Alkaline Hydrolysis of Y-Substituted Phenyl Phenyl Thionocarbonates: Effect of Changing Electrophilic Center from C=O to C=S on Reactivity and Mechanism

摘要

Second-order rate constants (k_(OH~-)) have been measured spectrophotometrically for reactions of Y-substituted phenyl phenyl thionocarbonates (4a-i) with Off in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. The k_(OH~-) values for the reactions of 4a-i have been compared with those reported previously for the corresponding reactions of Y-substituted phenyl phenyl carbonates (3a-i) to investigate the effect of changing the electrophilic center from C=0 to C=S on reactivity and mechanism. Thionocarbonates 4a-i are less reactive than the corresponding carbonates 3a-i although 4a-i are expected to be more reactive than 3a-i. The Bransted-type plot for reactions of 4a-i is linear with β_(lg) = - 0.33, a typical β_(lg) value for reactions reported to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step (RDS). Furthermore, the Hammett plot correlated with σ~° constants results in much better linearity than that correlated with σ~- constants, indicating that expulsion of the leaving group is not advanced in the RDS. Thus, alkaline hydrolysis of 4a-i has been concluded to proceed through a stepwise mechanism with formation of an intermediate being RDS, which is in contrast to the forced concerted mechanism reported for the corresponding reactions of 3a-i. Enhanced stability of the intermediate upon modification of the electrophilic center from C=O to C=S has been concluded to be responsible for the contrasting mechanisms.