O-acylation mechanism of p-substituted phenols with various alkanoyl chlorides under phase transfer catalysis conditions

The phenolic ester synthesis between equimolecular amounts of phenols and various alkanoyl chlorides in the presence of aqueous sodium hydroxide solution in a two-phase system (20 mL of each 10% NaOH and dicloromethane, respectively) was reinvestigated under PTC conditions (10 mmol of tetrabutyl ammonium chloride, BNC). The esterification was complete within several minutes and the obtained ester was in high purity. For example, phenyl acetate was obtained quantitatively at 0degreesC in 5 minutes using a stoichiometric amount of BNC. This acylation procedure is technically simple and does not require anhydrous conditions. A bimolecular electrophilic attack of alkanoyl chloride against the ion-pair in the organic phase, which was formed between the quaternary ammonium cation and the phenoxide anion, must be a rate-determining step, because a plot of the logarithm of relative rate constants of various alkanoyl chlorides vs Taft's polar substituents constants (taking into account the steric constants, Es) gave a good linear relationship. We have found a simple, rapid, convenient, easily separating and efficient PTC technique to synthesize phenolic esters.