Theoretical Analysis of Acetyltriazene and the Mechanistic Implications of Its Reaction with Acid

Theoretical calculations have been carried out on the protonation of 3-acetyltriazene in order to provide insight into the mechanism of the acid-catalyzed decomposition of acyltriazenes. We have previously reported the results of calculations on a series of alkyltriazenes. Ab initio RH F calculations were carried out at the 3–21G level to determine the optimized SCF energies and geometries of the neutral molecule and some site-specific protonated species. This allowed an estimate of the proton affinity at each site. Experimental studies on alkyltriazenes indicated that initial protonation at the N₃ site was critical in the acid catalysis mechanism, even though the calculated proton affinities indicated that N₁ was a more basic site. In the case of the acyltriazenes, the calculations showed that the proton affinity at N₃ was much lower than that at N₁ or at the carbonyl oxygens. The geometrical changes produced by protonations at the various sites indicated that the N₂-N₃ bond shows a propensity for cleavage upon protonation at either N₃ or at the carbonyl oxygen. These results suggest that acid-catalyzed decomposition of acetyltriazene would involve the breakage of the N₂-N₃ bond, rather than the hydrolysis of the acyl group. Subsequent experimental data supported this conclusion. A linear scaling method was applied to the geometric and energetic results from the semiempirical AM1 code to predict the results of the 3–21 G calculations with a surprising degree of success. A predictor function to allow AM1 geometries and proton affinities to provide a good estimate of 3–21 G results is given, and the limitations are discussed.