Kinetics and Mechanism of the Pyridinolysis of Phenyl Chloroformates in Acetonitrile

Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 °C. Both the Hammett and Brönsted plots are linear with enhanced substituent constants, σp-, and basicities, pKa-, for strong para π-acceptor X-substituents, p-CN and p-CH3CO. This indicates that the electron-rich formate (O−C−O) moiety overlaps with the pyridine ring π-system enabling, through conjugation with the para π-acceptors, the rate-limiting formation of a tetrahedral intermediate. The difference in the aminolysis mechanism between methyl, II, and phenyl chloroformate, III, is attributed to the much stronger electron-donating polarizability effect of C6H5 than of CH3. The proposed mechanism is supported by a relatively small βX (≅0.3) and by the lower ΔH⧧ (6.7 kcal mol-1) and ΔS⧧ (−44 eu) values for a stronger donor Y (Y = p-CH3O) coupled with a stronger para π-acceptor (X = p-CN) in pyridine.