Br Bond Research Articles

Flipping the Switch on Palladium-Catalyzed Carboiodination: Accessing Kinetic and Thermodynamic Products

A palladium-catalyzed epimerization reaction of stereogenic alkyl iodides is reported. This transformation involves use of an air-stable precatalyst that efficiently epimerizes the C–I bond from the exo to the endo face of [2.2.1] bicyclic compounds. Mechanistic experiments support the stereoinversion of the C–I bond via reversible bond formation to generate the antiproduct bearing an endo iodide. Density functional theory studies were conducted to support a thermodynamically driven epimerization. Stoichiometric experiments suggested that irradiation of isolable alkyl–Pd(II) complexes promoted the C–I reductive elimination, which could even be applied to C–Br bond formation.

Ipso Substitution of Aromatic Bromine in Chlorinated Waters: Impacts on Trihalomethane Formation.

Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C-Br bonds into C-Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br-) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters.

Electrochemical Rearrangement for Remote Functionalizations of Unactivated Alkenes.

An electrochemical strategy for the dual functionalizations of unactivated alkenes through an intramolecular migration process was realized in the absence of sacrificial chemical oxidants and noble-metal catalysts under mild reaction conditions. The electrochemistry enabled a (hetero)aryl migration while providing access to alkenyl/alkynyl-migration products. Thus, electricity was employed for the formation of functionalized fluoroalkyl radicals through activation of C-H/C-Br bonds from fluorinated esters. Thereby, we obtained a variety of di and mono-fluorinated alkyl products with good functional group tolerance as well as chemo, and regioselectivities. Likewise, defluorinative allylation of α-carbonyl alkyl bromides proved viable. The reaction mechanism was established by experiments and computations.

Halogen Bond‐Catalyzed Oxidative Annulation of N‐Alkyl Pyridinium Salts and Alkenes with Air as a Sole Oxidant: Metal‐free Synthesis of Indolizines

AbstractThe utilization of ambient air as a sole oxidant is highly desired for the sustainability of chemical industry. Herein, we reported a mild and metal‐free synthesis of indolizines using pyridinium salts and alkenes with ambient air as a sole oxidant. In this reaction, a halogen bond donor (perfluoro‐1‐iodooctane, 10 mol%) was successfully applied to catalyze this desired oxidative [3+2] annulation. The C−I bond of halogen bond donor is crucial for this transformation, while the corresponding C−Br bond and C−F bonds gave much lower yields. Moreover, the phenolic hydroxyl increases the amount of side‐product, probably due to the strong interaction of phenolate anion and the halogen bond donor. Finally, with the rapid development of halogen bond catalysis, this halogen bond‐catalyzed strategy could continuously promote the synthesis of indolizines in a sustainable manner.

Degradation efficiency and mechanism of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) by thermally activated persulfate system

Polybrominated diphenyl ethers (PBDEs) as a typical brominated flame retardant (BFR) have attracted worldwide attention due to the high environmental risk and resistance to conventional remediation processes. In this study, thermally activated persulfate (TAP) process was applied to degrade 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), which is the most toxic and representative PBDEs in e-waste dismantling sites. Impact factors such as PDS dosage, heating temperature, and initial pH were evaluated. Results showed that BDE-47 can be 100% degraded within 180 min under the condition of PDS:BDE-47 = 1000:1, 60 °C, and pH = 7. Quenching experiments combined with EPR analysis further proved the important role of SO4·− in oxidating BDE-47. According to high-resolution mass spectrometry (HRMS) analysis, only one oxidation product of low toxicity was detected during the oxidation process. Theoretical calculations further revealed that the oxidation process mainly involved radical attack at C–Br bond, cleavage of C–Br bond, and fission of ether bond, and HSO4· may also play an important role in BDE-47 degradation in TAP system. In addition, TAP system exhibited universality as all selected PBDE congeners can be degraded, and the degradation rate of PBDEs was greatly affected by the number of substituted Br atoms in a negative trend. Overall, these findings indicate that TAP can be applied as an effective method for removal of PBDEs, and we provide a new insight for the practical application of TAP technology in BDE-47 degradation from experimental and theoretical aspects.

Mechanistic characterization of anaerobic microbial degradation of BTBPE in coastal wetland soils: Implication by compound-specific stable isotope analysis

As a novel brominate flame retardants, 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) has been extensively used in various consumer products, and frequently detected in various environmental matrices. However, the microbial degradation of BTBPE remains unclear in the environment. This study comprehensively investigated the anaerobic microbial degradation of BTBPE and therein stable carbon isotope effect in the wetland soils. BTBPE degradation followed the pseudo-first-order kinetic, with degradation rate of 0.0085 ± 0.0008 day−1. Based on identification of degradation products, stepwise reductive debromination was the main transformation pathway of BTBPE, and tended to keep the stable of 2,4,6-tribromophenoxy group during the microbial degradation. The pronounced carbon isotope fractionation was observed for BTBPE microbial degradation, and carbon isotope enrichment factor (εC) was determined to be −4.81 ± 0.37‰, indicating cleavage of C–Br bond as the rate-limiting step. Compared to previously reported isotope effects, carbon apparent kinetic isotope effect (AKIEC = 1.072 ± 0.004) suggested that the nucleophilic substitution (SN2 reaction) was the potential reaction mechanism for reductive debromination of BTBPE in the anaerobic microbial degradation. These findings demonstrated that BTBPE could be degraded by the anaerobic microbes in wetland soils, and the compound-specific stable isotope analysis was a robust method to discover the underlying reaction mechanisms.

Signatures of the Bromine Atom and Open-Shell Spin Coupling in the X-ray Spectrum of the Bromobenzene Cation

Tabletop X-ray spectroscopy measurements at the carbon K-edge complemented by ab initio calculations are used to investigate the influence of the bromine atom on the carbon core-valence transitions in the bromobenzene cation (BrBz+). The electronic ground state of the cation is prepared by resonance-enhanced two-photon ionization of neutral bromobenzene (BrBz) and probed by X-rays produced by high-harmonic generation (HHG). Replacing one of the hydrogen atoms in benzene with a bromine atom shifts the transition from the 1sC* orbital of the carbon atom (C*) bonded to bromine by ∼1 eV to higher energy in the X-ray spectrum compared to the other carbon atoms (C). Moreover, in BrBz+, the X-ray spectrum is dominated by two relatively intense transitions, 1sC→π* and 1sC*→σ*(C*-Br), where the second transition is enhanced relative to the neutral BrBz. In addition, a doublet peak shape for these two transitions is observed in the experiment. The 1sC→π* doublet peak shape arises due to the spin coupling of the unpaired electron in the partially vacant π orbital (from ionization) with the two other unpaired electrons resulting from the transition from the 1sC core orbital to the fully vacant π* orbitals. The 1sC*→σ* doublet peak shape results from several transitions involving σ* and vibrational C*-Br mode activations following the UV ionization, which demonstrates the impact of the C*-Br bond length on the core-valence transition as well as on the relaxation geometry of BrBz+.

Flow photochemical Giese reaction via silane-mediated activation of alkyl bromides

Organic halides play a key role as building blocks in synthesis because of their low cost and wide availability. In recent years, halogen-atom transfer (XAT) has emerged as a reliable approach to exploit these substrates in radical processes. Herein, we report a hydroalkylation of electron-poor olefins using alkyl bromides based on a UVA-induced silane-mediated XAT reaction. Our protocol is operationally simple, displays a broad scope and does not require a photocatalyst. Flow technology was used to reduce the reaction times and scale the process. Notably, a two-step protocol, combining the XAT protocol with a subsequent Horner-Wadsworth-Emmons reaction, has been developed to enable the allylation of C(sp3)–Br bonds.

CsPbBr3 in the Activation of the C–Br Bond of CBrX3 (X = Cl, Br) under Sunlight

A newly synthesized green-fluorescent, orthorhombic, bromide-rich, perovskite nanocrystal (Φ ∼ 0.93, τ ∼ 12.5 ns, Eox = +1.6 V) obtained from an unprecedented bromide precursor dibromoisocyanuric acid was found to be an excellent visible-light (sunlight or blue-light-emitting diode (LED)) photocatalyst toward the synthesis of gem-dihaloenones for the first time. The photoactivated CsPbBr3 catalyzed the homolytic cleavage of CBrX3 (X = Cl, Br) to generate the •CX3 radical, which underwent cascaded C–C cross-coupling with terminal alkynes into the corresponding gem-dihaloenones. Radical-trapping experiments and luminescence-quenching studies helped establish a single-electron-transfer (SET) mechanism. Interestingly, other highly stable CsPbBr3 NCs, obtained from N-bromosuccinimide (NBS) and dibromohydantoin (DBHT) precursors, are unable to carry out these transformations. These results not only enrich the CsPbBr3 synthetic methodology but also encourage the research community to develop efficient and cost-effective photocatalytic materials.

DC slice ion imaging of the ultraviolet photodissociation of 2-bromohexane

The ultraviolet photodissociation dynamics of 2‑bromohexane around 234 nm has been investigated using the DC slice velocity map imaging technique. Relative quantum yield of the ground state bromine atom is measured to be 0.961. By analyzing the slice images, we can obtain the speed and angular distributions of the ground state and excited state bromine atoms. The speed distribution contains two Gaussian components, which are attributed to two dissociation channels. The high translational energy component is formed via the direct dissociation along the C–Br bond. Nevertheless, the low translational energy component is formed via the multidimensional dissociation along the C–Br bond coupling with the bending modes. The alkyl radical branching promotes the multidimensional dissociation. What’s more, the contribution of the electric-dipole allowed3Q1, 3Q0 and 1Q1 states to the product formation is also discussed. The nonadiabatic 1Q1←3Q0 transition plays a significant role in the dissociation process.

Activation of C–Br Bond of CBr4 and CBrCl3 Using 9-Mesityl-10-methylacridinium Perchlorate Photocatalyst

Herein, we report the activation of the C-Br bond of CBrX3 (X = Cl, Br) using 9-mesityl-10-methylacridinium perchlorate as a visible-light (12W blue LED, 450-455 nm) photocatalyst for the synthesis of gem-dihaloenones from terminal alkynes. An electron transfer from CBrX3 to Mes-Acr-MeClO4 led to the formation of •+CBrX3 which subsequently resulted in the intermediate +CX3. Next, C-C cross-coupling of +CX3 with terminal alkynes was the key path to obtain the gem-dihaloenones. Radical trapping experiments with TEMPO, BHT, and Stern-Volmer quenching studies helped to understand that the reaction proceeded via the SET mechanism.

Carboxylic Acids as Adaptive Functional Groups in Metallaphotoredox Catalysis.

The development of palladium-catalyzed cross-coupling methods for the activation of C(sp2)-Br bonds facilitated access to arene-rich molecules, enabling a concomitant increase in the prevalence of this structural motif in drug molecules in recent decades. Today, there is a growing appreciation of the value of incorporating saturated C(sp3)-rich scaffolds into pharmaceutically active molecules as a means to achieve improved solubility and physiological stability, providing the impetus to develop new coupling strategies to access these challenging motifs in the most straightforward way possible. As an alternative to classical two-electron chemistry, redox chemistry can enable access to elusive transformations, most recently, by interfacing abundant first-row transition-metal catalysis with photoredox catalysis. As such, the functionalization of ubiquitous and versatile functional handles such as (aliphatic) carboxylic acids via metallaphotoredox catalysis has emerged as a valuable field of research over the past eight years.In this Account, we will outline recent progress in the development of methodologies that employ aliphatic and (hetero)aromatic carboxylic acids as adaptive functional groups. Whereas recent decarboxylative functionalization methodologies often necessitate preactivated aliphatic carboxylic acids in the form of redox-active esters or as ligands for hypervalent iodine reagents, methods that enable the direct use of the native carboxylic acid functionality are highly desired and have been accomplished through metallaphotoredox protocols. As such, we found that bench-stable aliphatic carboxylic acids can undergo diverse transformations, such as alkylation, arylation, amination, and trifluoromethylation, by leveraging metallaphotoredox catalysis with prevalent first-row transition metals such as nickel and copper. Likewise, abundant aryl carboxylic acids are now able to undergo halogenation and borylation, enabling new entry points for traditional, primarily palladium- or copper-catalyzed cross-coupling strategies. Given the breadth of the functional group tolerance of the employed reaction conditions, the late-stage functionalization of abundant carboxylic acids toward desired targets has become a standard tool in reaction design, enabling the synthesis of various diversified drug molecules. The rapid rise of this field has positively inspired pharmaceutical discovery and will be further accelerated by novel reaction development. The achievement of generality through reaction optimization campaigns allows for future breakthroughs that can render protocols more reliable and applicable for industry. This article is intended to highlight, in particular, (i) the employment of aliphatic and (hetero)aryl carboxylic acids as powerful late-stage adaptive functional handles in drug discovery and (ii) the need for the further development of still-elusive and selective transformations.We strongly believe that access to native functionalities such as carboxylic acids as adaptive handles will further inspire researchers across the world to investigate new methodologies for complex molecular targets.

Oxidative addition of 8-bromo-9-ethyl-1,N6-ethenoadenine to d10 metals

The 8-bromoadenine derivative 8-bromo-9-ethyl-1,N6-ethenoadenine 3 was prepared from adenine in three steps. Treatment of 8-bromo-9-ethyl-1,N6-ethenoadenine 3 with the zero-valent metal precursors [M0(PPh3)4] (M = Pd, Pt) in the presence of NH4BF4 as proton source gave exclusively the compounds trans-[4]BF4 and trans-[5]BF4. The obtained complexes bear a modified C8-bound adenine derivative with a protonated endocyclic nitrogen atom N7. A stepwise reaction sequence consisting of an initial oxidative addition of the C8–Br bond of 3 to [Pt0(PPh3)4] followed by treatment with HBF4·Et2O as proton source led to the dinuclear complex [6]2+ after a reaction time of several days. This complex features two adenine-derived NHC ligands coordinating to two PtII ions in a µ-κC8:κN7 bridging fashion. Protonation of the NHC ligands occurred at the nitrogen atom N6 instead of N7 of the purine scaffold. The molecular structures of the three complexes were determined by X-ray diffraction studies.

C−Br Bond Activation of Bromoalkanes with Gallanediyl LGa

AbstractMonovalent group 13 diyls are promising reagents for C−X bond activation. We here report on C−Br bond activation reactions of gallanediyl LGa (L=HC[C(Me)NAr)]2, Ar=2,6‐i‐Pr2C6H3)) with a variety of bromoalkanes. Reactions with alkylbromides R−Br gave the corresponding LGa(Br)R (R=Et 1, n‐Pr 2, i‐Pr 3), while the reaction with dibromomethane in 2 : 1 molar ratio occurred with geminal C−Br bond addition and formation of [LGa(Br)]2CH2 (4). In contrast, equimolar reaction of LGa with CHBr3 yielded LGa(Br)CHBr2 (5), which reacted with another equiv. of LGa with decomposition and formation of LGaBr2. Compounds 1–5 were characterized by heteronuclear (1H, 13C) NMR and IR spectroscopy, as well as single‐crystal X‐ray diffraction (sc‐XRD).

Theoretical study on pyrolysis mechanism of decabromodiphenyl ether (BDE-209) using DFT method

Decabromodiphenyl ether (BDE-209), as a brominated flame retardant (BFR), is widely applied to various consumer products due to its superior performance and affordable pricing to improve the flame resistance of materials. To better comprehend the pyrolysis behavior of BDE-209 and the evolution process of main pyrolysis products, the thermal degradation mechanism of BDE-209 was studied using density functional theory (DFT) method at the theoretical level of M06/cc-pVDZ, and thermodynamic parameters were calculated in this paper. Unimolecular degradation was dominated by cleavage of the ether linkage, which results in a high yield of hexabromobenzene, and fission of the ortho-position C–Br bond is the main competitive reaction channel. In the system of BDE-209 + H, the pyrolysis reaction is majorly characterized by debromination, leading to the formation of considerable HBr and low-brominated diphenyl ethers. Additionally, the hydrogen-derived splitting of the ether bond acts as a mainly competitive channel, which is the source of polybromophenols and polybromobenzenes. The formation of polybrominated dibenzofuran (PBDF) derives from the cyclization reaction of ortho-phenyl-type radicals, which are readily generated through the ortho-position Br atom abstraction by H radical. The formation of polybrominated dibenzo-p-dioxin (PBDD) involves the ortho-C-O coupling reaction of polybromophenoxy radicals, debromination reaction, and cyclization reaction. And the total yield of PBDD/Fs was significantly increased when H was involved. Results presented in this work will provide the helpful information for the treatment and reuse of BDE-209-containing waste plastics through using pyrolysis technology.

Iron-Catalyzed Cross-Electrophile Coupling of Inert C–O Bonds with Alkyl Bromides

Iron-Catalyzed Cross-Electrophile Coupling of Inert C–O Bonds with Alkyl Bromides

Structural characterization of zinc and cadmium complexes derived from N-(4-carboxybenzyl)pyridinium: Revisiting the structure of (Cbp)2ZnBr2 and influence of the metal on carboxylate coordination mode

The molecular structures of the zinc and cadmium compounds, (Cbp)2ZnBr2 and (Cbp)2CdBr2, which feature a zwitterionic ligand derived from N-(4-carboxybenzyl)pyridinium, have been determined by using X-ray diffraction. Comparison of the structures of (Cbp)2ZnBr2 and (Cbp)2CdBr2 demonstrates that whereas the zinc compound adopts unidentate coordination of the carboxylate ligand, the cadmium counterpart exhibits anisobidentate coordination. The Zn–Br [2.4339(8) Å] and Cd–Br [2.6357(6) Å] bond lengths differ by 0.20 Å, which is in accord with the difference in covalent radii of zinc and cadmium (0.22 Å); as such, it suggests that the previous structure of “(Cbp)2ZnBr2”, which possesses a Zn–Br bond length of 2.635(2) Å, actually corresponds to that of (Cbp)2CdBr2.

Synthesis of 3‐Amino‐1‐(difluoromethylidene)tetralins via Relay Photocatalytic Cascade Reactions of Arylalanines and 2‐Bromo‐3,3,3‐trifluoropropene

AbstractA relay photocatalytic cascade reaction for the synthesis of 3‐amino‐1‐(difluoromethylidene)tetralins was described. The reaction used readily available N‐Boc protected arylalanines and bulk industrial chemical 2‐bromo‐3,3,3‐trifluoropropene (BTP) as the starting materials combining two photoredox catalytic cycles with a single photocatalyst. In the first catalytic cycle, the defluorinative coupling of N‐Boc arylalanines with BTP provided 2‐bromo‐1,1‐difluoroalkenes. In the second catalytic cycle, 2‐bromo‐1,1‐difluoroalkenes underwent intramolecular radical cyclization by photoredox activation of C(sp2)−Br bond, affording the desired 3‐amino‐tetralins bearing an exo‐difluoroethylene unit in good yields. Moreover, the reactions of 3‐aryl propanoic acids and BTP for the construction of 1‐(difluoromethylidene)tetralins were also demonstrated.magnified image

Optimization and mechanism of Tetrabromobisphenol A removal by dithionite under anaerobic conditions: Response surface methodology and degradation pathway

In this study, dithionite (DTN) was used to degrade Tetrabromobisphenol A (TBBPA), a widely applied brominated flame retardants, under anaerobic conditions with the reaction terminator of nitrate. The optimization of reaction parameters including TBBPA concentration, DTN concentration and pH value were conducted by response surface methodology (RSM) based on central composite design (CCD). The degradation process could be simulated accurately by a quadratic model with the correlation coefficient R2 of 0.9550. The interaction between pH and DTN concentration was significant with the p-value of 0.0017. Moreover, the maximum TBBPA removal was 87.6 ± 3.2% and obtained at TBBPA concentration of 2.00 μM, the DTN concentration of 322.31 μM, and the pH of 6.14 under anaerobic conditions. It was found that the factors influenced TBBPA removal followed the order: pH > DTN concentration > TBBPA concentration. The major active products from DTN are SO32− and S2O32−. In addition, different inhibitions of natural water matrix including chloride, bicarbonate, sulfide and humic acid on TBBPA degradation had been confirmed. According to the identified six intermediates via gas chromatography-mass spectrometry (GC-MS), two steps of the degradation pathways were speculated, including the breakage of C–Br bond and C–C bond. This study provides a convenient way to degrade TBBPA.

Pressure-Driven Phase Transition in Two-Dimensional Perovskite MHy2PbBr4

The application of high pressure allows tuning physicochemical properties of materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies of a two-dimensional (2D) perovskite, MHy2PbBr4, comprising a very small spacer cation (methylhydrazinium, MHy+). This crystal exhibits highly desired ferroelectric and extraordinary multiple linear and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction shows that MHy2PbBr4 undergoes an unusual Pmn21 → P21 phase transition near 4 GPa, associated with the extrusion of some MHy+ cations from the interlayer space into voids located within the inorganic sheets, not reported for any 2D hybrid perovskite. The transport of counter cations leads to a significant increase of Pb–NH2 interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa–1 within the layers. The Raman data confirm the association of the phase transition with strong distortion of the crystal structure and reorganization of the hydrogen bond network, while the absorption spectra of the compressed ambient-pressure Pmn21 phase show the band gap narrowing, followed by its widening in the high-pressure P21 phase. A similar change in the pressure dependence from a red shift to a blue shift is also observed for the free-exciton (FE) photoluminescence (PL). Furthermore, the pressure-induced phase transition leads to a giant enhancement of PL intensity, especially pronounced for the broad-band emission attributed to the self-trapped excitons (STEx). We attribute the effects, observed in absorption and PL spectra, to the shortening of Pb–Br bonds in the ambient pressure phase and increased distortion of the inorganic layers and tilts of PbBr6 octahedra in the high-pressure phase. Overall, our results for a 2D hybrid compound comprising very small spacer cations extend the understanding of the pressure effect on the properties of 2D hybrid perovskites in general and demonstrate a very different behavior under compression compared to the analogues with large organic cations. They revealed that the structure–strain mechanism can be used for engineering new high-pressure phases with unusual structural, mechanical, and optoelectronic properties.