Effect of the Structure of the Acyl Group on the Kinetics of Exchange between O-Nucleophiles

We have studied the rate of 15 reactions of acyl transfer from O-acyl salts of 4-dimethylaminostyryl-4-pyridine N-oxide to 4-morpholinopyridine and 4-dimethylaminopyridine N-oxides in acetonitrile solutions. Analysis of the results based on the Shaik – Pross approach and the Marcus equation shows that if the structure of the acyl group is varied, then the reactivity is determined by such parameters as the resonance interaction in the transition state (B) or the internal barrier (G ₀) of the reaction.     

Rate and Equilibrium Constants of Dimethylcarbamoyl Transfer between Pyridine N-Oxides

Dimethylcarbamoyl transfer from N-acyloxypyridinium salts to pyridine N-oxides in acetonitrile occurs in one stage by the forced concerted S_N2 mechanism. The rate and equilibrium of the reaction are fairly described by the Brönsted equation. The Marcus equation provides a much higher quality of reactivity predictions.     

Rate and Equilibrium of Acetyl Group Exchange between Pyridines and Pyridine N-Oxides

22 Acetyl group exchange reactions between N-acetoxypyridinium salts and 4-dimethylaminopyridine, 4-morpholinopyridine, and N-methylimidazole in acetonitrile at 298 K have been studied. The rate constants varied from 10⁵ to 10^–4 L/mol·s, and the equilibrium constants ranged from 10⁹ to 10^–9. The rates and equilibrium constants of these reactions did not comply with the Brönsted equation. The kinetics of the acetyl exchange reactions are well described by a correlation equation containing squared terms.     

Rate and equilibrium constants of transfer of the dimethylcarbamoyl group between substituted pyridines and their N-oxides

Transfer of the dimethylcarbamoyl group from N-acyloxypyridinium salts to pyridines and from N-acylpyridinium salts to pyridine N-oxides was studied in acetonitrile. Equations relating the reaction rates and equilibria in the N-O and O-N acyl transfer series to the basicity of the nucleophile and leaving group were obtained. The reactions all are one-stage and occur by the forced concerted SN2 mechanism. The intersecting state model was used to obtain a modified Marcus equation that accounts for the asymmetry of the transition state with respect to reagents and products and allows uncontradictory analysis of the reaction mechanism.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1720–1727.Original Russian Text Copyright © 2004 by Rybachenko, Shreder, Gokhfeld, Chotii, Kovalenko, Lenska.This revised version was published online in April 2005 with a corrected cover date.     

Equilibrium of acyl transfer between pyridine N-oxides and their acylonium salts

Transfer of acyl groups from N-acyloxypyridinium salts to pyridine N-oxides in acetonitrile was studied. The equilibrium constants of acyl exchange were determined. These quantities vary in the range covering eight orders of magnitude, depending on the structure of the reagents, and are independent of the structure of the acyl group.     

Reaction of Acetyl and Benzoyl Chlorides with Pyridines and Pyridine N-Oxides

The rate of formation of N- and O-acetyl and benzoyl salts of pyridines and pyridine N-oxides in acetonitrile and methylene chloride and equilibria therein were studied. The process occurs in one step following the SN2 mechanism with a small degree of bond rupture in the transition state.     

Rate and Equilibrium Constants of Benzoyl Group Transfer between Pyridine N-Oxides

Kinetic characteristics of 19 transfer reactions of benzoyl group from N-benzoyloxypyridinium salts to pyridine N-oxides and 4-dimethylaminopyridine were studied in acetonitrile by the stopped-flow method. The rate of an identical reaction for 4-methoxypyridine was measured by dynamic NMR spectroscopy. For 5 other identical reactions the rates were estimated from Bronsted correlations. Equilibrium constants were estimated with the use of UV spectrophotometry (6), IR spectroscopy (2), from kinetic data (K _ij = k _ij /k _ji ) (2), and in one case as logK _i−j = logK _i−x − logK _j−x . The second order rate constants (k _ij ) varied in the range 10²–10⁵ l mol⁻¹ s⁻¹, the equilibrium constants (K _ij ) in the range 10²–10⁻²; the activation parameters (ΔH ^≠) were within 15–50 kJ mol⁻¹, (−ΔS ^≠) −20–110 J mol⁻¹ K⁻¹. The reactions under study occur in a single stage following the concerted S_N2 mechanism through an early associative transition state. The benzoyl groups exchange rate and equilibrium are well described by simplified Marcus equation (omitting the quadratic term).__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 5, 2005, pp. 788–792.Original Russian Text Copyright © 2005 by Rybachenko, Schroeder, Chotii, Lenska, Red’ko, Kovalenko.