Pyridinolyses of O-Propyl and O-Isopropyl Phenyl Phosphonochloridothioates in Acetonitrile

Abstract

Continuing the kinetic studies on the pyridinolyses of the phosphonochloridothioates, the nucleophilic substitution reactions of O-propyl (4) and O-isopropyl (5) phenyl phosphonochloridothioates with X-pyridines have been carried out kinetically in acetonitrile (MeCN) at 35.0 ± 0.1 ºC (Scheme 1). The kinetic results of the present work are compared with those of the pydidinolyses of1d phosphonochloridothioates, based on the reactivities, selectivity parameters, free energy correlations and reaction mechanisms. Results and DiscussionThe second-order rate constants [k 2 (M -1 s Table 1, together with the selectivity parameters, ρ X and β X . The pyridinolysis rates of 4 are 1.4-1.7 times faster than those of 5. The substituent effects of the nucleophiles on the rates are in consistent with those for a typical nucleophilic substitution reaction with a positive charge development at the nucleophilic N atom in the transition state (TS). The free energy correlations for both 4 and 5, however, exhibit biphasic concave upwards with a break point at X = H (Fig. 1). Similarities of the substituent effects of X on the pyridinolysis rates between 4 and 5 strongly suggest that the pyridinolyses of both 4 and 5 proceed through the same mechanism. The magnitudes of β X [= 2.09 (4) and 2.01 (5) ) and Selectivity Parameters (ρX and βX) of the Reactions of O-Propyl (4) and O-Isopropyl (5) Phenyl Phosphonochloridothioates with X-Pyridines in MeCN at 35.0