Synthesis of furan derivatives. LXXXIII. Kinetics of acyl azide formation of carboxylic acids with diphenyl phosphorazidate.

Abstract

5-Substituted 2-furoyl azides (7a, 7b and 7c), 5-substituted 2-thenoyl azides (7d, 7e and 7f), 2-, 3- and 4-pyridinemonocarbonyl azides (7j, 7k and 7l) and p-substituted benzoyl azides (7g, 7h and 7i) were prepared from the corresponding carboxylic acids with DPPA (4) in one step and in good yields, respectively. It was found through the kinetic studies that an electron-withdrowing group substituted at 5-position (furan and thiophene ring) or p-position (benzene ring) decreases the rate of the reaction, and reversely, an electron-donating substituted at 5- or p-position increases it. In addition, it looks to be reasonable that the most probable pathway is to form the above acyl azides via the activated six-membered cyclic complex (9) as an intermediacy and then to proceed in the concerted reaction, based on the activation energies (ΔE*) and entropies (ΔS*) obtained.