Bromomethyl Radicals Research Articles

Photoredox‐Catalyzed Cyclopropanation of 1,1‐Disubstituted Alkenes via Radical‐Polar Crossover Process

AbstractThe photoredox‐neutral catalyzed cyclopropanation of 1,1‐disubstituted alkenes via radical addition‐anionic cyclization cascade has been successfully developed. Another new protocol based on photocatalytic allylation and cyclopropanation cascade was also described between allylic halide and halomethyl radical. In addition to the successful use of bis‐catecholato silicates as the alkyl radical precursors, the acyl and alkyl radicals derived from 1,4‐dihydropyridines were also engaged in this radical‐polar crossover process. The competing experiments displayed that the 3‐exo‐tet mode of cyclization preferred over 4‐exo and 5‐exo cyclization modes, allowing for the selective 3‐exo‐tet cyclization. The superior nucleofuge character of bromide over chloride and tosylate has been demonstrated in the reaction of bromomethyl radical with homoallylic (pseudo)halides. This new protocol is characterized by its redox‐neutral process, broad substrate scope, mild conditions, and good functional‐group compatibility.magnified image

Femtosecond photolysis of CH 2Br 2 in acetonitrile: Capturing the bromomethyl radical and bromine-atom charge transfer complex through deep-to-near UV probing

Dibromomethane (CH 2Br 2) in acetonitrile is a suitable precursor to characterize the absorption signatures of the CH 2Br radical and solvent Br charge-transfer complexes. Following irradiation of CH 2Br 2 at 255 nm, the iso-H 2C Br Br isomer product rapidly converts back to the parent species, and transient absorption spectra reveal the bands of solvent-separated radical species, the CH 2Br radical peaking at 235 nm, as well as the CH 3CN·Br complex at 272 nm. The absorption of CH 2Br exhibits minor solvatochromic shifts upon going from acetonitrile to cyclohexane, and the molecular decadic extinction coefficient of CH 3CN·Br is estimated to be 1470 M −1 cm −1.

Hyperfine resolved spectrum of the bromomethyl radical, CH2Br, by Fourier transform microwave spectroscopy

Pure rotational spectra of the bromomethyl radical, CH(2)Br, were measured by using a Fourier transform microwave (FT-MW) spectrometer in order to fully resolve hyperfine structures arising from both the bromine and hydrogen nuclei. We detected a total of 124 lines for the (79)Br and (81)Br isotopomers, including K(a)=0 (ortho species) and K(a)=1 (para species). No hyperfine splitting due to the hydrogen nuclei was observed for the para species, directly confirming the planarity of the radical. We conducted a global analysis of our present FT-MW results and previous measurements in the millimeter-wave region and obtained an exhaustive list of molecular constants. The sign of the Fermi constant of the bromine nucleus was unambiguously determined to be positive, which is opposite to that found in previous work in the millimeter-wave region and in electron spin resonance experiment on this radical. The present study permitted a systematic comparison to be made of the hyperfine coupling constants of both the halogen and hydrogen nuclei for CH(2)X-type compounds, where X=F, Cl, and Br.

Reaction of N(2D) atoms with bromomethyl radicals: A theoretical study

The singlet potential energy surface for the N(2D)+CH2Br reaction has been studied employing both MP2 and DFT(B3LYP) methods. The energies of the involved species have been refined using the G2, CBS and CCSD(T) methods, respectively. The reaction proceeds through the formation of an initial intermediate which does not involve any activation barrier. Based on the singlet PES, the most exothermic products result from elimination of either HBr or H2, instead of elimination of either Br or H. The preferred channel is predicted to be HCN+HBr. The analysis of the possible spin crossing between the triplet and singlet [CH2NBr] potential surfaces suggests that the N(4S)+CH2Br reaction should take place with no change in the spin angular momentum.

On the reaction of ground-state nitrogen atoms with bromomethyl radicals: A computational study

A computational study of the reaction of N( 4S) with CH 2Br radicals has been carried out. The reaction starts through the formation of a relatively stable intermediate which does not involve an energy barrier. The two most exothermic products are those resulting from the release of a bromine atom, H 2C = N + Br and trans-HC = NH + Br. A kinetic study, within the framework of the statistical theories, shows that the most exothermic product, namely H 2C = N + Br, is also the preferred product under kinetic control, whereas only minor fractions of HBrCN + H are predicted at all temperatures. Therefore, the mechanistic characteristics observed for the N( 4S) + CH 2Br reaction are very similar to the N( 4S) + CH 2Cl reaction, but quite different from those found in the case of N( 4S) + CH 2F, where the preferred channel corresponds to formation of HFCN through elimination of a hydrogen atom. The rate coefficient for the title reaction is estimated to be about 9 × 10 −13 cm 3 s −1 molecule −1 at 300 K, showing that should proceed with a high efficiency.

Millimeter wave spectrum of bromomethyl radical, CH2Br

The rotational spectra of the two isotopic species of the bromomethyl radical, CH2 79Br and CH2 81Br, have been observed in their ground electronic state 2B1 in the 180-470 GHz frequency region, corresponding to a-type transitions from N=8-7 to N=21-20. The radical was produced by hydrogen abstraction of methylbromide (CH3Br) either by chlorine or by fluorine atoms in a free space cell. Hyperfine structure due to the bromine nucleus has been resolved in the observed spectra, and the rotational constants as well as the fine and hyperfine interaction constants were accurately determined for both isotopomers. The inertial defect was determined to be 0.028 96(20) and 0.028 95(20) amu A(2), for CH2 79Br and CH2 81Br, respectively, suggesting a planar structure. By fixing the [angle]HCH bond angle at 124.5 degrees , an effective molecular structure can be derived as r0(CBr)=1.848 A and r0(CH)=1.084 A. A comparison of the molecular structure of various halogen-substituted methyl radicals with respect to the planarity of these radicals is discussed.

Investigation of the Atmospheric Oxidation Pathways of Bromoform and Dibromomethane: Initiation via UV Photolysis and Hydrogen Abstraction

A computational study of the oxidation of CH2Br2 and CHBr3 initiated via both UV photolysis and abstraction of an H atom by OH/Cl radicals has been performed. We have calculated the energetics associated with the addition of O2 to the substituted bromomethyl radicals and the subsequent addition of NO to the peroxy radicals to form energized peroxy nitrite molecules. The peroxy nitrite molecules are predicted to dissociate rapidly to form alkoxy radicals (CH2BrO and CHBr2O) and NO2, and the kinetics of these reactions have been determined using Rice−Ramsperger−Kassel−Marcus/master equation calculations. We additionally find that the reaction of the peroxy radicals with HO2 may directly lead to significant production of alkoxy radicals, a pathway that is unimportant in nonbrominated analogues. We predict that the alkoxy radicals will dissociate rapidly via C−Br bond cleavage. The atmospheric implications of these results will be discussed.

First observation of the rotational spectrum of the bromomethyl radical, CH2Br

The first high resolution rotational spectrum of the bromomethyl radical in the gas phase has been observed. The radical was produced in a fast flow system by hydrogen abstraction of CH3Br by Cl or F atoms obtained from a 2450 MHz microwave discharge in Cl2 or in CF4, respectively. The rotational, quartic centrifugal distortion constants have been obtained as well as the electron spin–rotation interaction constants. The small positive inertial defect, Δ0 = 0.032 amu A2 deduced from the rotational constants of the CH279Br species clearly indicates that the radical is planar in its ground vibronic state.

Transient resonance Raman and density functional theory investigation of bromomethyl radical

We have obtained transient resonance Raman spectra of CH 2Br produced following A-band excitation of dibromomethane and performed density functional theory computations to estimate the vibrational frequencies and electronic transition energies of the CH 2Br radical. Our results indicate the CH 2Br radical has a C–Br bond longer than in the parent CH 2Br 2 molecule but shorter than in a typical bromoalkane. Both the experimental and calculated resonance Raman spectra which display a strong overtone progression in the nominal C–Br stretch mode suggest that the first excited electronic state of the CH 2Br radical experiences motion mostly along the C–Br bond in the Franck–Condon region.

Theoretical study of the CH 2Br, CHBr 2 and CBr 3 radicals

Equilibrium structures, vibrational spectra, inversion barriers, and adiabatic ionization energies are determined for the series of bromomethyl radicals CH 2Br, CHBr 2 and CBr 3 using both ab initio correlated and hybrid UB3LYP density functional theory (DFT) methods in conjunction with the large TZ2P and 6-311++G(3df,3pd) basis sets. In particular, the trends in the calculated molecular properties within the series are indicated and the performance of the two theoretical approaches is assessed.

Structures, Vibrational Frequencies, Thermodynamic Properties, and Bond Dissociation Energies of the Bromomethanes and Bromomethyl Radicals: an Ab Initio Study

Reported here is a theoretical study of the entire series of bromomethanes (CH4-nBrn) and bromomethyl radicals (CH3-mBrm) establishing a self-consistent set of structural and thermodynamic information. Ab initio molecular orbital calculations were performed to compute equilibrium geometries for the molecules and radicals initially at the (U)HF/6-31G* and (R)HF/6-31G* levels, respectively, and then refined at the MP2/6-31G* level. Vibrational frequencies were determined for all species at the HF/6-31G* level and comparison with infrared measurements and matrix isolation studies is favorable. Electron correlation contributions were performed by single-point calculations using fourth-order Møller−Plesset perturbation theory for derived MP2/6-31G* geometries. Enthalpies of formation were obtained from a consideration of applicable isodesmic reactions using the derived MP4/6-31G**//MP2/6-31G* total energies in conjunction with experimentally established enthalpies of formation for CH3Br, CH4, and CH3•. The calculations predict the following standard enthalpies of formation in kilocalories per mole (at 298 K and 1 atm): CH2Br2, 1.07 ± 0.6; CHBr3, 12.16 ± 0.7; CBr4, 25.23 ± 0.8; CH2Br•, 41.63 ± 0.4; CHBr2•, 48.11 ± 0.6; and CBr3•, 55.36 ± 0.7. These data are then used to tabulate ΔH°f,T, ΔG°f,T, and Kf,T for all species over the temperature range 0−1500 K. Comparison is made to existing thermochemical data through calculation of C−H and C−Br bond dissociation energies.

Absolute Rate Constants for the Reactions of Cl Atoms with CH3Br, CH2Br2, and CHBr3

The rate constants for the reactions of chlorine atoms with the complete series of the three bromomethanes CH3Br (1), CH2Br2 (2), and CHBr3 (3) were measured in a very low pressure reactor, employing a microwave discharge for the generation of Cl atoms with mass spectrometric detection of reactants and products. The experiments were performed in the temperature range 273−363 K and at total pressures ∼1 mTorr. The reactions proceed via hydrogen atom transfer leading to HCl product and the corresponding bromomethyl radicals. Their rate constant expressions are (in cm3 molecule-1 s-1): k1 = (1.66 ± 0.14) × 10-11 exp(−1072 ± 46/T), k2 = (0.84 ± 0.15) × 10-11 exp(−911 ± 101/T), and k3 = (0.43 ± 0.11) × 10-11 exp(−809 ± 142/T). The activation energy of the reaction decreases with additional bromine substitution, which is attributed to the gradual weakening of the corresponding C−H bond strength. Ab initio theoretical calculations performed at the MP2/6-31++G(2d,2p) level of theory suggest C−H bond strengths fo...

Unimolecular decomposition of [80Br]bromocyclopropane formed by reaction of recoil bromine atoms with cyclopropane or with bromocyclopropane

The reactions of recoil bromine, from the 79Br(n,γ)80Br reaction, with cyclopropane and with bromocyclopropane have been investigated as a function of total pressure. The primary product, cyclo-C3H580Br, of the 80Br-for-H reaction in the former and of the 80Br-for-Br reaction in the latter, is highly excited and undergoes unimolecular decomposition to CH2CHCH280Br. The decomposition product from the former reaction still has enough energy to decompose further to the [80Br]bromomethyl radical which is isolated as CH280BrI in the presence of I2 scavenger. On the other hand most of the CH2CHCH280Br from the 80Br-for-Br reaction is stabilized either by collisional de-excitation or release of the 80Br atom.

Matrix Infrared Spectrum and Bonding in the Dibromomethyl Radical

Simultaneous condensation of bromoform and lithium atoms at high dilution in argon on a CsI window at 15°K produces new infrared absorptions which are assigned to lithium bromide and the dibromomethyl radical. The identity of dibromomethyl is confirmed by comparison of spectra obtained from HCBr3, DCBr3, and HCBr2Cl precursors reacting with lithium and sodium. These new absorptions are assigned to the antisymmetric H–C–Br bending and C–Br stretching modes and the symmetric C–Br vibration. The antisymmetric vibrational assignments are supported by product-rule and normal-coordinate calculations which give the potential constants F55 = 3.10 ± 0.10 mdyn/Å, F56 = 0.32 ± 0.01 mdyn/rad, and F66 = 0.50 ± 0.05 mdyn·Å/rad2, while the symmetric C–Br mode yields an approximate force constant F22 = 3.7 ± 0.4 mdyn/Å. The C–Br valence force constants for bromomethyl radicals exceed normal C–Br values, while electronic stabilization for these radicals is indicated by bond dissociation energies. Similar results for chloromethyl radicals, in contrast to fluoromethyl radicals, suggest that (p–d)π bonding between the free-radical carbon orbital and the d orbitals on chlorine and bromine might account for the stabilization of chloromethyl and bromomethyl radicals, which cannot occur for fluoromethyl radicals or molecules with completely satisfied valence.

Matrix Infrared Spectrum and Bonding in the Dichloromethyl Radical

Simultaneous deposition of chloroform and lithium atoms at high dilution in argon on a CsI window at 15°K produces new infrared absorptions which are attributed to lithium chloride and the dichloromethyl radical. The identity of dichloromethyl is provided by comparison of spectra recorded after reactions of HCCl3, DCCl3, and HCCl2Br with lithium and sodium and the detection of (HCCl2)2 following diffusion and the decrease in HCCl2 absorptions. The new absorptions are assigned to the antisymmetric H–C–Cl bending and C–Cl stretching vibrations. These vibrational assignments are supported by product-rule and normal-coordinate calculations which give the potential constants F55 = 3.60 ± 0.10 mdyn/Å, F56 = 0.35 ± 0.01 mdyn/rad, and F66 = 0.56 ± 0.05 mdyn·Å/rad2. The carbon–chlorine valence force constant deduced here exceeds normal C–Cl values, while bond dissociation energies indicate that HCCl2 is electronically stabilized. Similar results for bromomethyl radicals, in contrast to fluoromethyl radicals, suggest that (p–d)π bonding between the free-radical carbon orbital and the d orbitals on chlorine and bromine may account for the stabilization of chloromethyl and bromomethyl radicals, which cannot occur for fluoromethyl radicals or molecules with completely satisfied valence.