Iridium-Catalyzed Annulation of o-Carboranyl Carboxylic Acids with Alkynes: Synthesis of Carborano-Isocoumarins

Abstract

An efficient iridium-catalyzed formal [4+2] annulation of carboranyl carboxylic acids with alkynes is developed, resulting in the facile synthesis of a new class of carboranoisocoumarin derivatives. The carboxyl group not only serves as a directing group to control the regioselectivity but also ingeniously becomes a part of the final products. The reaction mechanism involves sequential carboxyl-directed B(4)-H metalation, alkyne insertion, and reductive elimination.

Directing-Group-Assisted Transition-Metal-Catalyzed Selective BH Functionalization of o-Carboranes

“…Recently, the Xie group documented that utilizing 5 mol % of [Cp*IrCl2]2 as catalyst and 3 equiv of Cu(OPiv)2 as oxidant, an efficient formal [4+2] annulation reaction of carboranyl carboxylic acids 1 with internal alkynes could be achieved, in which the carboxyl group not only regulate the regioselectivity as a directing group but also participate annulation as a component (Scheme 5). 22 A batch of carboranoisocoumarins 14 with various substituents on the aryl ring, including alkyl, methoxyl and halo groups, were produced in good to excellent yields. As outlined in Scheme 5, the reaction is initiated by the carboxyl-directed B(4)-H activation to form a five-membered iridacycle 15, followed by alkyne insertion and reductive elimination to afford final products 14.…”

Unraveling origin of chemoselectivity and regioselectivity of iridium‐catalyzed B(4)–H functionalization of o‐carborane by alkyne

“…However, the decarboxylation is not an easygoing process because the system has to be heated above 100 C. Recently, Xie and co-workers developed an efficient tactic to furnish the carborano-isocoumarins via the iridium-catalyzed formal [4 + 2] annulation of carboranyl carboxylic acids with alkynes, which intercepts the decarboxylation to give the alkenylated carborane as the major product (Scheme 1C). [27] In addition, for either the annulation or the alkenylation, the B(4)-H is specifically activated. As part of our ongoing investigation into origin of the selectivities for o-carborane functionalization, [20,28] herein, possible pathways for decarboxylation and the chemoselectivity and regioselectivity of this iridiumcatalyzed reaction between carboranyl carboxylic acids with alkynes have been studied by using the density functional theory (DFT) method, [29,30] intending to provide theoretical guidance for synthesis of novel carborane derivatives, especially when using carboxyl as the directing group.…”