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

Second-order rate constants (k(OH-)) have been measured spectrophotometrically for reactions of Y-substituted phenyl phenyl thionocarbonates (4a-i) with OH- in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees 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=O 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 Bronsted-type plot for reactions of 4a-i is linear with beta(lg) = -0.33, a typical beta(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 sigma degrees constants results in much better linearity than that correlated with sigma(-) 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.