Electrochemical Cage Activation of Carboranes

Abstract

Carboranes are boron-carbon molecular clusters that possess unique properties, such as their icosahedron geometry, high boron content, and delocalized three-dimensional aromaticity. These features render carboranes valuable building blocks for applications in supramolecular design, nanomaterials, optoelectronics, organometallic coordination chemistry, and as boron neutron capture therapy (BNCT) agents. Despite tremendous progress in this field, stoichiometric chemical redox reagents are largely required for the oxidative activation of carborane cages. In this context, electrosyntheses represent an alternative strategy for more sustainable molecular syntheses. It is only in recent few years that considerable progress has been made in electrochemical cage functionalization of carboranes, which are summarized in this Minireview. We anticipate that electrocatalysis will serve as an increasingly powerful stimulus within the current renaissance of carborane electrochemistry.

Keywords: B−H Activation; Cage Activation; Carborane; Electrochemistry.

Scheme 1

B−H functionalization of carborane cages.

Scheme 2

Electrochemical B−H halogenation and thiocyanation of nido‐carboranes.

Scheme 3

Electrochemical B−H nitrogenation of nido‐carboranes.

Scheme 4

Competition experiment.

Figure 1

Cyclic voltammetry in DME with 0.1 m nBu₄NPF₆ under N₂ at a scan rate of 100 mV s⁻¹. A glassy carbon working electrode (disk, diameter: 3 mm), a coiled platinum wire counter electrode, and a non‐aqueous Ag/Ag⁺ reference electrode were employed. The Ag/Ag⁺ reference electrode was a silver wire in a MeCN solution of 100 mm TBAPF₆ and 10 mm AgNO₃.

Scheme 5

Proposed mechanism.

Scheme 6

Electrochemical B−H functionalization with various heteroatoms bearing a lone pair of electrons.

Scheme 7

Metal‐catalyzed electrochemical B−H functionalization.

Scheme 8

Copper‐catalyzed electrochemical B−H oxygenation of o‐carboranes.

Scheme 9

Control experiments for copper‐catalyzed electrochemical B−H oxygenation.

Scheme 10

Plausible reaction mechanism of the copper‐catalyzed electrochemical B−H oxygenation.

Scheme 11

Electrochemical C−H chalcogenation of o‐carboranes.

Scheme 12

Late‐stage diversification and control experiments of cupra‐electrocatalyzed C−H chalcogenation.

Scheme 13

Noncovalent interaction plots for the complex V and VI. Here, strong attractive interactions are given in blue, weak attractive interactions are given in green, and red corresponds to strong repulsive interactions. Ar=4‐MeOC₆H₄.

Scheme 14

Plausible mechanism of cupra‐electrocatalyzed cage C−H chalcogenation.