Vinylogous Pummerer pathway

More recently, two distinct mechanistic pathways for the Pummerer rearrangement have been described. These are the vinylogous Pummerer pathway, and the additive Pummerer sequence. The vinylogous pathway... 

The Pummerer rearrangement of sulfoxides with acid anhydrides has been extensively utilized as a method for the synthesis of a-substituted sulfides. When a,(3-unsaturated sulfoxides are used, the initial formed oxysulfonium ion may undergo two different pathways the additive Pummerer reaction or the vinylogous Pummerer reaction. The following sections will consider examples from both pathways. 

For the intramolecular vinylogous Pummerer reaction pathway, an electrophilic thionium ion intermediate (222) is formed by y-proton loss from 221 followed by sulfoxide S-O bond scission. This unsaturated thionium ion (222) is then intercepted by a nucleophile at the y-position to yield a vinyl sulfide product (223) (Fig. 11). 

As was introduced earlier in this chapter tScheme 20.22 and previous section), the additive pathway in Pummerer chemistry is in competition with the vinylogous path, and its application is still in some cases unpredictable. Fortunately, Yorimitsu, Oshima, and coworkers have published a sequence of papers providing excellent applications of the additive Pummerer pathway. They used a combination of two Pummerer approaches, such that the activation of sulfoxides was done as usual by an electrophilic reagent, but they used the beginning of a so-called interrupted Pummerer reaction (vide infra) to generate the a,(3-unsaturated sulfonium salt 121. This intermediate did not follow the traditional... 

Vinyl sulfoxides also partake in additive Pummerer reactions. Of particular interest in this category are cases where the sulfoxide activating agent also provides the internal nucleophile. Two general pathways for such vinylogous additive Pummerer reactions are presented in Scheme 36. ... 

Feldman et al. has recently demonstrated the utility of the Pummerer reaction for a controlled oxidative cyclization onto both indole and imidazole rings to produce indolenines and imidazolines [13-20]. There are two possible mechanisms for this reaction, shown in Scheme 1 an additive pathway, where the nucleophile adds to C-3 (path B) or a vinylogous pathway (path A). The question of the mechanism has been studied but neither pathway can be confidently rejected [19]. 

Applications of the additive mechanism have often remained speculative because both vinylogous and additive pathways can provide the same product and because the preferred mechanism can depend on delicate changes in reaction conditions. Nonetheless, some examples exist in the literature (see next section) concerning the application of additive Pummerer reactions, and we present an innovative application of this Pummerer variant which is preceded by an interrupted Pummerer activation in the next section of this chapter. 

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