Regiodivergent metal-catalyzed B(4)- and C(1)-selenylation of o-carboranes.

Regiodivergent transition metal-catalyzed B(4)- and C(1)-selenylation reactions of o-carboranes have been demonstrated. Namely, Ru(ii)-catalysis selectively generated B(4)-selenylated o-carboranes from the reaction of o-carborane acids with arylselenyl bromides with the release of carbon dioxide. In contrast, Pd(ii) catalysis provided exclusively C(1)-selenylated o-carboranes from the decarboxylative reaction of o-carborane acids with diaryl diselenides. In contrast to previous milestones in this area, these reactions demonstrate broad substrate scope with excellent yields. Combination of these methods leads to the formation of B(4)-C(1)-diselenylated o-carboranes. DFT studies revealed the mechanism of the Ru-process, with initial selenylation of the carborane cluster discovered to be essential for an energetically reasonable decarboxylation. This results in selenylation on the B(4) position prior to the decarboxylation event at C(1). This contrasted with the Pd-process in which the ready decarboxylation at C(1) leads to selenylation at C(1).

Synthesis of o-Carborane-Fused Pyrazoles through Sequential C-N Bond Formation.

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.

Iridium-Catalyzed Cage B(4)-Amidation Reaction of o-Carboranes with Dioxazolones: Selective Synthesis of Amidated o-Carboranes and Amidated and Methoxycarbonylated nido-Carboranes.

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.