Arenecarboxylic acid carbon dioxide

Some of the latest developments in terms of substrates for the Heck reactions are nitrophenyl benzoates [55], acyl benzoates (mixed anhydrides) [236], and substituted benzoic acids [53]. For the last two, the leaving groups are carbon dioxide and carbon monoxide, respectively. A variety of alkenes have been coupled with these substrates. While esters and anhydrides presumably react in a catalytic cycle like that of the classical Heck reaction [237], arenecarboxylic acids, when treated with an equimolar amount of a silver salt as reoxidant, appear to undergo a nonclassical Heck reaction as demonstrated by the coupling with 2-cyclohexenone to give a 3-arylcyclohexenone. 

In this process, carboxylic acids and olefins are converted to vinylarenes in the presence of palladium in catalytic amounts and silver in overstoichiomettic amounts (Scheme 4). It is initiated by formation of a silver arenecarboxylate salt, which then reacts with a palladium(ll) halide complex (a) to give the silver halide and palladium carboxylate b. The extrusion of carbon dioxide, and thus the conversimi of the carboxylate to the organometallic species c, occurs within the ligand sphere of palladium. The subsequent steps, i.e., insertion of the olefin into the palladium-carbon bond, internal rotation, and (3-hydride elimination, correspond to the classical Heck reaction [26]. An additional oxidation step converts the palladium(0) species f back to palladium(ll). It is this oxidation by silver that... 

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