Stevens -sigmatropic rearrangement

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

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. 

Woerpel and coworkers interpreted the results of these mechanistic experiments as evidence that the insertion of silylene into the C-O bond occurs through a [1,2]-Stevens rearrangement of oxonium ylide 198 and that a competitive [2,3]-sigmatropic rearrangement of 198 could account for allylic transposition. 

There are a few sigmatropic rearrangements with more than six electrons, such as the 10-electron doubly vinylogous Stevens rearrangement 6.86—>6.87 of the unsaturated ammonium salt 6.85, and the 10-electron benzidine rearrangement 6.88 —> 6.89 of the unsaturated ammonium salt 6.85. 

A single report on 1,4-oxathiocines has been found in the literature published since 1995. The 2-styryl-l,3-oxathilane 215 with the methyl diazoacetate, catalyzed by Rh2(OAc)4, produced a complex mixture of products. 1,4-Oxathiocine 217 and 1,3-oxathiolane 218 were isolated from the mixture in low yield and their stereochemistry was assigned by NMR analysis (Scheme 25) <2006T3610>. The proposed mechanism of this transformation may involve formation of the ylide 216, which presumably underwent [2,3]-sigmatropic rearrangement into 217, or Stevens rearrangement into 218 (cf. Scheme 24 (Section 14.06.6.6) and Scheme 26 (Section 14.06.8)). 

Stereoselective syntheses of di- and trisubstituted olefins, (5, 400-402 6, 30-31). The stereochemistry of [2,3] sigmatropic rearrangements has been reviewed (103 references). The review covers the rearrangements of allyl sulfoxides and allyl selenoxides, as well as Stevens and Wittig rearrangements. ... 

The Stevens rearrangement and the Wittig rearrangement (nonallylic version) (Chapter 4) can be classified as four-electron [1,2] sigmatropic rearrangements. The Woodward-Hoffmann rules state that for a four-electron sigmatropic rearrangement to be allowed, one of the components must be antarafacial, yet it is... 

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