CIR-FTIR Studies of Palladium-Catalyzed Carboalkoxylation Reactions
Palladium-catalyzed carboalkoxylation reactions, in which an aryl halide and an alcohol react in the presence of carbon monoxide and a tertiary amine, are potentially important both industrially and in the laboratory as alternatives to conventional routes to ester production. In spite of this, the current understanding of the mechanism of this reaction is limited. A novel in situ infrared technique, cylindrical internal reflectance-Fourier transform infrared spectroscopy (CIR-FTIR), has been utilized in this study to obtain a clearer picture of the principal steps involved in catalytic ester formation. Infrared spectra of the active reaction at high alcohol concentrations show that the dominant palladium complex does not contain a carbonyl group, consistent with a rate-limiting step involving oxidative addition of the aryl halide to a palladium-phosphine complex. At low methanol concentrations, the palladium-benzoyl (“acyl”) complex predominates and the rate is a strong function of the basicity of the amine used. Stoichiometric studies have shown that the acyl complex is quite stable unless both alcohol and amine are present, suggesting that the amine activates the alcohol by deprotonating it 138and thus converting it to a more reactive alkoxide ion. These observations are consistent with the mechanism presented in Fig. 8.