Kinetic Study on Quinuclidinolysis of O-Phenyl O-Y-substituted-Phenyl Thionocarbonates: Effects of Changing Nonleaving Group from Thionobenzoyl to Phenyloxythionocarbonyl on Reactivity and Transition-State Structure

Second-order rate constants (k(quin)) for reactions of O-3,4-dinitrophenyl O-phenyl thionocarbonate (2a) with a series of quinuclidine derivatives and for those of O-phenyl O-Y-substituted-phenyl thionocarbonates (2a-2h) with quinuclidine in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C are reported. Comparison of the kquin values in this study with those reported previously for the corresponding reactions of O-3,4-dinitrophenyl thionobenzoate (1a) and O-Y-substituted-phenyl thionobenzoates (1a-1j) has revealed that 2a is more reactive than 1a toward all the quinuclidine derivatives studied, while the thionocarbonate esters possessing a weak electron-withdrawing group in the leaving group (e.g., 2g and 2h) are less reactive than the corresponding thionobenzoate esters (e.g., 1g and 1h) toward quinuclidine. The Bronsted-type plots for the reactions of 2a with quinuclidine derivatives and for those of 2a-2h with quinuclidine are linear with beta(nuc) = 0.67 and beta(1g) = -0.85, respectively, indicating that the reactions proceed through a concerted mechanism with a loose transition state (TS). This is in contrast to the report that the corresponding reactions of 1a-1j proceed through a forced concerted mechanism with a tight TS on the basis of the linear Bronsted-type plots with beta(nuc) = 0.89 and beta(1g) = -0.37. Factors that control the reactivity of these esters toward quinuclidine are discussed in detail.