Boron-centered anion radicals

Hoefelmeyer and Gabai (2000) have synthesized l,8-bis(diphenylboryl)naphthalene and compared the structures of this molecule and the anion radical prepared from it. In the parent neutral molecule, the boron centers are separated by 3 nm. In the anion radical, the boron-boron distance is approximately 1 nm longer than that observed in compounds with single bonds between four-coordinated boron atoms. According to the ESR spectrum of the anion radical, the unpaired electron is preferentially suspended in the field of the two boron atoms. The one-electron a-bond emerges. This one-electron a-bond can obviously be viewed as resulting from the overlap of the formerly vacant and parallel p-orbitals of two... 

In the nitrogen and boron analogs depicted in Scheme 3-52, two methyl groups provide a sufficient shielding at the NR2 centers (R = Me), while two mesityl groups are needed for protection of the BR2 centers (R = 2,4,6-trimethyl phenyl). Electrochemical studies of l,4-bis(dimesitylboryl)benzene have shown two well-separated one-electron reduction processes, with the formation of the corresponding anion radicals and dianions, respectively (Fiedler et al. 1996). According to UV/vis/near-IR and ESR spectroscopic data... 

Naturally, the product of two-electron reduction of the para derivative can be depicted (Scheme 3-54) as an anionic diborataquinoid system. In contrast with the para-dib-orataquinoid dianion, an anionic meta-quinoid system is impossible. Indeed, the meta-sub-stituted isomer depicted in Scheme 3-54 has been characterized as a spin-unpaired triplet species with boron-centered spins (Rajca et al. 1995). This dianion diradical can be viewed as two stable borane anion radicals linked with a ferromagnetic coupling unit, i.e., 1,3-phenylene see Chapter 1. 

The boron-centered radical undergoes a rapid B—C bond cleavage to produce a benzyl radical/cyanoaromatic anion-radical pair which combines to produce... 

The study of boron-centered radicals dates back several decades with early reports describing the s)mthesis and characterization of anionic boryl radicals/ The use of carbenes to generate stable boron-centered radicals has brought recent attention to these paramagnetic species as promising new reagents for radical initiation and related reactions/ Whereas early reports primarily described anionic boron radicals, recent developments involving carbenes have led to the discovery of a variety of species which can be cationic or neutral. Typically, these radicals contain a boron(ii) center, and are obtained by reduction or oxidation of boron(iii) or boron(i) precursors, respectively. Additionally, carbene-stabilized diborenes may be oxidized to afford radical cations which feature B-B one-electron it bonds. 

Additionally, the electron rich B=B double bond in diborenes 62 could be readily oxidized to the corresponding diboron radical cation 93 using tro-pylium salts with bulky non-coordinating borate anions, or with a borole to provide access to the first example of a stable boron-centered radical-anion radical-cation pair 94. ... 

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