Conformational studies by dynamic nuclear magnetic resonance. Part 16. The mechanism of tautomerism in triazenes

Abstract

The kinetics of tautomerism of triazenes, R¹NH–NNR²⇌ R¹NN–NHR², have been determined by means of line-shape analysis of the ¹H n.m.r. spectra at various temperatures. When R¹= R²= Me the equilibrium reaction is second-order whereas when R¹= Me, R²= Ph or when R¹= R²=p-MeC₆H₄ the reaction follows a first-order course. Thermodynamic activation parameters have been determined for both situations for a variety of solvents. Theoretical INDO calculations helped in understanding the switch from a bimolecular to a unimolecular reaction pathway when aliphatic are replaced by aromatic substituents. The absence of solvent effects suggests a radical mechanism, in agreement with the conclusion of a theoretical charge population analysis.