Sigmatropic rearrangements yhdes

A useful method for ortho-alkylation of aromatic amines is based on [2,3] sigmatropic rearrangement of iS -anilinosulfon ium yhdes. These yhdes are generated from anilinosulfonium ions, which can be prepared from /V-chloroanilines and sulfides.189... 

As a C - C bond-forming process, [2,3]-sigmatropic rearrangements of sulfonium ylides merit particular interest. Indeed, since their discovery in the late 1960s [172-176], this type of yhde rearrangement has become the most extensively studied and applied. This activity is well dociunented in a niunber of excellent review articles, many of them published quite recently [9,177-179]. This demonstrates a vivid interest by the scientific community in these rearrangements which is strongly related to the advent of asymmetric and catalytic variants, as will be discussed later. 

Blid, J. and Somfai, P (2003) Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium yhdes. Tetrahedron Letters, 44, 3195-3162. 

Sulfide 2a was later employed by Tang and coworkers in the reaction of 4-chlorobenzaldehyde with 3-trimethylsilylallyl bromide for the synthesis of viny-loxiranes. In the presence of 20mol% of 2a, the trans vinyl epoxide was obtained as the major diastereoisomer in 40% yield and 37% ee [21]. Higher concentrations of both aldehyde and aUyUc bromide were found to be beneficial to the yields, because the high concentration of aUyHc bromide favors the formation of the sulfonium salt and the high concentration of aldehyde could probably allow capture of the sulfur yhde before the [2,3]-sigmatropic rearrangement (Scheme 20.6). This reaction with thiolane (THT) as a catalyst could also be carried out without solvent, but the yield was lower than that in t-BuOH. 

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