Phosphonium ylides with oxiranes

Sulfonium salts react in several ways. They may behave as a leaving group, undergoing substitution by a nucleophile or fragmenting with the formation of an alkene. However, the most important reaction of sulfonium salts involves the formation of an ylide in the presence of a base. The carbanion of this sulfur ylide is stabilized by the adjacent positively charged sulfonium ion. The reaction of the carbanion with a carbonyl group parallels that of a phosphonium ylide in the Wittig reaction. However, the decomposition of the intermediate dipolar species is different and leads to the formation of an epoxide (oxirane) rather than an alkene. 

Cyclopropanes are formed in the condensation of epoxides with ylides derived from phosphonates. Attempts to carry out similar reactions with phosphonium ylides were generally unsuccessful, an exception being the reaction of the phosphonium halide (25) with oxiran. ... 

CH acidities of sulfonium and phosphonium salts are in the range accessible for the PTC so they can be deprotonated in liquid-liquid and liquid-solid two-phase systems. Since in the generated ylides the charges are internally neutralized the PT catalysts suppl3dng lipophilic cations are not necessary for transfer of the reacting species into the organic phase. Nevertheless in many cases of the reactions of sulfonium and phosphonium ylides, PT catalyst was used and recommended. Thus in two-phase systems with or without catalyst, an efficient synthesis of oxiranes via reactions of sulfonium ylides with aldehydes is executed (eqs. 66 and 67). 

Stabilized and semi-stabilized ylides can also be produced under virtually neutral conditions by using oxiranes as proton acceptors [18]. This has a precondition that the anion of the phosphonium salt is a halide, since epoxide and phosphonium halide are in equilibrium with ylide and the corresponding halohydrin. The technique is particularly advantageous if base-labile functionalities are present. 

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