.the substitution pattern, and the site appears to depend on a balance of electronic and steric factors. [126]. Thus dimethylsulphonium cyclopentadienylide in trifluoroacetic acid is protonated at the 1-, 2- and 3-positions in the ratios of 11 56 33 [157]. 

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