The reactions of 4-nitrophenyl and 2,4- dinitrophenyl 3,5-dinitrothiolbenzoates (1 and 2 respectively) and of phenyl, 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates (3, 4 and 5, respectively) with a series of 3- and 4-substituted pyridines are subject to a kinetic study in 44 wt% ethanol-water, at 25 ºC and an ionic strength of 0.2 M. Under excess amine all the reactions show pseudo-first-rate constants and are first order in the pyridine.

The Brønsted-type plots (log k vs pK_a) for the reactions of 1 and 2 are non linear downwards, with the curvature centre at 11.1 and 10.4, respectively. The values of the slopes (β) at low pK_a (ca. 1) and high pK_a (0.1-0.2) are in agreement with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T^±), and a change in the rate-determining step, from T^± breakdown (low pK_a) to T^± formation (high pK_a).

The pyridinolysis of other 4-nitrophenyl and 2,4-dinitrophenyl 4-Y-substituted thiolbenzoates (Y= H, Cl, NO₂) also show curved Brønsted plots, with increasing pK_aº values as Y becomes more electron withdrawing.^1,2

For the reactions of the carbonates 3, 4, and 5 the Brønsted type plots are linear with slopes ca 0.6, consistent with a concerted mechanism.

4-Methylphenyl and 4-chlorophenyl 4-nitrophenyl carbonates react with a series of secondary alicyclic amines (SAA) in water solution by a stepwise mechanism with biphasic Brønsted plots with pK_aº at 10.5 and 10.6 respectively.³ It is known that both the change to a better nucleofugue in the substrate and a change of SAA to pyridines shift the pK_aº to smaller values.

The absence of break in the Brønsted-type plots at the pK_a range studied (2.4 -11.5) allow to us confirm a concerted mechanism.

We thank FONDECYT of Chile (projects 1020254 and 1060593) for financial support. R.C.A. thanks DIPUC of Pontificia Universidad Católica de Chile for a Doctoral scholarship.

References:

[1] E. A. Castro, M. Vivanco, R. Aguayo, J. G. Santos, J. Org. Chem. 2004, 69, 5399.

[2] E. A. Castro, R. Aguayo, J. Bessolo, J. G. Santos, J. Org. Chem. 2005, 70, 3530.

[3] E. A. Castro, M. Andujar, A. Toro, J. G. Santos, J. Org. Chem. 2003, 68, 3608.

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