Factors affecting rates of thermal decomposition of 5-azidopyrazoles: a comparison with other aromatic azides

Abstract

2-Methoxyazidobenzene has been used as a model compound for α-azido five-membered heterocycles, and undergoes thermal loss of nitrogen 16 times as fast as azidobenzene and 3.6 times as fast as its para isomer. These rates identify an important electrostatic stabilization within a charge-separated transition state. It is argued that this stabilization would be very much larger in thermolyses of α-azido five-membered heterocycles and removes the need to postulate that their high rates are due to appreciable ring-opening at the transition state. Electronic effects of ring substituents in 5-azidopyrazoles are consistent with this electrostatic model. The electron distribution required is not compatible with the operation of a neighbouring group effect, and none is found for 5-azido-4-nitro-pyrazole.