Stabihzed yhdes

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. 

However, an interesting variant (way b) avoiding the loss of one equivalent of the starting yhde as a consequence of the transyhdation reaction was recently developed using a zinc promoted acylation of stabihzed ylides by acyl chlorides [18]. 

The azomethine ylides described above all bear an ester or other electron-withdrawing group in order to promote yhde formation. Non-stabihzed azomethine ylides are less common, but can be produced, for example by decarboxylation or desilylation. The parent azomethine yhde 233 can be conveniently prepared by desilylation of an a-amino-silane, in which a leaving group (alkoxide, cyanide, benzotriazole, etc.) is present at an a position (3.147). Formation of the non-stabilized ylide 233 in the presence of an alkene or alkyne dipolarophile leads to the 2,5-unsubstituted pyrrolidine or dihydropyrrole product. 

Aggarwal et al. [ 199] reported the preparation of phosphoranes derived from phosphites, generating these previously unknown species, by carbene transfer using decomposition of diazo derivatives catalyzed by iron porphyrins. This new class of yhdes led to a high level of E selectivity with semi-stabihzed ylides in Wittig olefinations. 

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