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

Abstract

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 Bro / nsted 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 ij = logK ix logK jx . The second order rate constants (k ij ) varied in the range 10 2 10 5 l mol -1 s -1 , the equilibrium constants (K ij ) in the range 10 2 10 2 ; the activation parameters (DH ¹ ) were within 1550 kJ mol -1 , (DS ¹ ) 20110 J mol -1 K -1 . The reactions under study occur in a single stage following the concerted S N 2 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).