Cleavage Of Carbon-bromine Bond Research Articles

Raman Monitoring of the Electro-Optical Synergy-Induced Enhancements in Carbon-Bromine Bond Cleavage, Reaction Rate, and Product Selectivity of p-Bromothiophenol.

Electro-optical synergy has recently been targeted to improve the separation of hot carriers and thereby further improve the efficiency of plasmon-mediated chemical reactions (PMCRs). However, the electro-optical synergy in PMCRs needs to be more deeply understood, and its contribution to bond dissociation and product selectivity needs to be clarified. Herein, the electro-optical synergy in plasmon-mediated reduction of p-bromothiophenol (PBTP) was studied on a plasmonic nanostructured silver electrode using in situ Raman spectroscopy and theoretical calculations. It was found that the electro-optical synergy-induced enhancements in the cleavage of carbon-bromine bonds, reaction rate, and product selectivity (4,4'-biphenyl dithiol vs thiophenol) were largely affected by the applied bias, laser wavelength, and laser power. The theoretical simulation further clarified that the strong electro-optical synergy is attributed to the matching of energy band diagrams of the plasmonic silver with those of the adsorbed PBTP molecules. A deep understanding of the electro-optical synergy in PBTP reduction and the clarification of the mechanism will be highly beneficial for the development of other highly efficient PMCRs.

Carbon-bromine bond cleavage – A perspective from bromine and carbon kinetic isotope effects on model debromination reactions

In this work, we explore the effect of solvent on 13C and 81Br kinetic isotope effects (KIEs) during elimination of bromine substituent from brominated organic compounds promoted by hydroxyl anion. In the present study, we investigate HBr elimination from 2-bromoethylbenzene in three different polar media (water, ethanol, and acetonitrile) as well as bromide ion elimination from 1,2-dibromoethane upon nucleophilic substitution by the hydroxyl ion in aqueous solution using carbon and bromine isotope analysis as mechanistic tools. We reconsider the hypothesis that the magnitude of leaving group halide KIE should visibly depend on the solvent and bond-breaking in a protic solvent should be accompanied by hydrogen bonding which would result in less zero-point energy loss than in an aprotic solvent. Modeling the elimination reaction using the available popular theoretical methods along with different approaches for including environment effects we demonstrate in the presented study no interpretable effect of the solvent on the transition state structure and hence on the theoretically predicted KIEs. The comparison of the magnitudes of carbon and bromine kinetic isotope effects for two different mechanistic pathways (elimination vs substitution) is also discussed.

Base-Promoted Vinyl Carbon–Bromine Bond Cleavage by Group 9 Metalloporphyrin Complexes

Base-promoted vinyl carbon–bromine bond cleavage of styryl bromide by group 9 metalloporphyrin complexes was achieved to give the metal(III) porphyrin styryls M(ttp)(styryl) (ttp = 5,10,15,20-tetra-p-tolylporphyrinato dianion). Mechanistic studies suggest that [MII(ttp)]2 (M = Rh, Ir) cleaves the vinyl C–Br bond via an addition–elimination mechanism. The much less reactive CoII(ttp) undergoes a radical recombination with styryl radical which is generated by the hydroxide reduction of styryl bromide to give a radical anion with subsequent C–Br cleavage.

Steric Effects in the Reaction of Electrogenerated Ligand-Reduced Nickel Salen with Organic Halides

Nickel salen catalysts have been extensively used for the electrochemical reduction of various organic halides. In general, nickel(II) salen (1) would undergo one-electron reduction to generate either the metal-reduced nickel(I) salen (2) or the ligand-centered radical-anion (3, Scheme 1), which can subsequently transfer an electron to the organic halide substrate to produce a radical and a halide ion (1-3). In this catalytic process, the ligand-reduced nickel salen (3) may also react with alkyl halide to form a nickel(II) salen that is alkylated at the imino bonds of the ligand (2,3). Nevertheless, when the sterically hindered dimethylated nickel(II) salen is employed as the catalyst, the corresponding alkylation reaction could be minimized (4).In this study, nickel salen-catalyzed reduction of benzyl bromide and 1-bromomethylnaphthalene was examined by cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) for comparison. Furthermore, 1-bromooctane and bromodiphenylmethane were also used as the substrates for the reaction with electrochemically reduced nickel salen species (2 and 3). The results show that the reaction of ligand-reduced nickel salen (3) with organic halides can be significantly affected by the substrate structure due to steric effects.Fig. 1 depicts the CVs recorded at a scan rate of 100 mV s–1 for the reduction of nickel(II) salen in the presence of four different concentrations of benzyl bromide in dimethylformamide (DMF) containing 0.050 M tetramethylammonium tetrafluoroborate (TMABF4). As shown by curves B-E, the first cathodic peak is approximately 100-150 mV more positive than that for reduction of nickel(II) salen by itself (curve A), attributed to the catalytic cleavage of carbon-bromine bond of the substrate. The magnitude of this peak increases with the substrate concentration and eventually it merges with the reduction peak of nickel salen (second cathodic wave at -1.70 V). Moreover, the anodic peak due to oxidation of nickel(I) salen disappears. These features are generally characteristic of the catalytic reduction of organic halides (3,4). Large scale electrolysis of 20 mM benzyl bromide with 2.0 mM nickel salen at reticulated vitreous carbon electrodes held at -1.55 V gives toluene, bibenzyl, and benzyl ether as the major products in a total yield of 94%, which is higher than that for the catalytic reduction of 1-bromooctane (86%) (5), suggesting the reaction of 3 with benzyl bromide should be less prominent than with 1-bromooctane to form the alkylated nickel salen. Similar CV and CPE results were obtained for the catalytic reduction of 1-bromomethylnaphthalene, mostly due to steric effect caused by the bulky substrate.Additionally, the reactivity of electrogenerated ligand-reduced nickel salen (3) towards the four organic halides follows the sequence of 1-bromooctance > benzyl bromide > 1-bromomethylnaphthalene > bromodiphenylmethane, as revealed by Fig. 2. The alkylation of 3 diminishes as the substrate molecules turn to be more bulky and therefore sterically hinder the corresponding nucleophilic reactions (4,5). ReferencesMiranda, J. A.; Wade, C. J.; Little, R. D. J. Org. Chem. 2005, 70, 8017-8026.Goken, D. M.; Peters, D. G.; Karty, J. A.; Reilly, J. P. J. Electroanal. Chem. 2004, 564, 123-132.Raess, P. W,; Mubarak, M. S.; Ischay, M. A.; Foley, M. P.; Jennermann, T. B.; Raghavachari, K.; Peters, D. G. J. Electroanal. Chem. 2007, 603, 124-134.Foley, M. P.; Du, P.; Griffith, K. J.; Karty, J. A.; Mubarak, M. S.; Raghavachari, K.; Peters, D. G. J. Electroanal. Chem. 2010, 647, 194-203.Ji, C.; Day, S. E.; Silvers, W. C. J. Electroanal. Chem. 2008, 622, 15-21.

Base-Promoted Aryl Carbon–Iodine and Carbon–Bromine Bond Cleavage with Rhodium Porphyrin Complexes: Scope and Mechanism

Base-promoted aryl carbon–iodine and carbon–bromine bond (Ar–X, X = I, Br) cleavage by rhodium porphyrin complexes was achieved to give the rhodium(III) porphyrin aryls Rh(ttp)Ar (ttp = tetra-p-tolylporphyrinato dianion). Mechanistic studies showed that RhII2(ttp)2 is the intermediate for Ar–X (X = I, Br) cleavage. The Ar–X cleavage process goes through a rhodium(II) porphyrin radical mediated ipso-substitution mechanism.

Photoinduced electron-transfer reaction of α-bromomethyl-substituted benzocyclic β-keto esters with amines: selective reaction pathways depending on the nature of the amine radical cations

Photoinduced electron-transfer reaction of α-bromomethyl-substituted benzocyclic β-keto esters with tertiary amines was investigated. Debrominated β-keto esters and ring-expanded γ-keto esters were obtained as major products. On the basis of mechanistic experiments it was concluded that these products are formed via a reaction sequence of selective carbon–bromine bond cleavage and subsequent competitive hydrogen abstraction and Dowd–Beckwith ring-expansion of the resulting primary alkyl radicals. The characteristic product distribution observed for the type of amine used is rationalized on the basis of selective reaction pathways of generated radical intermediates that depend on the nature of the amine radical cations.

Some Reactions of gem‐Dibromocyclopropanes and Metal Carbonyls

AbstractA series of gem‐dibromocyclopropanes were treated with various metal complexes. Among the metal complexes, Ru(CO)2(PPh3)3, Ru(CO)3(PPh3)2, and Mo(CO)6 were able to remove a bromine atom from 1,1‐dibromo‐2‐phenylcyclopropanes (1) to yield a series of corresponding of 1‐bromo‐2‐phenylcyclopropanes (2). Upon the treatment of 1 with Cr(CO)6 in DMSO, a series of allenes were obtained in good yields. The correlation between the rate of formation of allenes and the substituents on the benzene gives a negative coefficient which suggests the dibromocyclopropanes possesses as an electrophile toward to Cr(CO)6. In the presence of Cr(CO)6, gem‐dibromobicyclo[n,1,0]alkanes (4) in DMF or DMSO solution underwent the cleavage of carbon‐bromine bond followed by ring‐expansion and coupling reaction to form bicycloalkenes 7.

Carbon–Chlorine and Carbon–Bromine Bond Cleavage in the Catalytic Hydrodehalogenation of Halogenated Aromatics

The catalytic hydrodehalogenation of chlorobenzene (CB), bromobenzene (BB), CB/BB mixtures, and the three chlorobromobenzene (CBB) isomers was studied over the temperature range 473 K≤T≤603 K using Ni/SiO2 where the Ni loading was varied from 6.2 to 15.2% wt/wt. Each catalyst was 100% selective in terms of hydrodehalogenation and there was no evidence of any aromatic ring reduction. Steady-state conversion was readily achieved but there was a decided decline in activity with prolonged catalyst use. The catalyst samples were characterized, before and after use, by TEM, CO chemisorption/TPD, TPO, and potentiometric analysis. We recorded the levels of reversibly and irreversibly held Cl and/or Br on the spent catalysts. Appreciable Ni particle growth during catalysis was observed and this is attributed to a halide-induced metal agglomeration, while CO TPD revealed a significant disruption to the Ni particle electronic structure. Long-term deactivation is ascribed to a less effective activation of the haloarene and/or hydrogen reactant(s) allied to a halide-promoted carbonaceous deposition; the surface coke is characterized as predominantly amorphous. Reactivity increased in the order 2-CBB<3-CBB<4-CBB<BB<CB, which is taken to be diagnostic of an electrophilic mechanism where the presence of a second electron withdrawing a halogen substituent lowers the overall dehalogenation rate and steric hindrance governs reactivity in the case of the CBB isomers. In the hydrotreatment of CB/BB mixtures and CBB isomers, the fractional debromination increased due to a surface hydrochlorination and Cl exchange with the Br substituent on the aromatic ring. The reaction orders with respect to CB, BB, and H2 were determined, while the experimentally determined rate/pressure profiles were subjected to standard Langmuir–Hinshelwood kinetic modeling. The best overall (meaningful) fit to the experimental data was achieved with a model based on the associative adsorption of the haloarene and the dissociative adsorption of hydrogen on the same sites with no product inhibition.

Carbon–Chlorine and Carbon–Bromine Bond Cleavage in the Catalytic Hydrodehalogenation of Halogenated Aromatics

The catalytic hydrodehalogenation of chlorobenzene (CB), bromobenzene (BB), CB/BB mixtures, and the three chlorobromobenzene (CBB) isomers was studied over the temperature range 473 K≤ T≤603 K using Ni/SiO 2 where the Ni loading was varied from 6.2 to 15.2% wt/wt. Each catalyst was 100% selective in terms of hydrodehalogenation and there was no evidence of any aromatic ring reduction. Steady-state conversion was readily achieved but there was a decided decline in activity with prolonged catalyst use. The catalyst samples were characterized, before and after use, by TEM, CO chemisorption/TPD, TPO, and potentiometric analysis. We recorded the levels of reversibly and irreversibly held Cl and/or Br on the spent catalysts. Appreciable Ni particle growth during catalysis was observed and this is attributed to a halide-induced metal agglomeration, while CO TPD revealed a significant disruption to the Ni particle electronic structure. Long-term deactivation is ascribed to a less effective activation of the haloarene and/or hydrogen reactant(s) allied to a halide-promoted carbonaceous deposition; the surface coke is characterized as predominantly amorphous. Reactivity increased in the order 2-CBB<3-CBB<4-CBB<BB<CB, which is taken to be diagnostic of an electrophilic mechanism where the presence of a second electron withdrawing a halogen substituent lowers the overall dehalogenation rate and steric hindrance governs reactivity in the case of the CBB isomers. In the hydrotreatment of CB/BB mixtures and CBB isomers, the fractional debromination increased due to a surface hydrochlorination and Cl exchange with the Br substituent on the aromatic ring. The reaction orders with respect to CB, BB, and H 2 were determined, while the experimentally determined rate/pressure profiles were subjected to standard Langmuir–Hinshelwood kinetic modeling. The best overall (meaningful) fit to the experimental data was achieved with a model based on the associative adsorption of the haloarene and the dissociative adsorption of hydrogen on the same sites with no product inhibition.

Repositioning the catalytic triad aspartic acid of haloalkane dehalogenase: effects on stability, kinetics, and structure.

Haloalkane dehalogenase (DhlA) catalyzes the hydrolysis of haloalkanes via an alkyl-enzyme intermediate. The covalent intermediate, which is formed by nucleophilic substitution with Asp124, is hydrolyzed by a water molecule that is activated by His289. The role of Asp260, which is the third member of the catalytic triad, was studied by site-directed mutagenesis. Mutation of Asp260 to asparagine resulted in a catalytically inactive D260N mutant, which demonstrates that the triad acid Asp260 is essential for dehalogenase activity. Furthermore, Asp260 has an important structural role, since the D260N enzyme accumulated mainly in inclusion bodies during expression, and neither substrate nor product could bind in the active-site cavity. Activity for brominated substrates was restored to D260N by replacing Asn148 with an aspartic or glutamic acid. Both double mutants D260N+N148D and D260N+N148E had a 10-fold reduced kcat and 40-fold higher Km values for 1,2-dibromoethane compared to the wild-type enzyme. Pre-steady-state kinetic analysis of the D260N+N148E double mutant showed that the decrease in kcat was mainly caused by a 220-fold reduction of the rate of carbon-bromine bond cleavage and a 10-fold decrease in the rate of hydrolysis of the alkyl-enzyme intermediate. On the other hand, bromide was released 12-fold faster and via a different pathway than in the wild-type enzyme. Molecular modeling of the mutant showed that Glu148 indeed could take over the interaction with His289 and that there was a change in charge distribution in the tunnel region that connects the active site with the solvent. On the basis of primary structure similarity between DhlA and other alpha/beta-hydrolase fold dehalogenases, we propose that a conserved acidic residue at the equivalent position of Asn148 in DhlA is the third catalytic triad residue in the latter enzymes.

Facile new method for synthezising N-polyfluoroalkylated heterocycles – molecular structure of N-(bromodifluoromethyl)-4-dimethylaminopyridinium bromide

Carbon-bromine bond cleavage is observed when 4-dimethylaminopyridine is treated with CF2Br2 and BrCF2CF2Br to yield N-(bromodifluoromethyl) and N-(2-bromo-1,1,2,2-tetrafluoromethyl)pyridinium bromides, 1-Br and 2-Br, which are reductively debrominated using Bu3SnH and fluorinated by anhydrous Me4N+F–; the molecular structure of 1-Br is determined by single crystal X-ray crystallography to reveal a partial quinoidal character in the pyridine system and a hypervalent Br–⋯BrCF2 pairing.

Palladium-catalysed hydrostannylations of 1-bromoalkynes. A practical synthesis of (E)-1-stannylalk-1-enes

A practical synthesis of (E)-1-stannylalk-1-enes containing a range of oxygen and nitrogen functionality is highlighted, involving hydrostannylation followed by palladium-catalysed carbon–bromine bond cleavage reactions of 1-bromoalkynes.

Pulsed laser photolysis of bis(2-bromobenzoyl) peroxide. Structure of 2-bromobenzoyloxyl radicals and carbon-bromine bond cleavage

On pulsed laser photolysis of bis(2-bromobenzoyl) peroxide (2-BrBPO) a unique absorption band was observed around 400 nm as well as a broad band due to 2-BrC 6H 4CO 2 · radicals in the 500–800 nm region. The 400 nm band was also attributed to 2-BrC 6H 4CO 2 · on the basis of the similarity of both absorption in time profiles and behavior towards quenchers. Time-resolved EPR measurements provided clear evidence for some interaction between the CO 2 · radical center and the ortho-bromine atom in 2-BrC 6H 4CO 2 · and for the formation of ortho-substituted phenyl radicals, 2-XC 6H 4 by way of CBr bond cleavage in excited state 2-BrBPO.

Tandem radical cyclisations : Synthesis of lysergic acid derivatives

A novel free radical cyclisation approach for the synthesis of lysergic acid analogues has been investigated. The homolytic cleavage of carbon-bromine bond, mediated by tri-n-butyltin hydride, led to the development of a method for the construction of 3,4-disubstituted dihydroindoles via single cyclisation; hexahydrobenz[cd]indoles via double tandem cyclisations and both octahydroindolo[6,5,4-cd]indoles and decahydroindolo[4,3-fg]quinolines via triple radical cyclisations. A successful tandem double 5-exo-trig,6-endo-trig cyclisation of aryl radical generated from N-3-[3-(N-acetyl-N-allylamino)-2-bromophenyl]-5-(carbomethoxy)-1,4,5,6-tetrahydro-N-methylpyridine afforded methyl 1-acetyl-2,3,9,10-tetrahydrolysergate.

Electrooxidation of bromoalkanes on platinum in liquid hydrogen fluoride. Selective displacement of bromine by fluorine

Carbon—bromine bond cleavage was investigated in HF as a privileged route to the formation of electrophilic carbon ions and subsequent obtention of alkyl fluorides. Nucleophilic attack of fluoride ions on electrochemically generated radical cations, R—Br + , or carbenium ions, R +, proved selective and quantitative in the case of bromomethane, 1,2-dibromoethane, 1,2-dibromopropane, 1,3-dibromo propane and 1,2,3-tribromopropane. For bromoethane, 1-(or 2-)bromopropane, 1-(or 2-)bromobutane, 1-bromo-2-methyl propane and 1-bromo-3-methyl butane, the observed lack of halogenation selectivity is discussed in terms of radical side reactions.

ChemInform Abstract: CARBON‐BROMINE BOND CLEAVAGE BY ELECTROOXIDATION OF ALKYL BROMIDES

Chemischer InformationsdienstVolume 8, Issue 21 Article ChemInform Abstract: CARBON-BROMINE BOND CLEAVAGE BY ELECTROOXIDATION OF ALKYL BROMIDES J. Y. BECKER, J. Y. BECKERSearch for more papers by this authorM. MUENSTER, M. MUENSTERSearch for more papers by this author J. Y. BECKER, J. Y. BECKERSearch for more papers by this authorM. MUENSTER, M. MUENSTERSearch for more papers by this author First published: May 24, 1977 https://doi.org/10.1002/chin.197721081Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume8, Issue21May 24, 1977 RelatedInformation

Carbon-bromine bond cleavage by electrooxidation of alkyl bromides

Carbon-bromine bond cleavage by electrooxidation of alkyl bromides