Br Bond Research Articles

Chiral Iridium Complexes of Anionic NCP Pincer Ligand for Asymmetric Transfer Hydrogenation of 1,1-Diarylethenes with Ethanol.

Chiral iridium complexes ligated by anionic oxazoline-bearing NCP-type pincer ligands were developed and applied to the asymmetric transfer hydrogenation (ATH) of diarylethenes using environmentally benign ethanol as the hydrogen donor. High enantioselectivities could be achieved for substrates bearing ortho-Me, ortho-Cl, or ortho-Br substituents on one of the aryl groups. The ATH of ortho-Br-substituted diarylethenes is particularly attractive due to the propensity of the C(aryl)-Br bond to undergo various new bond-forming events.

Multiphoton-excited dynamics of the trans or cis structural isomer of 1,2-dibromoethylene.

Photofragmentation dynamics of cis and trans isomers of 1,2-dibromoethylene (1,2-DBE) have been investigated by multiphoton excitation using a picosecond (ps) laser pulse. It has been found that the Br2 + product ion preferentially originates from the cis isomer rather than from trans. The Boltzmann-type isotropic low kinetic energy components of the Br+ and Br2 + product state distributions seem to be most likely from the unimolecular reactions of the vibrationally hot cationic ground state generated by the three-photon absorption at the photon energy below ∼38 000cm-1. The highly anisotropic kinetic energy components of Br+ and Br2 + start to appear at the photon energy above ∼38 000cm-1, where the Dn (n ≥ 1) - D0 transition is facilitated within the same ps laser pulse as the parent molecule is efficiently ionized by the two-photon absorption. The transition dipole moment of the D4 - D0 transition of the strongest oscillator strength has been theoretically predicted to be parallel to the C-Br bond or C=C bond axis for the trans or cis isomer, respectively. The fast anisotropic with the (β ∼ +2) component in the Br+ product distribution is thus likely from the trans isomer, whereas that of Br2 + (β ∼ -0.5) should be the consequence of the photodissociation of the cis isomer. The isomer-specific reactivity found here in the picosecond multiphoton excitation of 1,2-DBE provides a nice platform for the better understanding of the structure-reactivity relationship under the harsh condition of the strong or ultrashort optical field.

Probing the Role of Local Structure in Driving the Stability of Halide Perovskites CH3NH3PbX3

Organic–inorganic lead halide perovskites have rapidly emerged as one of the leading contenders in optoelectronic and photovoltaic technology, but they show stability issues. Among methylammonium (MA) lead bromide and MA lead iodide, the Br counterpart is reported to exhibit improved stability under ambient conditions. However, the underlying fundamentals of such an enhancement are not fully understood. Using temperature-dependent X-ray diffraction and extended X-ray absorption fine structure measurements, we find relatively reduced value of linear coefficient of thermal expansion, elevated Einstein temperature, and reduced dynamic disorder in MAPbBr3 compared to MAPbI3 which implies that a relatively rigid Pb–Br bond offers steric hindrance to the migration of bulky MA ion resulting in improved stability of MAPbBr3.

Functional Porphyrinic Metal–Organic Framework as a New Class of Heterogeneous Halogen‐Bond‐Donor Catalyst

AbstractBiomimetic metal‐organic frameworks have attracted great attention as they can be used as bio‐inspired models, allowing us to gain important insights into how large biological molecules function as catalysts. In this work, we report the synthesis and utilization of such a metal‐metalloporphyrin framework (MMPF) that is constructed from a custom‐designed ligand as an efficient halogen bond donor catalyst for Diels–Alder reactions under ambient conditions. The implementation of fabricated halogen bonding capsule as binding pocket with high‐density C−Br bonds enabled the use of halogen bonding to facilitate organic transformations in their three‐dimensional cavities. Through combined experimental and computational studies, we showed that the substrate molecules diffuse through the pores of the MMPF, establishing a host‐guest system via the C−Br⋅⋅⋅π interaction. The formation of halogen bonds is a plausible explanation for the observed boosted catalytic efficiency in Diels–Alder reactions. Moreover, the unique capability of MMPF highlights new opportunities in using artificial non‐covalent binding pockets as highly tunable and selective catalytic materials.

Functional Porphyrinic Metal-Organic Framework as a New Class of Heterogeneous Halogen-Bond-Donor Catalyst.

Biomimetic metal-organic frameworks have attracted great attention as they can be used as bio-inspired models, allowing us to gain important insights into how large biological molecules function as catalysts. In this work, we report the synthesis and utilization of such a metal-metalloporphyrin framework (MMPF) that is constructed from a custom-designed ligand as an efficient halogen bond donor catalyst for Diels-Alder reactions under ambient conditions. The implementation of fabricated halogen bonding capsule as binding pocket with high-density C-Br bonds enabled the use of halogen bonding to facilitate organic transformations in their three-dimensional cavities. Through combined experimental and computational studies, we showed that the substrate molecules diffuse through the pores of the MMPF, establishing a host-guest system via the C-Br⋅⋅⋅π interaction. The formation of halogen bonds is a plausible explanation for the observed boosted catalytic efficiency in Diels-Alder reactions. Moreover, the unique capability of MMPF highlights new opportunities in using artificial non-covalent binding pockets as highly tunable and selective catalytic materials.

Successive Free-Radical C(sp 2 )–C(sp 2 ) Coupling Reactions to Form Graphene

Successive Free-Radical C(sp ² )–C(sp ² ) Coupling Reactions to Form Graphene

Theoretical insight into electronic and molecular properties of halogenated (F, Cl, Br) and hetero-atom (N, O, S) doped cyclooctane

Undoubtedly, cyclooctane has been investigated theoretically and experimentally, however, considering the vast application of cycloalkanes and limited research for the higher members, the reactivity and structural investigation of cyclooctane, fluorocyclooctane, bromocyclooctane, chlorocyclooctane, azocane, oxocane and thiocane have been explored using density functional theory (DFT) method. In this work, we reported elaborately on some molecular and electronic properties. Interestingly, the condensed dual descriptors (Δf); Br24 of bromocyclooctane compound and N22 of azocane compound has the highest positive values of 0.3561eV and 0.1618eV respectively and hence, potential sites for nucleophilic reaction compared to chlorocyclooctane, fluorocyclooctane, oxocane, thiocane and the parent cyclooctane. In the UV-VIS spectroscopic analysis, σ-π* nor π to π* transitions where observed for all the studied molecules and these molecules are expected to be colorless due to the absence of an absorption band in the visible region (380–760 nm). In all the substituted cyclooctane molecules investigated, C–F bond of fluorocyclooctane is more polar than C–O, C–S, C–N, C–Cl, C–Br bonds of other compounds. The interactions between the lone pair (LP (1) N22) and antibonding; σ*(1)C4–C5 of azocane gave the highest stabilization energy (10.44 kcal/mol) compared to other compounds investigated. We believe our detail reported work on this eight-membered ring compound and its derivatives will widen the scope of these molecules to other researchers.

2,6-Dimethoxybenzyl Bromide

The unstable title compound has been characterized for the first time. Its melting point, UV, IR, 1H and 13C-NMR and high-resolution mass spectra are presented. The X-ray structure has also been determined and shows a rather long C–Br bond perpendicular to the otherwise planar molecule.

Examining a Transition from Supramolecular Halogen Bonding to Covalent Bonds: Topological Analysis of Electron Densities and Energies in the Complexes of Bromosubstituted Electrophiles.

The transition from weak (noncovalent) interactions to fully developed covalent bonds is examined using the quantum theory of atoms in molecules in a series of halogen-bonded (XB) complexes of bromosubstituted electrophiles, RBr, with 1,4-diazabicyclo[2.2.2]octane (DABCO) and Cl– and Br– anions. The gradual decrease in the XB lengths in these associations, dBr···Y (where Y = Cl–, Br–, or N), was accompanied by the exponential increase in the binding energies and charge transfer, as well as electron densities and magnitudes of the kinetic and potential energy densities at the bond critical points (BCPs) on the Br···Y bond path. These indices, as well as characteristics of the adjacent bonds in the XB donor, followed remarkably close trend lines when plotted against the normalized XB length RBrY = dBr···Y/(rBr + rY) (where rBr and rY are the van der Waals radii) regardless of the methods [MP2/6-311+G(d,p) or M062X/6-311+G(d,p)], media (gas phase or dichloromethane), and nucleophiles (Cl–, Br–, or DABCO). In the systems with an RBrY higher than about 0.78, the energy densities H(r) at BCPs at the Br···Y bond path were small and positive, and XBs did not substantially affect the characteristics of the adjacent R–Br covalent bond in the XB donor. Accordingly, the XB can be identified as noncovalent in this range. In the complexes with RBrY values between about 0.67 and 0.78, energy densities H(r) at Br···Y BCPs were negative, and their magnitudes increased with the decrease in the Br···Y separation. In this range, formation of XBs was accompanied by the increase in the R–Br bond length in the XB donor and the decrease in the magnitude of the (negative) H(r) values at the BCPs of the R–Br bonds. XBs can be classified as partially covalent in this RBrY range. At an RBrY less than about 0.67, electron densities were larger, and energy densities were more negative at BCPs of the Br···Y bond than those at BCPs of the R–Br bond in the XB donor. This indicates that Br···Y bonds were stronger than R–Br bonds, and these (Br···Y) XBs can be regarded as essentially covalent. The synchronous change of a variety of (R–Br and Br···Y) bonding characteristics with RBrY suggests that the normalized XB bond length can be used as a basic parameter in the identification of the type of intermolecular interaction. A continuity of these characteristics suggests an inherent relationship between limiting (covalent and noncovalent) types of XBs and thus an onset of molecular–orbital interactions in the weaker bonds.

The Dynamics of Br Formation from the Primary and Secondary C–Br Bond Dissociation of Oxalyl Bromide near 265 and 234 nm

The Dynamics of Br Formation from the Primary and Secondary C–Br Bond Dissociation of Oxalyl Bromide near 265 and 234 nm

Radical 1,4-Aryl Migration Enabled Remote Cross-Electrophile Coupling of α-Amino-β-Bromo Acid Esters with Aryl Bromides.

We report an unprecedented, efficient nickel-catalysed radical relay for the remote cross-electrophile coupling of β-bromo-α-benzylamino acid esters with aryl bromides via 1,4-aryl migration/arylation cascades. β-Bromo-α-benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce-Smiles rearrangement. This reaction enables the formation of two new C(sp3 )-C(sp2 ) bonds using a bench-stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox-neutral C(sp3 )-C(sp2 ) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross-electrophile coupling and radical 1,4-aryl migration through the generation of C(sp3 )-centred radical intermediates from the homolysis of C(sp3 )-Br bonds and the switching of the transient alkyl radical into a robust α-aminoalkyl radical.

Radical 1,4‐Aryl Migration Enabled Remote Cross‐Electrophile Coupling of α‐Amino‐β‐Bromo Acid Esters with Aryl Bromides

AbstractWe report an unprecedented, efficient nickel‐catalysed radical relay for the remote cross‐electrophile coupling of β‐bromo‐α‐benzylamino acid esters with aryl bromides via 1,4‐aryl migration/arylation cascades. β‐Bromo‐α‐benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce–Smiles rearrangement. This reaction enables the formation of two new C(sp3)−C(sp2) bonds using a bench‐stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox‐neutral C(sp3)−C(sp2) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross‐electrophile coupling and radical 1,4‐aryl migration through the generation of C(sp3)‐centred radical intermediates from the homolysis of C(sp3)−Br bonds and the switching of the transient alkyl radical into a robust α‐aminoalkyl radical.

Formation of Lead Halide Perovskite Precursors in Solution: Insight from Electronic‐Structure Theory

Understanding the formation of lead halide (LH) perovskite solution precursors is crucial to gain insight into the evolution of these materials to thin films for solar cells. Using density functional theory in conjunction with the polarizable continuum model, 18 complexes with chemical formula are investigated, where X = Cl, Br, I, and M are common solvent molecules. Through the analysis of structural properties, binding energies, and charge distributions, the role of halogen species and solvent molecules in the formation of LH perovskite precursors is clarified. It is found that interatomic distances are critically affected by the halogen species, whereas the energetic stability is driven by the solvent coordination to the backbones. Regardless of the solvent, lead iodide complexes are more strongly bound than the others. Based on the charge distribution analysis, it is found that all solvent molecules bind covalently with the LH backbones and that PbI and PbBr bonds lose ionicity in solution. The results contribute to clarify the physical properties of LH perovskite solution precursors and offer a valuable starting point for further investigations on their crystalline intermediates.

Mechanism and regio- and stereoselectivity in NHC-catalyzed reaction of 2-bromoenals with β-ketoamides

• The detailed reaction mechanisms were examined. • Free energies were computed at the SMD(CH 3 CN)M06-2X-D3/6-311++G(2df,2pd) level. • The origins of stereoselectivity were identified. • The roles of the catalyst were determined. • Substituent effects were explored. We have investigated an N-heterocyclic carbene (NHC)-catalyzed annulation reaction in order to determine the mechanisms and origins of stereoselectivity. DFT results show that the catalytic formation of spiro-glutarimides involves coordination of 2-bromoenal to NHC, followed by a bicarbonate aided proton transfer to form the Breslow intermediate. Subsequent debromination and proton transfer forms the α,β-unsaturated acylazolium intermediate. Then the reaction proceeds via Michael addition, proton transfer and intramolecular cyclization to deliver the cyclic intermediate, which finally releases NHC affording the spiro-glutarimide product. The Michael addition resulting in a new carbon−carbon bond is found to be the stereochemistry-controlling step and generates the SR-configurational product observed in experiment. The important role of noncovalent interactions (such as C−H•••O, C−H•••π, π•••π and lone pair (LP)•••π) in controlling stereoselectivity is elucidated by noncovalent interaction (NCI) analysis. Moreover, analysis of the global reactivity indexes (GRIs) and the computed C−Br bond lengths revealed that NHC plays an important role in activating C−Br bond. The detailed mechanism, regio- and stereoselectivity of NHC-catalyzed annulation between 2-bromoenals with β-ketoamides has been investigated using a DFT method.

Cover Feature: Organophotoredox‐Catalyzed Late‐Stage Functionalization of Heterocycles (Asian J. Org. Chem. 8/2021)

Visible light organophotoredox catalysis promoting radical-based synthetic transformations has received immense popularity in the last decade owing to mild reaction conditions, low cost, less toxicity, easy availability, and high functional group tolerance of employed organic dyes as photocatalysts (soil). This minireview delineates all the recent developments made in the organophotoredox-catalyzed (soil) late-stage functionalization of various heterocycles (tree stem) leading to the formation of various C−C and C−X (X=N, P, O, S, Se, Br) bonds (leaves) with representative examples and mechanistic insights. More information can be found in the Mini Review by S. Murarka, B. Sahoo and co-workers.

α-Bromoacrylic Acids as C1 Insertion Units for Palladium-Catalyzed Decarboxylative Synthesis of Diverse Dibenzofulvenes.

Herein α-bromoacrylic acids have been employed as C1 insertion units to achieve the palladium-catalyzed [4 + 1] annulation of 2-iodobiphenyls, which provides an efficient platform for the construction of diverse dibenzofulvenes. This protocol enables the formation of double C(aryl)-C(vinyl) bonds via a C(vinyl)-Br bond cleavage and decarboxylation. It is particularly noteworthy that the method features a broad substrate scope, and various interesting frameworks, such as bridged ring, fused (hetero)aromatic ring, and divinylbenzene, can be successfully incorporated into the products.

Efficient mineralization of TBBPA via an integrated photocatalytic reduction/oxidation process mediated by MoS2/SnIn4S8 photocatalyst

Currently, Tetrabromobisphenol A (TBBPA) has been regarded as an emerging organic pollutant and efficient TBBPA elimination technology has been attracting increasing attention. In this work, a novel photocatalyst, MoS2/SnIn4S8, was synthesized through hydrothermal method by introducing few-layer MoS2 nanosheets and then employed to establish an integrated photocatalytic reduction/oxidation system for the remediation of TBBPA under visible light. The characterization results demonstrated that the few-layer MoS2 nanosheets were well combined with SnIn4S8 and significantly lowered the recombination rate of the photo-induced electron and holes, leading to outstanding photocatalytic performance of MoS2/SnIn4S8 composite. Besides, the MoS2/SnIn4S8 composite also exhibited excellent reusability (over 10 runs) and stability. The TBBPA degradation experiments showed that the integrated photocatalytic reduction/oxidation system was able to completely degrade TBBPA and mineralize its byproducts (60.2 ± 2.9%). In the photocatalytic reduction, due to the cleavage of C–Br bonds by photo-induced electrons, TBBPA underwent stepwise debromination and finally transferred into BPA in 6 h. In the following photocatalytic oxidation, under the attack of reactive oxygen species (1O2, h+,OH and O2−), BPA was first decomposed into aromatic products (such as phenol, benzoic acid, p-hydroxybenzyl alcohol and so on) via C–C bond cracking and hydroxylation, and then further oxidized into organic acids like maleic acid and muconic acid through ring-opening, and finally mineralized into CO2 and H2O. What was noteworthy was that the final effluent from the photocatalytic reduction/oxidation system showed no toxicity to the luminescent bacteria.

Reassessing the Adsorption Behavior and on-Surface Reactivity of a Brominated Porphyrin on Cu(111)

On-surface coupling reactions and molecular conformation are essential processes for building tailored functional molecular nanostructures. Here, we study the thermal debromination and reactivity of free-base tetra(4-bromophenyl)porphyrin (H2TBrPP) on Cu(111) as a function of the substrate temperature. It has been previously reported in the literature that C–Br bonds remain intact at room temperature (RT) and that the Br···Cu(111) interaction induces a drastic surface reconstruction around the molecule periphery and a distortion in the adsorbate itself. However, based on a combination of STM and XPS experiments, supported by density functional theory (DFT) calculations, we instead demonstrate that debromination readily occurs at RT, leading to a new interpretation of both the adsorption behavior and the molecular coupling of H2TBrPP on Cu(111). For the molecules deposited on the metallic substrate held above RT, our STM measurements show the growth of ordered 2D metal–organic frameworks (MOFs).

TESTING OF THE OPTIMUM BASIS SET FOR MODELING THE HEXABROMOTELURATE ANION

Using the PRIRODA 19 program, the bond lengths Te–Br have been calculated for the hexabromotelurate anion [TeBr6]2– within the frameworks of the GGA PBE density functional theory and the Møller–Plesset second order perturbation theory. In order to find the optimum basis set, the λXa (where X = 1-4) basis sets developed by D. Laikov, with diffusion functions, were tested. It must be noticed that in the case of the λXa (where X = 1-4) basis sets, the anion [TeBr6]2– is described by different numbers of basis functions: 261 for the double-zeta basic set λ1a, 422 for the triple-zeta basic set λ2a, 660 and 989 for the basis sets λ3a and λ4a, respectively. As the system under investigation contains relatively heavy atoms, the relativistic effects were taken into account using the four-component scalar-relativistic Hamiltonian by Dirac and using the new two-component AAA Hamiltonian by Laikov.
 The optimization of the geometric parameters of the hexabromotelurate anion was carried out with no symmetry restrictions. In all cases, the optimized [TeBr6]2– geometry corresponds to the nearly perfect octahedral symmetry; so the average Te–Br bond lengths have been considered. In the case of the PBE/λXa method (where X = 1-4), an increase in the size of the basis set results in a shortening of the Te–Br bond lengths. It has been shown that the difference between the four-component and two-component relativistic approximations increases with an increase of the basic set used. In turn, in the case of the MP2 method, the difference between the four-component and two-component relativistic approximations decreases with an increase of the basic set, and goes through the minimum for the quadruple-zeta λ3a basis set. Thus, in the post-Hartree–Fock approach, such as the Møller–Plesset perturbation theory and the coupled clusters theory, the agreement between the above relativistic approximations is reasonably good, and the two-component scalar-relativistic AAA Hamiltonian by Laikov (which is characterized by much lesser recourse demands) is more optimal for a variety of «heavy» and time-consuming calculations at the higher levels of theory. For modeling the geometric parameters of hexabromotelurate anion with an accuracy of 0.01 Å, we recommend the use of the triple-zeta basis set with the scalar-relativistic Hamiltonian.

Intramolecular Electrochemical Oxybromination of Olefins for the Synthesis of Isoxazolines in Batch and Continuous Flow

AbstractA highly regioselective protocol for the synthesis of isoxazolines through cascade C−O and C−Br bond formation has been developed. The electrochemical approach uses traceless electrons as a sole source of oxidant, thus avoiding the use of stoichiometric organic or inorganic oxidants and provides a mild and environmentally benign alternative pathway for the synthesis of a wide range of valuable substituted isoxazolines from alkenyl oximes in good yields.