Kinetics and Mechanism of the Pyridinolysis of Phenyl Chloroformates in Acetonitrile
作者:Han Joong Koh、Kwang Lae Han、Hai Whang Lee、Ikchoon Lee
DOI:10.1021/jo9814905
日期:1998.12.1
Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 degrees C. Both the Hammett and Bronsted plots are linear with enhanced substituent constants, sigma(p)(-), and basicities, pK(a)(-), for strong para pi-acceptor X-substituents, p-CN and p-CH3CO. This indicates that the electron-rich formate (O-C-O) moiety overlaps with the pyridine ring pi-system enabling, through conjugation with the para pi-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 beta(X) (congruent to 0.3) and by the lower Delta H double dagger (6.7 kcal mol(-1)) and Delta S double dagger (-44 eu) values for a stronger donor Y (Y = p-CH3O) coupled with a stronger para pi-acceptor (X = p-CN) in pyridine.