Copper-catalyzed electrochemical
selective cage B–H oxygenation
of o-carboranes has been achieved for the first time.
Under a constant electric current (4.0 mA) at room temperature, copper-catalyzed
cross-coupling of carboranyl amides with lithium phenolates results
in the formation of B(4,5)-diphenolated o-carboranes
via direct B–H activation, whereas the use of lithium tert-butoxide affords B(4)-monooxygenated products. This
reaction does not require any additional chemical oxidants and generates
H2 and a lithium salt as byproducts. Control experiments
indicated that a high-valent Cu(III) species is likely involved in
the reaction process.
Carboranes are a class of carbon-boron molecular clusters, possessing extraordinary characteristics including three-dimensional aromaticity conjugated by σ-bonds, icosahedral geometry and inherent robustness. They are finding growing applications as valuable building blocks in boron neutron capture therapy agents, pharmacophores, nanomaterials, optoelectronic, organometallic/coordination chemistry and more. Therefore, the effective and controlled functionalization of carboranes has attracted enormous research interests, particularly in regio- and enantio-selective cage BH derivatization among ten chemically similar BH vertices of o-carboranes. Only in the recent few years, significant progress has been made in transition metal catalyzed vertex-specific BH functionalization. This review summarizes recent advances in this research realm.
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