Expansion of Azulenes as Nonbenzenoid Aromatic Compounds for C–H Activation: Rhodium- and Iridium-Catalyzed Oxidative Cyclization of Azulene Carboxylic Acids with Alkynes for the Synthesis of Azulenolactones and Benzoazulenes

Rhodium-catalyzed oxidative [4 + 2] cyclization reactions through the C–H activation of azulene carboxylic acids as nonbenzenoid aromatic compounds with symmetrical and unsymmetrical alkynes were developed under aerobic conditions, which produced azulenolactone derivatives with a wide substrate scope and excellent functional group tolerance. Interestingly, azulenic acids in reaction with alkynes underwent iridium-catalyzed [2 + 2 + 2] cyclization accompanied by decarboxylation to afford tetra­(aryl)-substituted benzoazulene derivatives. The reactivity order for C–H activation reaction is greater toward azulene-6-carboxylic acid, azulene-1-carboxylic acid, and azulene-2-carboxylic acid. For the first time, the expansion of azulenes having directing group as nonbenzenoid aromatic compounds for C–H activation was successful, indicating that nonbenzenoid aromatic compounds can be used as good substrates for the C–H activation reaction. Therefore, the research area of C–H activation will certainly expand to nonbenzenoid aromatic compounds in future.