Mechanistic Studies of the Palladium-Catalyzed Reaction of Methanol with Bromobenzene and CO To Produce Methyl Benzoate. 1. Stoichiometric Study

Article abstract of DOI:10.1021/ja00217a020

The mechanism of the palladium-catalyzed carbomethoxylation of bromobenzene to methyl benzoate in the presence of methanol and triethylamine has been investigated with Cylindrical Internal Reflectance Fourier Transform Infrared Spectroscopy (CIR-FTIR).The synthesis of the proposed catalytic intermediate Pd(Br)(C6H5)(P(C6H5)3)2 (1) was carried out, and the reaction of this species with CO to form Pd(Br)(COC6H5)(P(C6H5)3)2 (3) was confirmed.Both complexes were characterized by IR and ³¹P and ¹H NMR spectroscopy.The stability of 3 in the absence of CO suggests that the carbonylation of 1 is irreversible; its stability in the presence of triethylamine establishes that reductive elimination of benzoyl bromide is not facile.In the absence of CO, the conversion of 3 to methyl benzoate was found to be rapid when both methanol and triethylamine were present, but very slow in the presence of methanol alone.A series of experiments in which 3 was allowed to react with methanol and a variety of amines in the absence of CO demonstrated a direct relationship between the rate of reaction and the amine basicity.These observations point to a mechanism in which methanol is initially deprotonated by the amine, and the methoxide ion thus produced attacks the benzoyl complex directly yielding methyl benzoate.When reaction of the benzoyl complex with methanol and triethylamine was carried out with bromobenzene also present, regeneration of 1 was observed via CIR-FTIR and confirmed by IR and proton NMR spectroscopy.The carbomethoxy species, Pd(Cl)(CO2CH3)(P(C6H5)3)2, was shown to be stable in the presence of bromobenzene, triethylamine, and methanol at 90 deg C, suggesting that this complex is not an active catalytic intermediate.These experiments substantiate the mechanism in Figure 8.