Pyridinolysis of O,O-Diethyl S-Aryl Phosphorothioates Bull. Korean Chem. Soc. 2014, Vol. 35, No. 5 1329 http://dx.doi.org/10.5012/bkcs.2014.35.5.1329 Kinetics and Mechanism of Pyridinolysis of O,O-Diethyl S-Aryl Phosphorothioates Hasi Rani Barai and Hai Whang Lee * Department of Chemistry, Inha University, Incheon 402-751, Korea. * E-mail: hwlee@inha.ac.kr Received December 16, 2013, Accepted January 6, 2014 The kinetic studies on the reactions of O,O-diethyl Z-S-aryl phosphorothioates with X-pyridines have been carried out in dimethyl sulfoxide. The free energy correlations with X in the nucleophiles are biphasic concave upwards with a break point at X = H, while those for substituent Z variations in the leaving groups are linear. The negative sign of ρXZ implies that the reaction proceeds through a concerted mechanism for both the strongly and weakly basic pyridines. The biphasic concave upward free energy relationships with X are rationalized by a change in the nucleophilic attacking direction from frontside with the strongly basic pyridines to backside with the weakly basic pyridines. Key Words : Biphasic concave upward free energy correlation, Phosphoryl transfer reaction, Pyridinolysis, O,O-Diethyl S-aryl phosphorothioates, Cross-interaction constant Introduction The authors reported that the substituent effects of the nucleophiles (X) and/or substrates (Y) and/or leaving groups (Z) upon the kinetics and mechanism of the pyridinolyses in the phosphoryl and thiophosphoryl transfer are exceptional- ly significant. In the present work, the nucleophilic sub- stitution reactions of O,O-diethyl Z-S-aryl phosphorothioates (2) with X-pyridines are investigated kinetically in dimethyl sulfoxide (DMSO) at 85.0 ± 0.1 ºC (Scheme 1) to obtain further systematic information on the phosphoryl transfer reactions and substituent effects of the nucleophiles and leaving groups on the reaction mechanism, as well as to compare the relevant pyridinolyses of O,O-dimethyl [1: (CH 3 O) 2 P(=O)SC 6 H 4 Z] 1a and O,O-diphenyl [3: (C 6 H 5 O) 2 - P(=O)SC 6 H 4 Z] 1b Z-S-aryl phosphorothioates in DMSO and MeCN, respectively. Results and Discussion Tables 1-3 list the second-order rate constants (k 2 /M –1 s –1 ), Hammett (ρ X ) and Brönsted (β X ) coefficients with X, and Hammett coefficients (ρ Z ) with Z, respectively. The sub- stituent X and Z effects on the rates are in accord with those for a typical nucleophilic substitution reaction with partial positive charge development at the nucleophilic N atom and with partial negative charge development at the leaving group S atom in the transition state (TS). The Brönsted (Fig. Table 1. Second-Order Rate Constants (k2 × 10 4 /M –1 s –1 ) of the Reactions of 2 with X-Pyridines in DMSO at 85.0 ºC X \ Z 4-MeO 4-Me H 3-MeO 4-Cl 4-MeO 0.936 ± 0.001 1.47 ± 0.01 2.05 ± 0.01 2.77 ± 0.02 4.26 ± 0.01 4-Me 0.695 ± 0.001 1.06 ± 0.01 1.38 ± 0.02 1.91 ± 0.01 2.93 ± 0.03 H 0.387 ± 0.002 0.571 ± 0.001 0.774 ± 0.001 1.01 ± 0.01 1.51 ± 0.01 3-MeO 0.329 ± 0.001 0.462 ± 0.002 0.641 ± 0.001 0.798 ± 0.001 1.17 ± 0.01 3-Cl 0.233 ± 0.001 0.311 ± 0.001 0.404 ± 0.002 0.511 ± 0.001 0.723 ± 0.001 Table 2. Hammett (ρX) a and Brönsted (βX) b Coefficients with X of the Reactions of 2 with X-Pyridines in DMSO at 85.0 ºC X \ Z 4-MeO 4-Me H 3-MeO 4-Cl 4-MeO, 4-Me, H –ρX 1.43 ± 0.01 1.53 ± 0.01 1.55 ± 0.01 1.62 ± 0.01 1.67 ± 0.01 βX 0.31 ± 0.01 0.33 ± 0.01 0.34 ± 0.02 0.35 ± 0.01 0.36 ± 0.01 H, 3-MeO, 3-Cl –ρX 0.60 ± 0.01 0.71 ± 0.01 0.77 ± 0.01 0.80 ± 0.01 0.86 ± 0.01 βX 0.09 ± 0.02 0.11 ± 0.03 0.12 ± 0.02 0.13 ± 0.03 0.14 ± 0.04 a Correlation coefficients, r, are better than 0.999. b r ≥ 0.987. Scheme 1. Pyridinolysis of O,O-diethyl Z-S-aryl phosphorothio- ates (2) in DMSO at 85.0 ºC.