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.