Sulfoxides, Pummerer rearrangement mechanism

The mechanism of the Pummerer rearrangement consists of four steps 1) acylation of the sulfoxide oxygen to form an acyloxysulfonium salt 2) loss of a proton from the a-carbon to afford an acylsulfonium ylide 3) cleavage of the sulfur-oxygen bond to give sulfur-substituted carbocation (RDS) and 4) capture of the nucleophile by the carbocation. 

Another interesting stereospecific transformation is the conversion of enantiomerically pure a-Li alkyl sulfoxides to vicinal chloroamines (eq 29). The nonoxidative chloro-Pummerer rearrangement was proposed as the mechanism. The final products can be converted to the corresponding aziridines by treatment with sodium borohydride followed by sodium hydride. 

Sigmatropic rearrangements have been described above as part of the proposed mechanism of additive Pummerer rearrangements such as the reaction between a,P-unsaturated sulfoxides and dichloroketene [220]. De Lucchi et al. have observed a similar type of rearrangement with isopropenyl acetate under acidic conditions [232]. 

Posner et al. reported the first example of an asymmetric additive Pummerer rearrangement in their total synthesis of (-)-methyl jasmonate, a perfume essence.Enantiomerically pure sulfoxide 98 was treated with diehloroketene (generated in situ from dichloroacetyl chloride and triethylamine) to form a,y5-disubstituted sulphide 99. The mechanism is thought to involve a [3,3]-sigmatropic rearrangement of the doubly charged intermediate 100. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

The synthesis of the benzoimidazo[l,2- ][l,2,3]thiadiazole 61 can be explained using the same mechanistic model to that used for the Hurd-Mori reaction. The amino benzimidazole 58 when treated with thionyl chloride at reflux affords the benzoimidazo[l,2-r ][l,2,3]thiadiazole 61. If, however, the reactant 58 is treated with thionyl chloride at room temperature, the chloromethyl derivative 59 is formed. This derivative was then transformed into product 61 on reflux with thionyl chloride. The proposed mechanism for the formation of product 61 is for the initial formation of the sulfoxide 60, which then undergoes a Pummerer-like rearrangement, followed by loss of SO2 and HC1 to give the c-fused 1,2,3-thiadiazole 61 (Scheme 7) <2003TL6635>. 

The reaction of phenyl vinyl sulfoxide 234 with isobutene, in the presence of trifluoroacetic anhydride, yielded the to-alkylated product 238 (Scheme 59).128 It was suggested that this reaction proceeded by a different mechanism than the usual additive Pummerer mechanism. The alkene reacts with the electrophilic sulfur atom of intermediate 235, giving, after loss of a trifluoroacetate ion and a proton, the sulfonium ion 236. Thio-Claisen rearrangement of the ion then gives the thonium ion 237 which reacts with a further molecule of isobutene to give the product 238. 

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