Elimination Of HBr Research Articles

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides.

Elimination of HBr from UV-photoexcited vinyl bromides can occur through both 3-centre and 4-centre transition states (TSs). The competition between these pathways is examined using velocity map imaging of HBr (v = 0-2, J) photofragments. The three vinyl bromides chosen for study have methyl substituents that block either the 3-centre or the 4-centre TS, or leave both pathways open. The kinetic energy distributions extracted from velocity map images of HBr from 193 nm photolysis of the three vinyl bromide compounds are approximately described by a statistical model of energy disposal among the degrees of freedom of the photoproducts, and are attributed to dissociation on the lowest electronic state of the molecule after internal conversion. Dissociation via the 4-centre TS gives greater average kinetic energy release than for the 3-centre TS pathway. The resonance enhanced multi-photon ionization (REMPI) schemes used to detect HBr restrict measurements to J ≤ 7 for v = 2 and J ≤ 15 for v = 0. Within this spectroscopic range, the HBr rotational temperature is colder for the 4-centre than for the 3-centre elimination pathway. Calculations of the intrinsic reaction coordinates and RRKM calculations of HBr elimination rate coefficients provide mechanistic insights into the competition between the pathways.

On the Solvent-Dependent Bromination of Dihydroazulenes

Bromination of 1,8a-dihydroazulene-1,1-dicarbonitriles (DHA) followed by elimination of HBr was previously shown to be an important protocol for functionalizing these molecular photoswitches in the seven-membered ring (at C7). Here we show systematically how the outcome of the bromination reaction depends on the electronic character of an aryl substituent at the C2 position of DHA, and the solvent polarity as the reaction can lead to either the addition product or to the corresponding 2-aryl-1-bromo-3-cyanoazulene.

ChemInform Abstract: An Unexpected Double Diels—Alder Reaction of (E)‐2‐Bromo‐4‐aryl‐1,3‐pentadiene Involving [1,5]‐Hydrogen Migration and HBr Elimination: Synthesis of Bicyclo[2.2.2]octene Derivatives.

An unexpected double Diels-Alder (DDA) reaction of (E)-2-bromo-4-aryl-1,3-pentadiene was developed and resulted in a series of butterfly-like bicyclo[2.2.2]octene derivatives in moderate to good yields without the need for a metal catalyst. The proposed mechanism involves a [1,5]-sigmatropic hydrogen migration and HBr elimination. Through this decisive [1,5]-hydrogen shift step, the electronic properties and steric hindrance of the conjugated diene substrate are completely altered and the DDA reaction of this potential diene synthon is successfully achieved.

An unexpected double Diels-Alder reaction of (E)-2-bromo-4-aryl-1,3-pentadiene involving [1,5]-hydrogen migration and HBr elimination: synthesis of bicyclo[2.2.2]octene derivatives.

An unexpected double Diels-Alder (DDA) reaction of (E)-2-bromo-4-aryl-1,3-pentadiene was developed and resulted in a series of "butterfly-like" bicyclo[2.2.2]octene derivatives in moderate to good yields without the need for a metal catalyst. The proposed mechanism involves a [1,5]-sigmatropic hydrogen migration and HBr elimination. Through this decisive [1,5]-hydrogen shift step, the electronic properties and steric hindrance of the conjugated diene substrate are completely altered and the DDA reaction of this potential diene synthon is successfully achieved.

ChemInform Abstract: Annulated Isoxazoles via [3 + 2] Cycloaddition of Alkenyl Bromides and Oximoyl Chlorides and Ag(I) Promoted Elimination.

AbstractIsoxazole heterocycles (VI) appended onto an oxabicyclo[4.4.0]decane ring are synthesized through a sequence involving substitution of 6‐bromo‐1‐cyclohexen‐1‐ol with aromatic aldehydes, conversion to oximoyl chloride intermediates, intramolecular [3 + 2] cycloaddition, and final elimination of HBr.

Stereochemistry of enzymatic transformations of (+)β- and (−)β-HBCD with LinA2 – A HCH-degrading bacterial enzyme of Sphingobium indicum B90A

LinA2, a bacterial enzyme expressed in various Sphingomonadaceae, catalyzes the elimination of HCl from hexachlorocyclohexanes (HCHs) and, as discussed here, the release of HBr from certain hexabromocyclododecanes (HBCDs). Both classes of compounds are persistent organic pollutants now regulated under the Stockholm Convention. LinA2 selectively catalyzes the transformation of β-HBCDs; other stereoisomers like α-, γ-, and δ-HBCDs are not converted. The transformation of (−)β-HBCD is considerably faster than that of its enantiomer. Here, we present the XRD crystal structure of 1E,5S,6S,9R,10S-pentabromocyclododecene (PBCDE) and demonstrate that its enantiomer with the 1E,5R,6R,9S,10R-configuration is the only metabolite formed during LinA2-catalyzed dehydrobromination of (−)β-HBCD. Formation of this product can be rationalized by HBr elimination at C5 and C6. A reasonable enzyme-substrate complex with the catalytic dyad His-73 and Asp-25 approaching the hydrogen at C6 and a cationic pocket of Lys-20, Try-42 and Arg-129 binding the leaving bromine at C5 was found from in silico docking experiments. A second PBCDE of yet unknown configuration was obtained from (+)β-HBCD. We predicted its stereochemistry to be 1E,5S,6S,9S,10R-PBCDE from docking experiments. The enzyme–substrate complex obtained from LinA2 and an activated conformation of (+)β-HBCD allows the HBr elimination at C9 and C10 leading to the predicted product. Both modeled enzyme–substrate complexes are in line with 1,2-diaxial HBr eliminations. In conclusion, LinA2, a bacterial enzyme of the HCH-degrading strain Sphingobium indicum B90A was able to stereoselectively convert β-HBCDs. Configurations of both PBCDE metabolites were predicted by molecular docking experiments and confirmed in one case by XRD data.

Rapid α-Amination of N-Substituted Indoles by Using DBU-Activated N-Haloimides as Nitrogen Sources

By using the <i>N</i>-haloimide/DBU protocol, the electrophilic imidation at C2-position of N-substituted indoles has been achieved in high efficiency. The dual activation of <i>N</i>-haloimide by DBU to simultaneously achieve a more electrophilic bromine and a more nucleophilic nitrogen atom, is demonstrated to be crucial in this transformation. The process involves tandem bromonium ion formation, electrophilic addition, and elimination of HBr. The protocol provides a novel, efficient, green, and complimentary access to α-imidated indoles under mild conditions, without the necessity of external nitrogen sources.

Regio-Specific Polyacetylenes Synthesized from Anionic Polymerizations of Template Monomers

Substituted polyacetylenes with alkylphenyl side groups and head-to-head regioregularity were prepared through anionic living polymerization of template monomers and subsequent dehydrogenation process. The template monomers have the structure of 2,3-disubstituted-1,3-butadienes prepared by palladium-catalyzed Kumada coupling of the corresponding vinyl bromides. Anionic polymerizations of the template monomers produced narrow disperse substituted polybutadiene precursors with exclusive 1,4-enchainment. The precursors were converted into soluble polyacetylene derivatives via two methods, e.g., bromination followed by elimination of HBr, and direct dehydrogenation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), both resulting in dark colored products with significant red shift in UV spectra. The obtained head-to-head polyacetylene derivatives exhibited highly thermal stability, possibly due to trans-rich and/or head-to-head chain configurations. The microstructures of the poly(2,3-dialkylphenyl butadiene...

LinA2, a HCH-converting bacterial enzyme that dehydrohalogenates HBCDs

Hexabromocyclododecanes (HBCDs) and hexachlorocyclohexanes (HCHs) are lipophilic, polyhalogenated hydrocarbons with comparable stereochemistry. Bacterial evolution in HCH-contaminated soils resulted in the development of several Spingomonadaceae which express a series of HCH-converting enzymes. We showed that LinB, a haloalkane dehalogenase from Sphingobium indicum B90A, also transforms various HBCDs besides HCHs. Here we present evidence that LinA2, another dehalogenase from S. indicum also converts certain HBCDs to pentabromocyclododecenes (PBCDEs). Racemic mixtures of α-, β-, γ-HBCDs, a mixture of them, and δ-HBCD, a meso form, were exposed to LinA2. Substantial conversion of (−)β-HBCD was observed, but all other stereoisomers were not transformed significantly. The enantiomeric excess (EE) of β-HBCDs increased up to 60% in 32h, whereas EE values of α- and γ-HBCDs were not affected. Substrate conversion and product formation were described with second-order kinetic models. One major (P1β) and possibly two minor (P2β, P3β) metabolites were detected. Respective mass spectra showed the characteristic isotope pattern of PBCDEs, the HBr elimination products of HBCDs. Michaelis–Menten parameters KM=0.47±0.07μM and vmax=0.17±0.01μmoll−1h−1 were deduced from exposure data with varying enzyme/substrate ratios. LinA2 is more substrate specific than LinB, the latter converted all tested HBCDs, LinA2 only one. The widespread HCH pollution favored the selection and evolution of bacteria converting these compounds. We found that LinA2 and LinB, two of these HCH-converting enzymes expressed in S. indicum B90A, also dehalogenate HBCDs to lower brominated compounds, indicating that structural similarities of both classes of compounds are recognized at the level of substrate-protein interactions.

Solar photo-thermochemical syntheses of 4-bromo-2,5-substituted oxazoles from N-arylethylamides

Solar photo-thermochemical C(sp3)–H bromination, conducted efficiently in a specially designed reactor, was reported recently. In the present study, the more complex formation of 4-bromo-2,5-substituted oxazoles from N-arylethylamides using this approach was achieved. The one-pot syntheses were carried out with N-bromosuccinimide–dichloroethane over 6 h (10.00 am to 4.00 pm) on sunny days. The isolated yields were in the range 42–82%. Benzylic bromination, followed by O–C bond formation through intramolecular nucleophilic substitution, and a second benzylic bromination followed by HBr elimination, gave the oxazole ring. A third bromination of the ring yielded the final product. The feasibility of synthesizing thiazole derivatives using a similar approach was also demonstrated. During the course of the reactions, succinimide and HBr were co-generated in the aqueous phase. Treatment with NaBrO3 and additional acid returned the reagent in 57% isolated yield upon chilling. The overall methodology was greener as a result.

Synthesis and characterization of ether bridged polymers and their fluorescent, thermal, conductivity, optical and electrochemical properties

This paper describes the synthesis of novel ether bridged polymers containing carbazole units via HBr elimination. Also, these polymers include the ether bridged aliphatic chains at different lengths in their structures. The synthesized polymers were characterized by FT-IR, NMR and UV–Vis spectroscopy, and their thermal stabilities were determined by TGA measurements. Additionally, the oxidation peak potentials of compounds were demonstrated using cyclic voltammetry method. Fluorescence measurements were performed using dimethylformamide solution, and the optimization of concentrations to obtain the maximal emission intensity was also investigated in dimethylformamide solution. The morphologic properties of the polymers were obtained by using scanning electron microscopy at different amplifications. Moreover, the number average molecular weight, weight average molecular weight and polydispersity index values of polymers were determined by size exclusion chromatography technique. As a result, some remarkable properties related to the fluorescence and thermal measurements of the polymers were obtained. Because of the fluorescence and thermal properties of these polymers, they could be used in various application fields, such as fluorescence sensors and space crafts.

Case of the Missing Isomer: Pathways for Molecular Elimination in the Photoinduced Decomposition of 1,1-Dibromoethane

We report an experimental and computational study of the photodecomposition pathways of a prototypical gem-dihalide, 1,1-dibromoethane (1,1-EDB), in the condensed phase. Following photolysis of the matrix isolated parent compound in Ar at 5 K, photoproducts are observed corresponding to Br2 elimination (+ C2H4 or C2H2) and HBr elimination (+ vinyl bromide). The elimination products are observed in the matrix as complexes. In contrast to our recent studies of the photolysis of matrix isolated polyhalomethanes, no evidence for the iso-1,1-EDB species is found, although studies of the matrix isolated 1,1-dibromo-2,2,2-trifluoroethane analogue show that the isomer is the dominant photoproduct. These results are examined in the light of theoretical studies that have characterized in detail the 1,1-EDB potential energy surface (PES). For Br2 elimination, a pathway from the isomer on the singlet PES is found which involves a simultaneous Br2 loss with 1,2-hydrogen shift; this pathway lies lower in energy than a concerted three-center elimination from the parent 1,1-EDB. For HBr elimination, our previous theoretical studies [Kalume, A.; George, L.; Cunningham, N.; Reid, S. A. Chem. Phys. Lett. 2013, 556, 35-38] have demonstrated the existence of concerted (single-step) and sequential pathways that involve coupled proton and electron transfer, with the sequential pathway involving the isomer as an intermediate. Here, more extensive computational results argue against a simple radical abstraction pathway for this process, and we compare experimental and computational results to prior results from the photolysis of the structural isomer, 1,2-EDB. These steady-state experiments set the stage for ultrafast studies of the dynamics of this system, which will be important in unraveling the complex photodecomposition pathways operative in condensed phases.

Synthesis and characterization of aromatic and aliphatic ether bridged polymers containing carbazole moieties

Abstract In this study, the polymers were synthesized ether bridged containing carbazole moieties via HBr elimination. Also, these polymers include the aliphatic chains at the different lengths and aromatic groups in their structures. However, the polymers were characterized by FT-IR, NMR and UV–vis spectroscopy, and their thermal stabilities were determined via TGA measurements. Fluorescence measurements were performed using N,N′-dimethyl formamide solutions and also, the optimization of the concentrations to obtain maximal emission intensity was investigated in N,N′-dimethyl formamide. The morphologic properties of the polymers were investigated at different particle sizes by using Scanning Electron Microscopy (SEM). Additionally, the number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) values of the polymers were determined by size exclusion chromatography (SEC) technique. Resultantly, the remarkable properties related to the fluorescence and thermal measurements of the polymers were obtained. Therefore, these polymers could be used in various application fields because of the fluorescent and thermal properties.

One‐Pot Method for Regioselective Bromin­ation and Sequential Carbon–Carbon Bond‐Forming Reactions of Allylic Alcohol Derivatives

AbstractAn efficient one‐pot method for the regioselective bromination of allylic alcohol derivatives (two‐step reaction sequence) followed by Sonogashira, Negishi, or Suzuki–Miyaura coupling reactions in the same reaction vessel (three‐step reaction sequence) has been developed. The key reaction in these one‐pot systems is the regioselective DBU‐promoted trans HBr elimination of vicinal dibromides bearing an adjacent O‐functional group.

Elucidating the Decomposition Mechanism of Energetic Materials with Geminal Dinitro Groups Using 2-Bromo-2-nitropropane Photodissociation

These experiments photolytically generate two key intermediates in the decomposition mechanisms of energetic materials with nitro substituents, 2-nitropropene, and 2-nitro-2-propyl radicals. These intermediates are produced at high internal energies and access a number of competing unimolecular dissociation channels investigated herein. We use a combination of crossed laser-molecular beam scattering and velocity map imaging to study the photodissociation of 2-bromo-2-nitropropane at 193 nm and the subsequent unimolecular dissociation of the intermediates above. Our results demonstrate that 2-bromo-2-nitropropane has four primary photodissociation pathways: C-Br bond fission yielding the 2-nitro-2-propyl radical, HBr elimination yielding 2-nitropropene, C-N bond fission yielding the 2-bromo-2-propyl radical, and HONO elimination yielding 2-bromopropene. The photofragments are formed with significant internal energy and undergo many secondary dissociation events, including the exothermic dissociation of 2-nitro-2-propyl radicals to NO + acetone. Calculations at the G4//B3LYP/6-311++g(3df,2p) level show that the presence of a radical at a nitroalkyl center changes the mechanism for and substantially lowers the barrier to NO loss. This mechanism involves an intermediate with a three-center ring rather than the intermediate formed during the traditional nitro-nitrite isomerization. The observed dissociation pathways of the 2-nitro-2-propyl radical and 2-nitropropene help elucidate the decomposition mechanism of larger energetic materials with geminal dinitro groups.

Synthesis and antifungal activity of C-21 steroids with an aromatic D ring

Six analogues of salpichrolides with a simplified side chain (6–11) were synthesized using a new methodology to obtain steroids with an aromatic D-ring. The key step was the elimination of HBr in a vicinal dibromo D-homosteroid by treatment with 1,4-diazabicyclo[2.2.2]octane (DABCO). All new compounds were completely characterized by 2D NMR techniques and tested on two fungal pathogenic species, Fusarium virguliforme and Fusarium solani.

Halonium-initiated double oxa-cyclization cascade as a synthetic strategy for halogenated furo[3,2-c]pyran-4-ones

The reaction of 1-alkenoylcyclopropane carboxylic acids with NBS or NIS was investigated, which provides an efficient route to biologically important 7-halogenated furo[3,2-c]pyran-4-ones in a one-pot transformation. The major pathway for the formation of the O-O heterocycles was proposed as a halo-oxa-cyclization, HBr elimination, cyclopropane ring-opening and recyclization (intramolecular oxa-cyclization), and bromination cascade. The double-oxa-cyclization represents a novel synthetic strategy towards functionalized furo[3,2-c]pyranones.

Unsaturated Organic Derivatives of Phosphaferrocene – Synthesis and Reactivity of Vinyl‐ and Alkynylphosphaferrocenes

Starting from 2-formyl-3,4-dimethylphosphaferrocene, a series of vinyl-substituted phosphaferrocenes was prepared by Wittig and related olefination reactions. The parent 3,4-dimethyl-2-vinylphosphaferrocene (2) could be selectively converted to either the terminal or branched alcohols 8b and 8a by hydroboration/oxidation or protonation in aqueous solution, respectively. Elimination of HBr from 2-bromovinylphosphaferrocene 9 provided access to the 2-ethynylphosphaferrocene derivative 10, which could be converted to other alkynes by deprotonation and electrophilic quench. Additionally, the syntheses of the 2-cyano- (15) and 2-(ethoxycarbonyl)phosphaferrocene derivative 13 are reported. Several new phosphaferrocenes were converted into their respective Fe(CO)4 complexes, which were investigated by IR spectroscopy. The CO stretching vibrations were found to depend on the nature of the substituent in the 2-position and the cyano compound 15 was identified as the least electron-donating ligand. Reaction of the alkynylphosphaferrocenes 10 and 11 with Co2(CO)8 afforded the respective dimetallatetrahedrane complexes 24 and 25. The molecular structures of compounds 12, 21 and 25 have been determined by X-ray diffraction analysis.

Synthesis of a New 2,3‐Diaminoconduritol with Conduritol F Structure

AbstractA new 2,3‐diaminoconduritol derivative with the conduritol F structure was prepared starting from cyclohexa‐1,4‐diene. Initially, a lactam, prepared by the cycloaddition of chlorosulfonyl isocyanate (CSI) to cyclohexa‐1,4‐diene, was converted into an amino acid. The conversion of the acid functionality into an isocyanate resulted in the formation of an imidazolidinone derivative, formed by an intramolecular cyclization. In the second part of this work, the known anhydride, 3a,4,7,7a‐tetrahydroisobenzofuran‐1,3‐dione was successfully converted into the desired bis(carbamate). The bromination of the double bond in the six‐membered ring followed by a DBU‐induced (DBU = 1,8‐diazabicyclo[5.4.0]undec‐7‐ene) HBr elimination furnished the symmetrical diene. The photooxygenation of the diene unit afforded the bicyclic endoperoxide. The reaction of the endoperoxide with thiourea followed by acetylation resulted in the formation of a syn‐configured diacetate. The deprotection of the urethane and acetate groups gave the new 2,3‐diaminoconduritol with the conduritol F structure.

Ice photolysis of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100): Laboratory investigations using solid phase microextraction

Here, we report for the first time a laboratory investigation into the photochemical degradation of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100) in ice solid samples using an artificial UV light source. Solid phase microextraction (SPME) was used as a sensitive extraction technique for monitoring trace amounts of the hydrophobic pollutant and its photoproducts. The results showed that ice photolysis kinetics for BDE-100 is similar to the one observed in the aqueous counterpart. The eight photoproducts identified consisted of brominated diphenyl ethers with lower bromine content and polybrominated dibenzofurans, suggesting two important photodegradation pathways for BDE-100 in ice solid samples: (i) stepwise reductive debromination and (ii) intramolecular elimination of HBr. Similarities in photochemical product arrays observed in the ice and water photolysis of BDE-100 were attributed to a similar mechanism for photochemical decomposition for both phases. Possible involvement of the water molecules in the reactions has been excluded by performing photolysis in D2O ice solid and water samples. Taking advantage of the high preconcentration factor obtained with SPME at low temperatures, a SPME fiber cooled with liquid carbon dioxide down to 0°C was used as a photoreaction support for BDE-100 allowing the identification of a greater number of photoproducts.