Stevens rearrangement halides

A related process involves fluoride-induced desilylation in HMPA at room temperature of benzyldi-methyl(trimethylsilylmethyl)ammonium halides (20) to (21 62-84% yields R = H, 2-Me, 4-Me, 2-C1,4-Cl, 4-OAc X = Cl, Br).9 100 Compunds (20) were prepared by reaction of the corresponding benzyl halide derivatives with (dimethylaminomethyl)trimethylsilane, Me3SiCH2NMe2. The product of Stevens rearrangement (22) formed competitively and predominantly from precursors (20) having strong electron-withdrawing substituents (R = 2-COMe, 2-CN, 4-CN, 2-NO2, and 4-NO2).100... 

The base-induced 2,3-shift in a benzyltrialkylammonium halide (Sommelet-Hauser synthesis) efrects an orr/io-selective aminomethylation of aromatics. A variation (orr/to-selective formylation) is shown in equation (37). Unfortunately, the 1,2-Stevens rearrangement (— 10% 143) of the intervening ylide competes with the 2,3-pathway (— 142). A reliable tool to circumvent this very general difficulty is still unknown. Nonetheless, 2,3-rearrangement pathways can be very efficient as demonstrated by the synthesis of the cyclic amine (144 equation 38). ... 

Sato, Y., Yagi, Y., Koto, M. Ylide reactions of benzyldimethyl[(triorganosilyl)methyl]ammonium halides. J. Org. Chem. 1980,45, 613-617. Zhang, J. J., Schuster, G. B. Photo-Stevens rearrangement of 9-dimethylsulfonium fluorenylide. J. Org. Chem. 1988, 53, 716-719. 

The yield and ratio of the two Stevens rearrangement products are dependent on both the base and the solvent however, the ortho-rearrangement product was only detected when butyllithium was used in hexane. Variation of the halide ion had only a small effect on the ratio of rearrangement products. Several mechanisms were considered 93) as routes to the formation of the rearrangement products, 1) ion-pairs, 2) cationic rearrangement, 3) carbenoid, 4) dimetallated intermediates, 5) free-radical, 6) predissociation of the ammonium salt and, 7) sigmatropic shift. 

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. 

The reactions of sulphimides studied recently concern the kinetics of the reaction between PhSH and JV-toluene-p-sulphonyl-sulphimides - to give PhSSPh, or RSPh (other differences exist between the results from these two studies) the formation of vinyl sulphides by treatment of JV-toluene-p-sulphonyl aryl ethyl sulphimides with Bu OK [TsN=S(Et)Ar -> CHg= CHSAr] the halide-ion-induced Stevens-type rearrangement to the sulphen-amide [TsN=SRAr -> RN(Ts)SAr] and the S-substitution or Pummerer rearrangement products from iV-toluene-/j-sulphonyl-sulphimides by cleavage with NaOH or NaOMe in MeOH. °... 

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