A couple potential radical initiation mechanisms are given below, where recent research the radical species and reactions that occur the chain reaction is highly complicated and even dependent on the amount of oxygen present.
Since then, organoboranes have demonstrated wide applicability in radical chemistry as chain-transfer reagents, radical precursors, reducing agents, and more. However, most of the relevant reagents containing boron in these reactions involve a radical species in which the radical itself is localized on an atom that is not boron. As a result, they are not considered boryl radicals. 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.
The study of boryl radicals starts with the investigation of ligated 4c-7e species like trialkylborane given in the figure. The ligated forms of the boryl radical such as the one above were successfully identified in the 80s by Roberts et. al. through electron spin resonance spectroscopy (ESR, now also referred to as electron paramagnetic resonance spectroscopy or EPR) and investigated in its reaction pathways through the decay of the EPR signal under reaction conditions. In the two decades following the first studies, the literature presents a considerable lack of interest for boryl radical species. The beginning of a new wave of recent interest in studying the radical species can be correlated with the successful isolation and structural analysis of persistent radicals starting in 2007 with Gabbai's acridinium moiety attached to a boryl radical. Around the same time, NHC-boranes were identified as atomic hydrogen donors in the radical deoxygenation of xanthanes in a cornerstone investigation by Curran et al. Following mechanistic studies led to the eventual identification of a boron centered radical intermediate. This resulted in a multitude of NHC-stabilized boryl radicals to be designed by exploiting the tunable electronic and steric characteristics of the carbene. The stability of the radicals has been successfully engineered to achieve kinetic persistency and thermodynamic stability such that they now allow for an investigation of radical containing organoboron complexes that are relevant for potential applications in materials. 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. Incorporating radicals onto functional boranes such as borafluorene may unlock new areas of materials research, by combining the luminescence and reactivity observed in borafluorene based materials. These properties can be potentially electronically addressable due to the presence of energetically accessible electrons provided by the radical species.
The simplest type of boryl radicals are sigma-type 3c-5e radicals that have only been observed under extremely specialized conditions inside a specially constructed mass spectrometer. 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. Since it is not stable, it is not a useful species other than for spectroscopic investigations.
As highlighted in the above sections, the breakthrough was the stability introduced through the formation of a Lewis acid-Lewis base adduct whereby the electrons donated by the Lewis base stabilized the boryl radical. This was because the complex, now a 4c-7e boryl radical with the unpaired electron in a vertical p-type orbital. This suggested that the interaction between the ligand orbitals parallel to the out of plane p-orbital of the boryl radical was critical in determining its stability, and potentially with the alkyl framework surrounding the boron atom (though limited in correlation compared to the interaction with the lewis base).
Assuming the alkyl framework surrounding the boron center is flexible, if the lewis base donates a lot of electron density to the boryl radical via hyperconjugation, this manifests itself as an interaction which favors the trigonal pyramidal center over the trigonal planar at the boron center. While this is one of the electronic interaction that determines the geometry at the boryl radical center, there are other effects such as the electronic nature of the substrates covalently bonded to the boron, as well as back bonding to the ligand π-system (if present), both of which favor a trigonal planar geometry. In these cases, significant portion of the radical density is delocalized onto the Lewis base itself. Some possible interactions summarized as synergistic interactions by Lu et al, are given below. It is a combination of these which ends up deciding the geometry, stability and reactivity of a boryl radical at hand.
Tertiary amine boryl radicals can be generated from the reaction of a tertiary amine boryl adduct with a heteroatom centered radical.
Even though the tertiary-amine-borylradical and alkyl radicals are isoelectronic with one another, the amine-boryl radical demonstrates reactivity similar to metalloidyl radicals such as trialkylsilyl and trialkylstannyl analogues. Even through they appear isoelectronic, the difference in reactivity can be attributed to the nature of the bonds and the orbitals involved in the dative interaction. A recent extensive survey by Nagib et al in 2024 quantifies the nucleophilic nature of the boryl radicals when compared with other radical species. The amine-boryl radicals also demonstrate distinct reactivity compared to the other metalloidyl radicals such as beta-scission and hydrogen atom transfer(HAT). Some of the reactivities observed are presented below as seen in publications by Roberts et al. as compiled by the Knowles group.
Phosphine-boryl radicals and dialkyl sulfide-boryl radicals were found to be distinct in their reactivities. Phosphine-boryl radicals were less reactive towards halogen atom transfer (XAT) , β-scission and addition reactions and the dialkyl-sulfide-boryl radicals were very reactive for XAT and β-scission, where XAT was faster.
The investigation into the more stable ligated-boryl radical species and their reactivity therefore showcased a variety of interesting reactions and established more stable and tunable analogues.
The first isolable example of a boryl radical ligated by a carbene analogue was a 9-borylated acridinyl-radical synthesized by Gabbai in 2007, lending credibility to the idea that persistent boryl radicals could be obtained through other strongly binding ligand systems with steric and electronic tunability. The use of N-heterocyclic carbenes (NHCs) as a way to stabilize boryl radicals was first demonstrated as a part of the mechanism involving the radical deoxygenation of xanthanes using NHC-boranes, which had previously been synthesized by Kuhn. 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. The product, rate and EPR experiments supported the presence of a boryl radical with spin delocalization onto the NHC-π-system orbitals whereby the radical demonstrated π-character.
This then led to the synthesis of a persistent NHC-boryl radical from a NHC-borenium cation reduced with magnesium. It was identified as a part of the reaction mixture, but unfortunately could not be isolated and stored. The first isolable NHC-stabilized boryl radical was synthesized by Braunschweig from an NHC-borolyl anion reduced by a single electron using triphenyltin chloride as a reducing agent. Cyclic alkyl amino carbene (CAAC) ligands are similarly strongly σ donating carbene ligands like NHCs, but are missing a nitrogen atom adjacent to the carbene, and as a result are much better π-acceptors compared to NHCs. CAAC based boryl radicals were also synthesized. An example to a CAAC analogue by Braunschweig and was synthesized via a reduction of an adduct with decamethylchromocene. One of the radicals with the largest spin density on the boron atom itself, as revealed by the hyperfine coupling observed in the EPR spectra, is the following diarylboryl radical stabilized by an NHC. The spin density on the boron is found to be diminished and highly delocalized to the CAAC system which lacks one of the nitrogens in the heterocycle, while NHCs with their significantly less π-accepting character appear to showcase the larger spin densities localized on the boron atom. In the decade following, there have been many persistent and isolable boryl radicals synthesized through a multitude of reactions, most of which appear to involve the redox chemistry of a stable precursor anion or cation, where the increased stability allows for increased tunability of the reactivity of the radical complex.
The newly synthesized NHC and CAAC-boryl radicals and their analogues have been implemented as radical mediators, reactants and boron sources in various contexts.
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. An example of this is demonstrated in the context of radical cycloisomerization reactions.
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. The net spin density located on the CAAC (0.38 electrons) was calculated to be more than double that of NHC (0.16 electrons), in agreement with lesser π accepting character of NHC induced by the additional N lone-pair.
Another system that has recently been explored is the borepin which is a 7 membered ring with 6π-electron B-PAH. It was also easy to synthesize its anion analogue through a single electron reduction. These investigations not led to a new boryl radical environment to model aromatic systems, but also suggested that a stable radical can act as an intermediate to access the corresponding reduced or oxidized anion species. These electronically addressable molecules are projected to electronically and optically tune materials they are implemented in.
Larger and more complicated polycyclic aromatic hydrocarbon frameworks can also be accessed as of 2023, where Gilliard et. al. reported a new Bis(9-Boraphenanthrene) and its biradical through a facile, one-pot synthesis. It also interestingly showcases an open-shell singlet boron-doped biradical with usually high 95% biradical character when using a CAAC ligand.
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Metrano, Anthony (2018-12-06). "Ligated Boryl Radicals: Structure, Properties, and Reactivity" (PDF). Presentations - The Knowles Group. Retrieved 2024-11-20. https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-12-06-AJM-Ligated_Boryl_Radicals-GM_v2-1.pdf
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Metrano, Anthony (2018-12-06). "Ligated Boryl Radicals: Structure, Properties, and Reactivity" (PDF). Presentations - The Knowles Group. Retrieved 2024-11-20. https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-12-06-AJM-Ligated_Boryl_Radicals-GM_v2-1.pdf
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Lu, Dongmei; Wu, Chao; Li, Pengfei (2014). "Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals". Chemistry – A European Journal. 20 (6): 1630–1637. doi:10.1002/chem.201303705. ISSN 1521-3765. PMID 24435980. https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.201303705
Lu, Dongmei; Wu, Chao; Li, Pengfei (2014). "Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals". Chemistry – A European Journal. 20 (6): 1630–1637. doi:10.1002/chem.201303705. ISSN 1521-3765. PMID 24435980. https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.201303705
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Yang, Wenlong; Krantz, Kelsie E.; Freeman, Lucas A.; Dickie, Diane A.; Molino, Andrew; Frenking, Gernot; Pan, Sudip; Wilson, David J. D.; Gilliard Jr., Robert J. (2020). "Persistent Borafluorene Radicals". Angewandte Chemie International Edition. 59 (10): 3850–3854. doi:10.1002/anie.201909627. ISSN 1521-3773. PMC 7064902. PMID 31816143. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7064902
Metrano, Anthony (2018-12-06). "Ligated Boryl Radicals: Structure, Properties, and Reactivity" (PDF). Presentations - The Knowles Group. Retrieved 2024-11-20. https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-12-06-AJM-Ligated_Boryl_Radicals-GM_v2-1.pdf
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Metrano, Anthony (2018-12-06). "Ligated Boryl Radicals: Structure, Properties, and Reactivity" (PDF). Presentations - The Knowles Group. Retrieved 2024-11-20. https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-12-06-AJM-Ligated_Boryl_Radicals-GM_v2-1.pdf
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Metrano, Anthony (2018-12-06). "Ligated Boryl Radicals: Structure, Properties, and Reactivity" (PDF). Presentations - The Knowles Group. Retrieved 2024-11-20. https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-12-06-AJM-Ligated_Boryl_Radicals-GM_v2-1.pdf
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Metrano, Anthony (2018-12-06). "Ligated Boryl Radicals: Structure, Properties, and Reactivity" (PDF). Presentations - The Knowles Group. Retrieved 2024-11-20. https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-12-06-AJM-Ligated_Boryl_Radicals-GM_v2-1.pdf
Baban, Jehan A.; Roberts, Brian P. (1987-01-01). "Homolytic reactions of ligated boranes. Part 6. Electron spin resonance studies of dialkyl sulphide–boryl radicals". Journal of the Chemical Society, Perkin Transactions 2 (4): 497–505. doi:10.1039/P29870000497. ISSN 1364-5471. https://pubs.rsc.org/en/content/articlelanding/1987/p2/p29870000497
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