Circumambulatory migrations of arylazo groups in the systems of pentamethoxycarbonyland pentamethylcyclopentadiene

According to quantum chemical calculations CAM-B3LYP/6-311++G(d,p) intramolecular migrations of arylazo groups in 5-arylazo-1,2,3,4,5-pentamethoxycarbonylcyclopentadienes occur by a dissociative mechanism through the intermediate formation of tight ionic pairs with low energy barriers: Δ E ≠ZPE, benzonitrile, 14.0 (Ar = C6H4NO2-4) and 16.5 [Ar = C6H3(NO2)2-2,4] kcal/mol. In 5-arylazo-1,2,3,4,5-pentamethylcyclopentadienes, 1,5-sigmatropic shifts of arylazo groups occur along the perimeter of the five-membered ring in conformers with the exo position of the azo group relative to the cyclopentadiene ring with significantly higher activation barriers: Δ E ≠ZPE, benzonitrile, 30.3 (Ar = C6H4NO2-4) and 27.5 [Ar = C6H3(NO2)2-2,4] kcal/mol. The calculations are in good agreement with the data for these compounds obtained by us earlier by dynamic NMR.