Effect of Changing Electrophilic Center from CO to CS on Rates and Mechanism:  Pyridinolyses of O-2,4-Dinitrophenyl Thionobenzoate and Its Oxygen Analogue

Abstract

Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Brønsted-type plots obtained are nonlinear with beta(1) = 0.26, beta(2) = 1.07, and pK(a) degrees = 7.5 for the reactions of 1 and beta(1) = 0.40, beta(2) = 0.90, and pK(a) degrees = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T(+/-) with a change in the rate-determining step at pK(a) degrees = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK(a) = 11.30). The k(1) value is larger for the reactions of 1 than for those of 2 in the low pK(a) region, but the difference in the k(1) value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k(2)/k(-1) ratio than 2 in the low pK(a) region but the difference in the k(2)/k(-1) ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pK(a) above ca. 7.2 but less reactive in the pK(a) below ca. 7.2. The k(1) value is slightly larger, but the k(2)/k(-1) ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pK(a) region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines.

Pyridinolysis of 2,4-Dinitrophenyl Phenyl Thionocarbonate: Effect of Changing Electrophilic Center from C=O to C=S on Reactivity and Mechanism

“…1-10 Aminolysis of carboxylic esters has generally been proposed to proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate T ± on the basis of curved Brønsted-type plots reported for reactions of esters possessing a good leaving group (e.g., 2,4-dinitrophenoxide). [1][2][3][4][5][6][7][8][9][10] The rate-determining step (RDS) has been suggested to be dependent on the basicity of the incoming amine and the leaving group, i.e., it changes at pK a o , defined as the pK a at the center of the Brønsted curvature, 9,10 from breakdown of T ± to its formation as the incoming amine becomes more basic than the leaving group by 4 to 5 pK a units. [1][2][3][4][5][6][7][8][9][10] Aminolysis of thiono esters has been reported to proceed through one or two intermediates (i.e., T ± and its deprotonated form T -) depending on the reaction conditions.…”

“…[1][2][3][4][5][6][7][8][9][10] The rate-determining step (RDS) has been suggested to be dependent on the basicity of the incoming amine and the leaving group, i.e., it changes at pK a o , defined as the pK a at the center of the Brønsted curvature, 9,10 from breakdown of T ± to its formation as the incoming amine becomes more basic than the leaving group by 4 to 5 pK a units. [1][2][3][4][5][6][7][8][9][10] Aminolysis of thiono esters has been reported to proceed through one or two intermediates (i.e., T ± and its deprotonated form T -) depending on the reaction conditions. 5 Castro et al have reported that reactions of 4-nitrophenyl phenyl thionocarbonate with strongly basic amines proceed through T ± while those with weakly basic amines proceed through T ± and T -.…”

“…Pyridinolyses of 2,4-dinitrophenyl benzoate (1) and thionobenzoate (2) have been reported to proceed through a stepwise mechanism with a change in RDS on the basis of curved Brønsted-type plots. 7 Interestingly, the pK a o has been reported to be strongly dependent on the nature of the electrophilic center, i.e., 9.5 and 7.5 for the reactions of 1 and 2, respectively. 7 We have recently reported that pyridinolysis of 2,4-dinitrophenyl phenyl carbonate (3) proceeds through a stepwise mechanism with a change in RDS at pK a o = 8.5.…”

“…7 Interestingly, the pK a o has been reported to be strongly dependent on the nature of the electrophilic center, i.e., 9.5 and 7.5 for the reactions of 1 and 2, respectively. 7 We have recently reported that pyridinolysis of 2,4-dinitrophenyl phenyl carbonate (3) proceeds through a stepwise mechanism with a change in RDS at pK a o = 8.5. 8 Our study has been extended to pyridinolysis of 2,4-dinitrophenyl phenyl thionocarbonate (4) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and mechanism including pK a o .…”

“…Such curved Brønsted-type plots are typical for reactions reported to proceed through a stepwise mechanism with a change in RDS. [1][2][3][4][5][6][7][8][9][10] In fact, the nonlinear Brønsted-type plot for the reaction of 3 has been reported as evidence for a change in RDS. 8 Thus, one can propose that the reactions of 4 proceed also through a stepwise mechanism with a change in RDS as shown in Scheme 1 on the basis of the curved Brønsted-type plot.…”