Rearrangement of Allylic Sulfonium and Ammonium Ylides

Allylic sulfonium ylides readily undergo [2,3]-sigmatropic rearrangement.280 The ylides are usually formed by deprotonation of the S-allyl sulfonium salts. 

The reaction proceeds best when the ylide has a carbanion-stabilizing substituent. This reaction results in carbon-carbon bond formation and has found synthetic application in ring-expansion sequences for generation of medium-sized rings. 

Sulfonium ylides can also be generated by in situ alkylation with diazo compounds. The alkylation can be carried out by reaction of a diazo compound with HBF4 and DBU.281 The reagents are added alternately in small portions and the reaction presumably proceeds by trapping of the carbocation generated by dediazonization and deprotonation. 

Reactions of this type lead to preferential formation of the anti stereochemistry at the new C-C bond. 

Rhodium catalysis have been used for formation of ylides by intramolecular reactions. 

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