Oxidative Generation of Boron-Centered Radicals in Carboranes

Abstract

We report the first indirect observation and use of boron vertex-centered carboranyl radicals generated by the oxidation of modified carboranyl precursors. These radical intermediates are formed by the direct oxidation of a B–B bond between a boron cluster cage and an exopolyhedral boron-based substituent (e.g., −BF3K, −B­(OH)2). The in situ generated radical species are shown to be competent substrates in reactions with oxygen-based radicals, dichalcogenides, and N-heterocycles, yielding the corresponding substituted carboranes containing B–O, B–S, B–Se, B–Te, and B–C bonds. Remarkably, this chemistry tolerates various electronic environments, providing access to facile substitution chemistry at both electron-rich and electron-poor B–H vertices in carboranes.

The Mechanochemistry of Carboranes

“…We conducted the sonication of P1 in the presence of 32 mM TEMPO for 2 h, [10] which is known to add efficiently to in situ generated boron‐centered radicals [11] . 11 B NMR spectroscopy with a characteristic chemical shift ( δ =16 ppm, polyhedral 11 B−O bonds, Figure 3a) revealed the formation of a boron cluster‐containing species corresponding to a TEMPO adduct [11] . Sonication of P1 in the presence of a coumarin‐TEMPO (CT) adduct led to incorporation of multiple chromophores onto the boron cluster‐containing fragments due to multiple homolytic steps during o ‐carborane dissociation (Figure S3), consistent with a homolytic dissociation mechanism.…”

Synthesis and Structure of an o‐Carboranyl‐Substituted Three‐Coordinate Borane Radical Anion

“…According to Wade's rules, such an increase of skeletal electrons (SE) results in a transition from a 2n+2 closo ‐ towards a 2n+4 nido ‐structure [134,135] . Reports of rare 2n+3 SE structures have always sparked curiosity throughout the cluster community [136–148] . The 2n+3 closo ‐ nido cluster intermediates of o ‐carboranes offer a structural comparison with calculated S 1 CT excited state geometries as shown by Weber and Fox et al [104,147,149] .…”

“…[134,135] Reports of rare 2n + 3 SE structures have always sparked curiosity throughout the cluster community. [136][137][138][139][140][141][142][143][144][145][146][147][148] The 2n + 3 closo-nido cluster intermediates of o-carboranes offer a structural comparison with calculated S 1 CT excited state geometries as shown by Weber and Fox et al [104,147,149] Reduction of o-carboranes results in a 2n + 3 SE system and typically requires a conjugated πsystem to stabilize the resulting radical. [150][151][152][153] Few such crystal structures have been reported, all of which are dianionic [144,147] or double bonded, [146] resulting in diamagnetism.…”

Rh‐Catalyzed Decarbonylative Cross‐Coupling between o‐Carboranes and Twisted Amides: A Regioselective, Additive‐Free, and Concise Late‐Stage Carboranylation

“…Traditionally some precursors for B(3)−C coupling were employed (Figure 1 B). From a synthetic standpoint, these precursors are hard to prepare, most often by a multistep chemical sequences [13] and toxic [13a, d] or explosive [13b, c] syntheses. In addition, precursor 1 is sensitive to oxygen as well as moisture, and diazonium salt 3 is not stable.…”