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 % H 2 O /20 mol % DMSO at 25.0 ± 0.1°C are reported. Comparison of the k quin 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 Brønsted‐type plots for the reactions of 2a with quinuclidine derivatives and for those of 2a–2h with quinuclidine are linear with β nuc = 0.67 and β lg = −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 Brønsted‐type plots with β nuc = 0.89 and β lg = −0.37. Factors that control the reactivity of these esters toward quinuclidine are discussed in detail.