Carbon-palladium bonds acylpalladium complexes

The catalytic process (Figure 2-4) usually begins with the oxidative addition of an aryl halide or sulfonate onto the active form of the catalyst. In the presence of carbon monoxide the formed palladium-carbon bond breaks up with the concomitant insertion of a CO unit to give an acylpalladium complex. Such complexes might also be formed by the oxidative addition of acyl halides onto palladium. 

Acylpalladium complexes are readily prepared through oxidative addition of Pd° complexes to acid chlorides. PdL4 compounds, where L is a tertiary phosphine, react with acid chlorides at room temperature to give trani-L2Pd(COR)Cl complexes. Since carbon monoxide does not insert into palladium acyl bonds, Pd(C0C02R) complexes are made from oxidative addition of oxalyl chloride monoesters. 

The following mechanism was proposed for the carbonylation of olefin-palladium chloride complex (10). The first step is coordination of carbon monoxide to the complex. Insertion of the coordinated olefin into the palladium-chlorine bond then forms a -chloroalkylpalladium complex (IV). This complex undergoes carbon monoxide insertion to form an acylpalladium complex (V), as has been assumed for many metal carbonyl-catalyzed carbonylation reactions. The final step is formation of a )8-chloroacyl chloride and zero-valent palladium by combination of the acyl group with the coordinated chlorine. 

Dhawan et al. reported that the reaction of the imine 476 with benzoyl chloride and carbon monoxide in the presence of the palladium catalyst 478 gave miinchnone 477 (Scheme 147).209 This reaction proceeds through formation of the acyliminium salt 479, the insertion of CO into the C—Pd bond of the intermediate 480, dehydrochlorination from the resulting acylpalladium complex 481, and cyclization by attack of the oxygen to the electron-deficient carbonyl coordinated to palladium (see 482). 

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). 

The intramolecular version of Pd(II)-catalyzed alkoxycar-bonylation is an investigated transformation because of the facility of its exploitation in the stereoselective construction of oxaheterocycles. The generally accepted mechanism of this domino process involves a few steps intramolecular nucleophilic attack of hydroxyl group to ifi -alkene-Pd(II) complex to afford a u-alkyl palladium(ll) intermediate. Exposure of this unstable intermediate to the atmospheric pressure of carbon monoxide results in the insertion of CO into the Pd C bond to provide the a-acylpalladium(ll) complex, which is finally trapped by alcohol to release the oxaheterocyclic carboxylate (Scheme 15.2). The required... 

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