Modification of Both the Electrophilic Center and Substituents on the Nonleaving Group in Pyridinolysis of O-4-Nitrophenyl Benzoate and Thionobenzoates

Abstract

A kinetic study is reported for reactions of 4-nitrophenyl benzoate (1c) and O-4-nitrophenyl X-substituted thionobenzoates (2a-e) with a series of pyridines in 80 mol % H2O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 +/- 0.1 degrees C. O-4-Nitrophenyl thionobenzoate (2c) is more reactive than its oxygen analogue 1c toward all the pyridines studied. The Brønsted-type plot is linear with beta(nuc)=1.06 for reactions of 1c but curved for the corresponding reactions of 2c with beta(nu)c decreasing from 1.38 to 0.38 as the pyridine basicity increases, indicating that the reaction mechanism is also influenced on changing the electrophilic center from C=O to C=S. The curvature center of the curved Brønsted-type plots (defined as pK(a)(o)) occurs at pKa = 9.3 regardless of the electronic nature of the substituent X in the nonleaving group. The Hammett plot for reactions of 2a-e with 4-aminopyridine is nonlinear, i.e., the substrates having an electron-donating substituent exhibit negative deviations from the Hammett plot. However, the Yukawa-Tsuno plot for the same reactions exhibits good linear correlation, indicating that the negative deviations shown by these substrates arise from stabilization of the ground state through resonance interaction between the electron-donating substituent X and the C=S bond.

Kinetic and Theoretical Studies on Pyridinolysis of 2,4-Dinitrophenyl X-Substituted Benzoates: Effect of Substituent X on Reactivity and Mechanism

“…However, the k 2 /k -1 ratios for the reactions of substrates 1a-c are almost identical for a given pyridine. This is consistent with our previous report that the electronic nature of the substituent X in the nonleaving group does not influence the k2/k-1 ratio for pyridinolysis of O-4-dinitrophenyl X-substituted thionobenzoates 3 and aminolysis of 2,4-dinitrophenyl X-substituted benzenesulfonates. 7a The effect of pyridine basicity on k1 is illustrated in Figure 3.…”

“…In contrast, we have shown that pKa o and k2/k-1 ratio are not influenced by the electronic nature of substituent X in the nonleaving group for pyridinolysis of O-4-nitrophenyl X-substituted thionobenzoates 3 and aminolysis of 2,4-dinitrophenyl X-substituted benzenesulfonates in 80 mol % H2O/20 mol % DMSO. 7a It has been found that the Hammett plots for these reactions are not linear, e.g., the ρ value decreases as the substituent changes from EDGs to EWGs.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Curved Brøn-sted-type plots reported for aminolyses of esters possessing a weakly basic leaving-group (e.g., 2,4-dinitrophenoxide) have been taken to be diagnostic of a change in rate-determining step (RDS). [1][2][3][4][5][6][7] It has been proposed that RDS changes at pKa o , defined as the pKa at the center of Brønsted curvature.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Curved Brøn-sted-type plots reported for aminolyses of esters possessing a weakly basic leaving-group (e.g., 2,4-dinitrophenoxide) have been taken to be diagnostic of a change in rate-determining step (RDS). [1][2][3][4][5][6][7] It has been proposed that RDS changes at pKa o , defined as the pKa at the center of Brønsted curvature. 4,5 It is now firmly understood that RDS is dependent on the basicity of the incoming amine and the leaving group, i.e., it changes from breakdown of a zwitterionic tetrahedral intermediate T ± to its formation as the amine becomes more basic than the leaving group by 4 to 5 pKa units.…”

Reactions of Carboxylic, Phosphoric and Sulfonic Acids and their Derivatives

“…The curvature centre (pK a 0 ) of the curved Brønsted-type plots occurred at pK a = 9.3 regardless of the electronic nature of the substituent X in the non-leaving group. 48 Boron trifluoride and indium(III) triflate were found to catalyse efficiently the isomerization of thionolactones (26) to thiolactones (27) in good yields. When applied to an optically active γ -thionolactone [(R)-28], the isomerization reaction proceeded with complete inversion of configuration to [(S)-29] by using BF 3 .OEt 2 (Scheme 9).…”