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Kinetics and Mechanism of the Pyridinolysis of Thiolbenzoates and Carbonates

José G. Santos 1Enrique A. Castro Raúl C. Aguayo Magdalena Acuña 

1. Pontificia Universidad Catolica de Chile (PUC), Vicuna Mackenna 4860, Santiago 6094411, Chile

Abstract

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 pKa) 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 pKa (ca. 1) and high pKa (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 pKa) to T± formation (high pKa).

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 pKaº 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 pKaº 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 pKaº to smaller values.

The absence of break in the Brønsted-type plots at the pKa 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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Related papers

Presentation: poster at 18th Conference on Physical Organic Chemistry, Posters, by José G. Santos
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-25 20:43
Revised:   2009-06-07 00:44