The Influence of Boryl Substituents on the Formation and Reactivity of Adjacent and Vicinal Free Radical Centers

Abstract

Radicals containing R-boronate substituents were generated by bromine abstraction from 1-bromoalkyldioxaborolanes (boronic esters), by addition to vinyl boronate, and by hydrogen abstraction from alkyldioxaborolanes and observed by EPR spectroscopy. Unsymmetrically substituted R-boronate radicals displayed selective line broadening in their low-temperature spectra from which barriers to internal rotation about • CH 2 -B(OR′)OR bonds were found to be 3 ( 1 kcal mol -1 . Use of an empirical relationship between barrier height and bond dissociation energy led to BDE[(RO) 2 BCH 2 -H] ) 98.6 kcal mol -1 . Rate constants for hydrogen abstraction from 2,4,4,5,5-pentamethyl-1,3,2-dioxaborolane by tert-butoxyl radicals were determined from competitive EPR and product studies and found to be relatively small, comparable to those of unactivated methyl groups. Hydrogen abstraction from bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-methane was found to be extremely difficult. The structures and energetics of R-boronate radicals were computed by DFT methods (B3LYP/6-31G*). This predicted reductions in the rotation barriers of X 2 B-CH 2 • radicals for increasing alkoxy substitution at B (X ) Me or MeO) and corresponding increases in the X 2 BCH 2 -H bond dissociation energies. The B3LYP-computed BDE[(MeO) 2 BCH 2 -H] was in excellent agreement with the analogous value derived from the experimental rotation barrier. Radicals containing -boronate substituents were generated from the corresponding 2-bromoalkylboronic esters and characterized by EPR spectroscopy. At higher temperatures the main product from trialkyltin and triethylsilyl radical promoted reactions of 2-(2-bromohexyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane was 1-hexene. This was best accounted for by a mechanism involving initial S H 2 attack on the borolane and subsequent bromine atom elimination from the displaced 2-bromohexyl radical.Boronic acids and esters sustain a high reputation as versatile and convenient reagents for stereocontrolled syntheses 1 and for aromatic coupling processes. 2 A recent development in their chemistry, stemming from the discovery of efficient syntheses of alkenylboranes 3 and haloalkylboranes, 4 has been the emergence of promising carbon-carbon bond formation and cyclization processes mediated by boron-containing free radicals. For example, sulfonyl halides, 5 alkyl halides, and O-acyl-Nhydroxypyridine-2-thiones added efficiently to vinyl boronic esters. Likewise, suitably unsaturated haloalkylboronic esters underwent exo-ring closures in high yields. 6 Functionalized alkenyl diamino-and dialkoxyboranes were prepared regio-and stereoselectively by radical addition to the corresponding alkynylboranes. 7 Furthermore, R-boryl radicals derived from 1-haloalkylboronic esters were shown to add to alkenes to afford homoallylboronic esters in high yields. 8 A recent ingenious innovation employed boron-tethered radical cyclizations to provide convenient syntheses of 1, 3-, 1,4-, and 1,5-diols. 9 The key intermediates in most of these proc...

Enantioselective Radical Addition/Cross‐Coupling of Organozinc Reagents, Alkyl Iodides, and Alkenyl Boron Reagents

“…[7,8] If the intermediate radical might instead react with the Ni complex furnishing D,t hen catalyst-based stereocontrol might be realized even though the process involves radical intermediates.T he underlying principles that would enable such ap rocess are aligned with other metal-catalyzed intermolecular dicarbofunctionalizations [9] of alkenes,w hich take place either through carbometalation/cross-coupling [10] pathways or through radical addition/cross-coupling cascades. To address this obstacle,we targeted formation of an eutral a-boryl radical (C,S cheme 1c), which would be more stable and less prone to engage in aS ET-based radical propagation reaction.…”

(α‐Haloalkyl)boronic Esters in Asymmetric Synthesis

“…However, some examples of high diastereoselectivity have been found. There is substantial evidence that α-boryl radicals are stabilized by π-bonding involving the vacant p-orbital of boron [82]. Free radical additions to alkenylboronic esters provided the first access to (α-haloalkyl)boronic esters [8].…”

Recent Advances in Radical Addition to Alkenylboron Compounds

“…Utilizing low temperature EPR spectroscopy, Walton and Carboni established that the barrier to internal rotation for several α‐boryl radical species is 3 ± 1 kcal/mol . Calculations conducted by the same group on the hydrogen transfer reaction of simple CH 3 BL 2 derivatives indicate that increasing the number of oxygen substituents on boron decreases the radical stabilization energy (Scheme ).…”