1754 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 6 Hasi Rani Barai and Hai Whang Lee http://dx.doi.org/10.5012/bkcs.2014.35.6.1754 Dual Substituent Effects on Pyridinolysis of Bis(aryl) Chlorothiophosphates in Acetonitrile Hasi Rani Barai and Hai Whang Lee * Department of Chemistry, Inha University, Incheon 402-751, Korea. * E-mail: hwlee@inha.ac.kr Received February 6, 2014, Accepted February 21, 2014 The nucleophilic substitution reactions of bis(Y-aryl) chlorothiophosphates (1) with X-pyridines are investigated kinetically in acetonitrile at 35.0 o C. The free energy relationships with both X and Y are biphasic concave upwards with a break point at X = 3-Ph and Y = H, respectively. The sign of cross-interaction constants (CICs; ρ XY ) is positive with all X and Y. Proposed mechanism is a stepwise process with a rate-limiting leaving group departure from the intermediate with all X and Y. The kinetic results of 1 are compared with those of Y- aryl phenyl chlorothiophosphates (2). In the case of Y = electron-withdrawing groups, the cross-interaction between Y and Y, due to additional substituent Y, is significant enough to change the sign of ρ XY from negative with 2 to positive with 1, indicative of the change of mechanism from a rate-limiting bond formation to bond breaking. Key Words : Dual substituent effects, Cross-interaction constant, Pyridinolysis, Bis(Y-aryl) chlorothiophos- phates, Thiophosphoryl transfer reaction Introduction The kinetic studies on the reactions of Y-aryl phenyl chlorothiophosphates [2; (YC 6 H 4 O)(C 6 H 5 O)P(=S)Cl] with X-pyridines in acetonitrile (MeCN) were reported earlier by this lab. 1 Herein, (i) the free energy relationships with X were biphasic concave upwards while those with Y were biphasic concave downwards; (ii) proposed mechanism was a stepwise process with a rate-limiting step change from bond breaking with the weaker electrophiles to bond formation with the stronger eletrophiles based on the sign of cross-interaction constants (CICs; ρ XY ); 2 and (iii) nonlinear free energy correlations of biphasic concave upward plots with X were rationalized by a change in the attacking direction of the nucleophile from a backside with the weakly basic pyridines to a frontside attack with the strongly basic pyridines. In the present work, the nucleophilic substitution reactions of bis(Y-aryl) chlorothiophosphates [1; (YC 6 H 4 O) 2 - P(=S)Cl] with substituted pyridines are investigated kineti- cally in MeCN at 35.0 ± 0.1 o C (Scheme 1). The purpose of this work is to study the dual substituent effects on the reac- tivity and mechanism by adding the very same substituent Y in the other phenyl ring based on the selectivity parameters and CICs. The difference between 1 and 2 is nothing but one substituent Y in the other phenyl ring, i.e., substrate 1 has one more same substituent Y compared to substrate 2. 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 Y ) with Y, respectively. The ρ Y values are calculated from the plots of log k 2 against σ Y although all the studied substrates contain two Y-substituted phenyl rings with same substituent Y. Figures 1 and 2 show the Hammett and Brönsted plots with X, respectively, and Figure 3 shows the Hammett plots with Y. The substituent effects on the reaction rates with X and Y are compatible with a typical nucleophilic substitution reaction. The stronger nucleophile leads to the faster rate and a more electron- withdrawing substituent Y in the substrate leads to the faster rate. However, all the free energy relationships with X and Y are biphasic concave upwards with a break point at X = 3-Ph and Y = H, respectively. In the case of 2, the free energy relationships with X are the same as in 1, but those with Y are biphasic concave downwards with a break point at Y = H. 1 For convenience, henceforth, the substituents X in the nucleophiles and Y in the substrates are divided into two blocks, respectively, as follows: (i) u-block with X = (4- MeO, 4-Me, H, 3-Ph); (ii) d-block with X = (3-Ph, 3-Ac, 4- Ac); (iii) l-block with Y = (4-MeO, 4-Me, H); and (iv) r- block with Y = (H, 3-MeO, 4-Cl). 3 Thus, there are four blocks with X and Y: u,l-, d,l-, u,r- and d,r-block. The magnitudes of the ρ X and β X values with u-block are 4-6 times larger than those with d-block. The magnitudes of the ρ X values with 1 are somewhat smaller than those with 2: ρ X = –6.86 to –6.39 (u-block) and –1.54 to –1.22 (d-block) with 1 while ρ X = –7.31 to –6.62 (u-block) and –2.72 to –1.24 (d- Scheme 1. Pyridinolysis of 1 [bis(Y-aryl) chlorothiophosphates] in MeCN at 35.0 o C.