Evidence for Extensive Single‐Electron‐Transfer Chemistry in Boryl Anions: Isolation and Reactivity of a Neutral Borole Radical

Abstract

Despite the synthesis of a boryl anion by Yamashita et al. in 2006, compounds that show boron-centered nucleophilicity are still rare and sought-after synthetic goals. A number of such boryl anions have since been prepared, two of which were reported to react with methyl iodide in apparent nucleophilic substitution reactions. One of these, a borolyl anion based on the borole framework, has now been found to display single-electron-transfer (SET) reactivity in its reaction with triorganotetrel halides, which was confirmed by the isolation of the first neutral borole-based radical. The radical was characterized by elemental analysis, single-crystal X-ray crystallography, and EPR spectroscopy, and has implications for the understanding of boron-based nucleophilic behavior and the emergent role of boron radicals in synthesis. This radical reactivity was also exploited in the synthesis of compounds with rare B-Sn and B-Pb bonds, the latter of which was the first isolated and structurally characterized compound with a "noncluster" B-Pb bond.

Easy access to nucleophilic boron through diborane to magnesium boryl metathesis

“…5a), which preserves a monomeric constitution through the retention of the coordinated DMAP ligand. Although we cannot discount the possibility that the mechanism of boron methylation takes place via a radical-based pathway52, it is notable that a further reaction of compound 11 and MeI performed in the presence of the potential radical trap 9,10-dihydroanthracene provided identical conversion to compound 12 and MeBpin and evidenced no consumption of the aromatic hydrocarbon (Supplementary Fig. 14).…”

From Borane to Borylene without Reduction: Ambiphilic Behavior of a Monovalent Silylisonitrile Boron Species

“…[18] Diffraction analysis on poor-quality crystals of plumbyl derivative 6 provided additional proof of connectivity.…”

“…While an umber of germyl-, stannyl-, and plumbylboranes have been reported, they are usually synthesized by salt metathesis of ah aloborane with aL iER 3 precursor [17] rather than by the inverse-polarity reaction of ab oron nucleophile with ClER 3 . [18] In solution the Pb derivative 6 displayed no 207 Pb NMR resonance due to quadrupolar broadening and slowly decomposed to amixture of 1,PbPh 4 ,Pb 2 Ph 6 ,and Pb 0 , presumably through radical processes.T he stable Ge and Sn derivatives displayed 11 BNMR doublets in the region of À32 ppm ( 1 J 11B-1H = 99 to 106 Hz) while the softer Pb analogue presented abroad resonance at À26.7 ppm.…”

“…[17e, 18] Attempts to deprotonate 5a and 5b with LiCH 2 t Bu to generate the corresponding (cyano)(stannyl)boryl anions resulted instead in the formation of 2 and ( t BuCH 2 )SnR 3 (Scheme 1d), suggesting ar ather labile B-Sn bond.…”

Isolation and Reactivity of 1,4,2-Diazaborole

“…However, it should be noted that electron transfer mechanism involving the radical intermediate cannot be ruled out for the formation of 5. 26 Scheme 2. Reactions of 3 with MeOTf and selectfluor.…”