Revisiting the pyrolysis of 1,5‐diaryl‐1,2,5‐triazapentadienes: A computational reaction mechanism study

Abstract

A computational study of the pyrolysis of 1,5-diaryl-1,2,5triazapentadiene derivatives is carried out employing density functional theory (DFT) and high-level coupled-cluster methods, such as coupled-cluster singles and doubles with perturbative triples [DLPNO-CCSD(T)]. Our results demonstrate that the reaction proceeds through the products formed by the electrocyclization, as opposed to the mechanism suggested by Mcnab (J. Chem. Soc., Chem. Commun. 1980, 422-423), in which the reaction proceeds by starting via formation of the iminyl radicals. Our results demonstrate that the products formed by electrocyclization are responsible for the formation of iminyl radicals, contrary to homolysis of NÀ N bond of 1,5-diaryl-1,2,5triazapentadiene derivatives proposed by Mcnab. Besides, our results showing that the reaction starts with the electrocyclization very well clarify why the formation of ortho-cyclization product, contrary to ipso-cyclization from 1,5-diaryl-1,2,5-triazapentadiene derivatives as the main product. Therefore, our results are in agreement with the experimental results for pyrolysis of 1,5-diaryl-1,2,5-triazapentadiene derivatives.