Boryl radicals

Organosiliconboranes having bulky substituents on the boron, e.g. R3SiB[N(CHMe)2]2, exhibit UV absorption at wavelengths longer than 300 nm. Photolysis of this band afforded a pair of silyl and boryl radicals that can be trapped quantitatively by nitroxide (TEMPO) as shown in Reaction (1.4) [10]. 

The EPR spectra of the NHC boryl radicals that were generated through HAT to the ferf-butoxyl radical clearly show the delocalized 7i-type nature of these intermediates postulated to be essential by calculations [10, 12]. It was also demonstrated that the decay of the EPR signals could be fitted to a second-order decay having 2kt = 9 x 106 M-1 s-1. In agreement with this kinetic analysis, the NHC boryl radicals ultimately dimerize to give bis-NHC diborane derivatives. With the aid of EPR spectroscopy it was also established that the NHC boryl radicals readily abstract bromine atoms from primary, secondary, and tertiary alkyl bromides. However, chlorine atom abstraction is much slower and useful only for benzyl chloride. 

Alkyl radicals stabilized with boron atom are easily generated and undergo addition and coupling reactions. Such cr-boryl radicals were generated in situ by treatment of a-chloroalkylboronic esters with CrCl2 (Equation (188))706 or with radical initiators such as AIBN (Equation (189)).707... 

Nucleophilic and electrophilic radicals abstract different hydrogen atoms from butyrolactone 7.10 the ferf-butoxy radical selectively abstracts a hydrogen atom from the methylene group adjacent to the oxygen atom, whereas a boryl radical abstracts a hydrogen atom from the methylene group a to the carbonyl group. 

Chain extension of electron-deficient alkenes. a-Boryl radicals generated from... 

Fig. 7.3 Interactions for the attack of /erf-butoxy and boryl radicals on butyrolactone...

Enantioselective hydrogen atom abstraction by chiral amine boryl radical 83 allows for kinetic resolution of racemic ester 84. In the specific example illustrated in Eq. (23), the enantioselectivity of the residual enantiomer of the substrate reached 74% (/ ,/ ). It is postulated that a transition state as shown in 86 could account for the asymmetric hydrogen atom abstraction. 

Free radical chemistry is not generally a useful way to introduce asymmetry, and this review is very brief. However, some examples of high diastereoselectivity have been found. There is substantial evidence that a-boryl radicals are stabilized by Ttbonding involving the vacant p-orbital of boron [82]. 

Intermolecular radical additions are facilitated if the alkene has a hydroxyl substituent that can form an ester to boron, so that the actual addition is intramolecular. Accordingly, bis(2-bromoethyl) alkenylboronates readily cyclize under radical conditions [85]. This concept was quickly extended to a-boryl radicals [86]. One of the more favorable examples is the reaction of diisopropyl (bromomethyl)boronate with 4-methylhex-l-en-3-ol and tris(trimethylsilyl)silane in the presence of azobis(isobuty-... 

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. 

To date there are no reported examples of the above molecules catalyzing a reaction, however Lacote, Curran et al. recently demonstrated that their NHC-stabilized boryl radicals (85, vide supra) can serve as radical initiators for acrylate polymerizations, radical deoxygenation reagents, radical reductions of alkyl halides and radical chain homolytic substitution reactions. ... 

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