Boryl radicals

Boryl <a href="/facts/Radical_(chemistry)/qFymuVoT">radicals</a> are defined as chemical species with an unpaired electron localized on the boron atom in a molecule. There is renewed interest in their discovery as they have recently showcased useful organic reactivities. While the first studies of boryl radicals involved <a href="/facts/Borane/pHQMPw5r">borane</a> radical anions, the study of overall neutral boryl radical species was unlocked through the investigation of what are referred to as <a href="/facts/Ligand/6etB3qeW">ligated</a> boryl radicals. 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). These descriptions found in the literature refer to the number of coordinated atoms that surround the boron atom plus the boron atom, and the number of electrons involved in the immediate bonding environment. For example, in the case of the 3c-5e boryl radical, the boron is covalently bonded to two atoms (two bonds with two electrons each) and is predicted to have its unpaired electron in the <a href="/facts/Hybridization_(chemistry)/Fl7sz8Jm">sp2-like</a> orbital (1 electron). This leads to a highly reactive radical and an empty <a href="/facts/Atomic_orbital/tKhgl6mL">p orbital</a> on the boron. In contrast, the ligated boryl radicals with a 4c-7e configuration have an additional, dative bond with a <a href="/facts/Lewis_acids_and_bases/qlLqRvg2">Lewis base</a>, such that the sp2 orbital is now filled. In this configuration, the radical occupies the p orbital and has the appropriate symmetry to interact with the coordinated groups and the ligand, allowing the otherwise strongly lewis basic radical to be stabilized. These structures, and the stabilizing interactions are showcased in the figure below.

While the definition of the boryl radical requires the unpaired electron density to be localized on the boron atom, in practice the extent at which the radical spin density is localized on the boron itself can vary greatly (0.15 electrons to 0.90 electrons). This leads to a diverse list of structures that are studied as boryl radicals, as long as the boron has some calculated/measured radical character or showcases radical type reactivity in corresponding organic reactions. Examples to these structures include sigma-type boron radical anions generated from borane, <a href="/facts/Amine/JvfYJfP6">trialkylamine</a>- and dialkylsulphide- ligated radicals,  boron-based heterocyclic radicals, <a href="/facts/Persistent_carbene/g4U9YPae">N-heterocyclic carbene</a>-stabilized boryl radicals, and a variety of ligated boryl <a href="/facts/Radical_anion/N4vFqKrV">radical anions</a> and <a href="/facts/Cation/wrbwhF3z">cations</a>. Studies have also revealed cations that can undergo electrochemical reduction to form a neutral boryl radical species.
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.  Recent breakthroughs in stable and isolable boryl radicals such as <a href="/facts/9-Borafluorene/nIAw0dPo">borafluorene</a> based radicals by the Gilliard group suggest a future where boryl radicals may find generalized use in new types of materials, as well as catalytic reactivities in a wider range of reactions.

Boryl radicals open-in-new

Boryl radicals