Acylpalladium derivatives oxidative addition

The catalytic cycle is common with the single carbonylation of aryl hafides in the steps of oxidative addition of aryl halide to a Pd(0) species and the subsequent CO insertion process. Since successive CO insertion into the acylpalladium bond is not thermodynamically favorable, incorporation of another CO molecule takes the route of coordination of the CO molecule to the acylpalladium(II) intermediate. Attack by a nucleophile such as secondary amine, alcohol, and water in the presence of an appropriate base, including the amine itself, gives bis acyl type intermediate which reductively efiminates a-keto acid derivatives [131]. 

Before discussing the double carbonylation processes it may be helpful to understand the mechanism of the single carbonylation of aryl halides into carboxylic acid derivatives (Heck processes). The first step in the catalytic process is oxidative addition of an aryl halide to Pd(0) species formed from a catalyst precursor to yield an arylpal-ladium halide intermediate (A) in Scheme 1. Insertion of carbon monoxide into the aryl-palladium bond in A gives an acylpalladium halide complex (B). Attack of a nucleophile such as alcohol, amine, and water assisted by a base on the acylpalladium complex yields carboxylic ester, amide, and carboxylic acid, although details of the mechanism have not been unequivocally established. The palladium(O) species regenerated in the process further undergoes oxidative addition to carry out the catalytic cycle (Scheme 1). 

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