Pummerer rearrangement mechanism

Chlorotrimethylsilane-induced Pummerer rearrangements effect the transformation of 4-ketothiane oxides into the corresponding a, /1-unsaturated thianes348, apparently via the formation and subsequent deprotonation of thiiranium intermediates rather than by the conventional sulfocarbonium mechanism depicted in equation 129. 

The influence of the classical anomeric effect and quasi-anomeric effect on the reactivity of various radicals has been probed. The isomer distribution for the deu-teriation of radical (48) was found to be selective whereas allylation was non-selective (Scheme 37). The results were explained by invoking a later transition state in the allylation, thus increasing the significance of thermodynamic control in the later reactions. Radical addition to a range of o -(arylsulfonyl)enones has been reported to give unexpected Pummerer rearrangement products (49) (Scheme 38).A mechanism has been postulated proceeding via the boron enolate followed by elimination of EtaBO anion. 

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. 

The mechanism of the Pummerer rearrangement is probably as shown in Scheme 16. The rearrangement involves four stages. First is the formation of an... 

A mechanism suggested for Swern-Moffatt oxidation with TFAA is shown in Scheme 8.6. In the first step, DMSO reacts with TFAA to form cationic reactive species I, which is known to be stable only below —At higher temperatures, rearrangement of I takes place to give species II. The reaction of II with an alcohol IQ upon treatment with a base leads to formation of a major by-product, trifluoroacetic acid (TFA) ester VII. Therefore, the first step should be carried out below —50 °C. In the second step, reactive species I is allowed to react with an alcohol HI at or below —50°C to obtain intermediate IV. IV may also undergo the Pummerer rearrangement to give a methyl thiomethyl (MTM) ether VI upon treatment with a base. In the third step, IV is treated with a base (usually triethylamine) to obtain the desired carbonyl compound V and dimethyl sulfide. 

An operationally simple halogenation of 4,5-dimethyl-2-arylthiazoles provides a regioselective approach to bromo- or chloro-methyl substituted thiazoles <04TL69>. Thus, treatment of 117 and its hydrochloride salt with NBS and NCS affords 4-bromothiazole 118 and 4-chlorothiazole 119, respectively, with >99% regioselectivity. The remarkable regioselectivity observed may arise from a Pummerer-type rearrangement mechanism via 120. 

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. 

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