Sigmatropic reactions Stevens rearrangement

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

The other major reaction pathway for oxonium ylide is [l,2]-shift (Stevens rearrangement). Compared with [2,3]-sigmatropic rearrangement, which is an orbital symmetry-allowed concerted process, the [l,2]-shift has higher activation barrier, [1,2]-Shift is generally considered as stepwise process with radical pair as possible intermediates. 

The [2,3]-Stevens rearrangement is a thermal sigmatropic rearrangement of an ammonium ylide (38) to form unnatural amino acid derivatives 39 (Scheme 12). Traditionally, the ammonium ylides have been formed through alkylation of aminoesters 36 with aUcyl halides 37 to form quaternary salts followed by treatment with base. Although effective, the harsh conditions lead to side products and limited substrate scope. More recently, the coupling of diazoesters 40 and allylic amines 41 in the presence of metals like copper, rhodium, and palladium has been developed for the direct constmction of ammonium ylides 38 via metal carbenoid intermediates. " Although this approach represented an advance over the traditional alkylation chemistry, the use of diazoesters still limits the synthetic utility of these reactions. 

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