Boryl radicals
Boryl radicals are defined as chemical species with an unpaired electron localized on the boron atom in a molecule.
[2][4] A boryl radical in its isolated form has a three-center-five-electron (3c-5e) configuration, while the ligation results in its transformation to a four-center-seven-electron complex (4c-7e).
In contrast, the ligated boryl radicals with a 4c-7e configuration have an additional, dative bond with a Lewis base, such that the sp2 orbital is now filled.
[11] Studies have also revealed cations that can undergo electrochemical reduction to form a neutral boryl radical species.
[12] Study of boryl radicals have also allowed for probing the phenomenon referred to as Polarity-reversal catalysis (PRC) by Roberts and his colleagues, where a normally slow single-step hydrogen atom abstraction (HAT) reaction from an electron rich C-H bond can be split into two steps where the radicals and substrates are polarity matched in the presence of a nucleophilic hydridic catalyst, making it faster.
[14] Investigations of organoboron compounds date as far back as 1860, when Sir Edward Frankland described a range of substitution reactions in which triethylborane is autoxidized in the presence of oxygen.
This is a very important distinction to make in the investigation and literature search associated with boron containing reagents in the context of radical based organic chemistry.
[26] This resulted in a multitude of NHC-stabilized boryl radicals to be designed by exploiting the tunable electronic and steric characteristics of the carbene.
A recent example from the Gilliard group in 2020 is the persistent borafluorene radical which is not simply an intermediate species but demonstrates solid state and solution stability.
[2] This was done under pyrolysis conditions at very high vacuum using a special quadrupole lens, wide slits and ionizing chamber open all sides to minimize wall collisions and a sensitive detector.
This way, Fehlner and Koski were able to preserve the highly reactive species long enough to confirmed the presence of ·BH2 as a pyrolysis product in 1964.
The hyperfine structure of the EPR spectra suggests that these are both closer to a trigonal planar geometry at the boron center.
[32] They were able to isolate NHC-(methylthiocarbonylthio)borane as a solid product from the reaction of xanthanes and NHC-borane with Et3B/O2 as a radical initiator.
One of the ways in which the tunable ligand environment directly presents itself as an advantage is found in the highly active area of asymmetric catalysis of organic reactions.
To understand these heterocyclic aromatic compounds better, model systems could be a good way to probe the underlying principles.
With this goal in mind, it is therefore valuable to synthesize "B-doped aromatic systems" with different electronic landscapes by introducing defects into the band structure.
With this goal in mind, persistent borafluorene radicals were recently synthesized using both NHC and CAAC ligands which yielded blue and purple crystals respectively, hinting at their tunable electronic environments.
