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.

Described herein is the iridium-catalyzed sequential C(2)-arylation reaction and intramolecular C–O bond formation from azulenecarboxylic acids and diaryliodonium salts, leading to the formation of 3-arylazulenofuranones. The sequential reaction proceeded smoothly through generation of 2-arylazulene-1-carboxylic acids derived from the iridium-catalyzed regioselective C(2)-arylation reaction without the decarboxylation reaction.

A direct B(4)–H monoacyloxylation via a Pd-catalyzed regioselective B(4)–H activation of o-carborane acids with phenyliodonium dicarboxylates was developed, and a series of B(4)–H monoacyloxylated o-carboranes decorated with active groups were synthesized with moderate to good yields as well as excellent selectivity. In addition, a direct B­(4,5)–H diacetoxylation from o-carborane acids with phenyliodonium diacetate was demonstrated.

Described is the Ir-catalyzed cage B(4)-amidation of o-carboranes with dioxazolones by carboxylic acid-assisted B(4)−H bond activation under extremely mild conditions, affording amidated o-carboranes and amidated and methoxycarbonylated nido-carboranes through sequential B(4)-amidation, O-methylation, and B(3)-deboronation in one pot. Carboxylic acid used as a directing group after the cage B(4)-amidation is efficiently trapped by trimethylsilyldiazomethane instead of undergoing decarboxylation. Mechanism studies demonstrated that the O-methylation through trapping of acid occurred first, followed by the B(3)-deboronation.

Transition-metal-free synthetic method for o-carborane-fused pyrazoles as a new scaffold has been developed from the reaction of B(4)-acylmethyl or B­(3,5)-diacylmethyl o-carborane with 2-azido-1,3-dimethyl­imidazolinium hexafluorophosphate (ADMP) in the presence of DBU in acetonitrile through sequential diazotization and cyclization reaction in one pot, consequently allowing twofold C–N bond formation under extremely mild conditions and high functional group tolerance.