Rates of thermolysis of azidobenzenes in solution: Large stabilizations of transition states by charge transfer from electron-donor substituents

Introduction of + R type para substituents into azidobenzenes causes very large increases in rate of thermolysis, up to 225-fold, The rates of nitrobenzene solutions at 120 degrees C follow a Hammett-type linear free energy relationship log k = -5.44 - 2,33 sigma(1) - 1.48R(+) which indicates conjugative stabilization of a nitrene-like transition state, ortho-substituents of the + R type causes still larger rate enhancements, up to 456-fold for 2-amino, which identify a special resonance proximity effect. It is suggested that the very high rates reported for such alpha-azidoheterocycles as 2-azidothiophene are due to similar resonance stabilizations and not to ring-opening concerted with nitrogen loss.