Visible light induced ‘on water’ benzylic bromination with N-bromosuccinimide (original) (raw)
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Tetrahedron, 2009
A H 2 O 2-HBr system and N-bromosuccinimide in an aqueous medium were used as a 'green' approach to electrophilic and radical bromination. Several activated and less activated aromatic molecules, phenylsubstituted ketones and styrene were efficiently brominated 'on water' using both systems at ambient temperature and without an added metal or acid catalyst, whereas various non-activated toluenes were functionalized at the benzyl position in the presence of visible light as a radical activator. A comparison of reactivity and selectivity of both brominating systems reveals the H 2 O 2-HBr system to be more reactive than NBS for benzyl bromination and for the bromination of ketones, while for electrophilic aromatic substitution of methoxy-substituted tetralone it was higher for NBS. Also, higher yields of brominated aromatics were observed when using H 2 O 2-HBr 'on water'. Bromination of styrene reveals that not just the structure of the brominating reagent but the reaction conditions: amount of water, organic solvent, stirring rate and interface structure, play a key role in defining the outcome of bromination (dibromination vs bromohydroxylation). In addition, mild reaction conditions, a straightforward isolation procedure, inexpensive reagents and a lower environment impact make aqueous brominating methods a possible alternative to other reported brominating protocols.
ChemInform, 2009
The visible-light-induced transformation of toluenes with N-bromosuccinimide (NBS) under solvent-free reaction conditions (SFRC) was studied. The reaction took place in spite of the very restricted molecular motion; toluenes could be regioselectively converted to benzyl bromides. Selective radical-chain reactions with NBS were carried out in liquid/liquid and in solid/solid systems; furthermore, reactions could be performed in the presence of air. The radical scavenger TEMPO (¼ 2,2,6,6tetramethylpiperidin-1-yloxy) completely suppressed the side-chain bromination of toluenes with NBS under SFRC. Electron-withdrawing groups decreased the reactivity of the toluenes, and the Hammett reaction constant 1 þ ¼ À 1.7 indicated involvement of polar radical intermediates with electrophilic character.
Helvetica Chimica Acta, 2009
The visible-light-induced transformation of toluenes with N-bromosuccinimide (NBS) under solvent-free reaction conditions (SFRC) was studied. The reaction took place in spite of the very restricted molecular motion; toluenes could be regioselectively converted to benzyl bromides. Selective radical-chain reactions with NBS were carried out in liquid/liquid and in solid/solid systems; furthermore, reactions could be performed in the presence of air. The radical scavenger TEMPO (¼ 2,2,6,6tetramethylpiperidin-1-yloxy) completely suppressed the side-chain bromination of toluenes with NBS under SFRC. Electron-withdrawing groups decreased the reactivity of the toluenes, and the Hammett reaction constant 1 þ ¼ À 1.7 indicated involvement of polar radical intermediates with electrophilic character.
Free radical bromination by the H2O2–HBr system on water
Tetrahedron Letters, 2006
An aqueous solution of hydrogen peroxide and hydrogen bromide illuminated by a 40 W incandescent light bulb serves as a source of bromine radicals. Various substituted toluenes (H, Me, tBu, Br, COOEt, COPh, NO 2) were brominated at the benzyl position. This haloperoxidase-like system for benzylic bromination does not require the presence of metal ions or an organic solvent for efficient conversion of methyl-arenes to benzyl bromides.
Organic Chemistry Frontiers, 2020
A highly efficient, site-selective, visible light-accelerated, remote C-H halogenation of unsymmetrical aromatic bisamides/amidoesters has been developed. Unprecedented selectivity was realized in this C-H functionalization reaction. The N,N'-(1,2-phenylene)bisamides/amidoesters, derived from various halogenated acids and aliphatic/aromatic acids, were selectively mono-brominated para to the more electron-deficient group. This unique, site-selective bromination of aromatic amine derivatives proceeded under mild and metal-free conditions, without the use of any oxidant or base. The present procedure benefits from short reaction times, avoids excessive bromination, and is air and moisture tolerant. Furthermore, the synthetic utility of the products has been demonstrated by preparing useful substrates and intermediates, including 2-perfluoroalkyl benzimidazole, para-bromoaniline derivatives and substituted biphenyls. A mechanistic investigation using experimental and theoretical means was undertaken to attempt to explain the observed phenomena. Based on DFT calculations, the mechanism involves the differential formation of nitrogen-centered (amidyl) radicals which leads to the observed regioselectivity. Additionally, when the radical stabilizing abilities of both amides are similar, the regioselectivity breaks down, leading to a mixture of products (2x-2z).
Benzylic Brominations with N-Bromosuccinimide in (Trifluoromethyl)benzene
Synthesis, 2009
A variety of benzylic brominations were performed by using N-bromosuccinimide in (trifluoromethyl)benzene with photochemical activation in the presence of 2,2¢-azobisisobutyronitrile, 1,1¢-azobis(cyclohexanecarbonitrile), or benzoyl peroxide as the radical initiator. This system provides clean, rapid, and high-yielding reactions with replacement of conventional solvents, such as tetrachloromethane, by less-toxic (trifluoromethyl)benzene.
Bulletin of The Korean Chemical Society, 2013
E-mail: Kazemi_f@iasbs.ac.irReceived October 27, 2012, Accepted November 19, 2012In this paper, in addition to introducing efficient method for bromohydrin and bromoether preparation, simple,green and efficient method to C-N bond formation from alkene and N,N'-Dibromo-N,N'-1,2-ethanediyl-bis(p-toluenesulfonamide) [BNBTS] in water was investigated. The reaction between alkenes, β-cyclodexterin, andBNBTS took place in water afterward, by making media basic; it will give the corresponding valuable buildingblocks in good yields (45-79%).Key Words : C-N bond formation, BNBTS, N-bromo sulfonamide, Bromo hydrin, Building blocksIntroductionThe regioselective electrophilic addition to alkenes is ofimportance process in organic synthesis and several meth-odologies have been described in the literature. The trans-formation of alkenes into the corresponding halohydrins andhaloethers is frequently practiced in organic synthesis.
Bromination of hydrocarbons with CBr 4 , initiated by light-emitting diode irradiation
Beilstein Journal of Organic Chemistry, 2013
The bromination of hydrocarbons with CBr 4 as a bromine source, induced by light-emitting diode (LED) irradiation, has been developed. Monobromides were synthesized with high efficiency without the need for any additives, catalysts, heating, or inert conditions. Action and absorption spectra suggest that CBr 4 absorbs light to give active species for the bromination. The generation of CHBr 3 was confirmed by NMR spectroscopy and GC-MS spectrometry analysis, indicating that the present bromination involves the homolytic cleavage of a C-Br bond in CBr 4 followed by radical abstraction of a hydrogen atom from a hydrocarbon.