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 v ery large increases in rate of thermolysis, up to 225-fold, The rates of nitrobenzene solutions at 120 degrees C follow a Hammett-type linea r free energy relationship log k = -5.44 - 2,33 sigma(1) - 1.48R(+) wh ich indicates conjugative stabilization of a nitrene-like transition s tate, ortho-substituents of the + R type causes still larger rate enha ncements, up to 456-fold for 2-amino, which identify a special resonan ce proximity effect. It is suggested that the very high rates reported for such alpha-azidoheterocycles as 2-azidothiophene are due to simil ar resonance stabilizations and not to ring-opening concerted with nit rogen loss.