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 a-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 x-azido five-membered heterocycles and removes the need t o postulate that their high rates are due t o 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.The high rates of thermal decomposition of a-azido fivemembered heterocycles are well known phenomena. Whereas simple azidobenzenes require temperatures of about 160 "C to achieve a half-life of 1 h,'T2 many a-azidoheterocycles can decompose to this extent at 60 "C, and some even at ambient temperatures. Qualitative reports of these facile decompositions abound in the literature for the a-azides of furans, thiophenes, oxazoles, pyrazoles, triazoles, benzofurans, benzothiophenes and benzopyrroles; recent reviews have summarized these data.3.4 A limited number of rate constants have been reported for 2-azidofurans, 2-a~idothiophenes,~*~ 5-azido-172,3-triazoles,8 2-a~idobenzothiophene,~" 2-azidobenzofuran 9b and 2azido-1 -m e t h y l i n d ~l e . ~~ 0-Azidothiophenes also lose nitrogen more rapidly than the azidobenzenes though the relative rate factors are much smaller than for ~a z i d o t h i o p h e n e s . ~~~* ~~In addition to the remarkably high rates of thermal decomposition, the thermolysis products of the a-azido fivemembered heterocycles are unusual. Most of them are the result of ring-opening to form nitrilic heterodienes which in some cases reclose to yield new heterocycles. A published reaction is shown in Scheme 1 .I 1 Scheme IIn view of the very high rates of nitrogen loss from the a-azidoheterocycles, recent reviewers 3.4 of the field have suggested that the ring-opening and nitrogen loss may be a concerted process in which the high-energy nitrene species 2 (Scheme 2) is never formed. In particular, Spinelli and Zanirato have reported that 3-azidothiophene undergoes (X = 0, S, Se, NR) Scheme 2 * t On sabbatical leave from the University of Newcastle, Australia.