CH Insertion Research Articles

Exploring C1-C3 variations and Fischer–Tropsch chemistry via electrochemical CO2 reduction on electrodeposited Cu/Ag and Ag/Cu electrodes

Ag and Cu bimetallic electrodes are extensively studied for producing C-C coupled C2+ reduction products through electrochemical CO2 reduction. In this study, we prepared electrodes with Cu electrodeposited on Ag supports (CuED/Ag) and Ag electrodeposited on Cu supports (AgED/Cu) via electrodeposition, demonstrating behaviors for reduction products under various electrochemical conditions. The products were categorized as H2, C1 (CO, CH4, formate, methanol), C2 (C2H4, ethanol, acetate, acetaldehyde, glycolaldehyde, and ethylene glycol), C3 (propanol, isopropanol), and Fischer–Tropsch (F-T) synthesis products (CnH2n and CnH2n+2, n ≥ 2). Notably, ethylene glycol was only observed over AgED/Cu. The reduction products dynamically changed with applied potentials and electrolyte concentrations. The mechanism was understood to involve surface H* adsorption, CO2 adsorption, C-C coupling, hydrogenation, and dehydration. Minor F-T synthesis chemistry was observed, explained by *CO and *CH2 insertion chain growth. The distinct product behaviors over the CuED/Ag and AgED/Cu electrodes provide valuable insights into the development of electrodes for producing value-added carbon products via CO2 utilization.

Two-dimensional modeling of diamond growth by microwave plasma activated chemical vapor deposition: Effects of pressure, absorbed power and the beneficial role of nitrogen on diamond growth

A two-dimensional (2D(r, z)) self-consistent model developed and validated in previous studies of diamond deposition processes in microwave (MW) plasma activated chemical vapor deposition reactors is applied in a systematic study of ways in which the gas pressure (over the range p = 75–350 Torr) and absorbed power (P = 1–3 kW) affect the plasma parameters, species distributions and diamond deposition processes from a gas mixture (1%CH4/H2 with 60 ppm added N2) at substrate temperatures (Ts = 1073 and 1323 K) and diameters (ds = 32–100 mm). A more limited set of process conditions for a 0.006%N2/4%CH4/H2 gas mixture and Ts = 1038–1153 K are investigated also. The study traces variations in the global distributions of electron concentration, electron and gas temperature, absorbed power density, key species concentrations and the hydrocarbon interconversion reactions, with particular focus on the radial profiles of CH3 radical and H atom concentrations just above the substrate and on predicted diamond growth rates, G(r). The results and the trends revealed when varying p and P should help guide future optimizations of deposition regimes. The absorbed power density is shown to exhibit quite steep gradients in both the radial (r) and axial (z) directions. This, and the finding of significant power absorption beyond the glowing plasma region, limits the utility of the oft-quoted ‘averaged power density’ as a parameter. The modeling also highlights the essential 2D character of the hydrocarbon interconversion and diffusion transfer processes, which challenges the applicability of any 1D modeling of such MW plasmas. Trace additions of N2 to a MW plasma activated CH4/H2 gas mixture have negligible effect on the plasma parameters or chemistry yet are known to boost the diamond growth rate. A semi-empirical expression for G(r) is developed further to explicitly include the effects of added N2 and a new mechanistic picture presented to account for the observed N-induced enhancements in G. This picture invokes stable moieties such as that formed by CH2 insertion into a CN dimer bond on the 2 × 1 reconstructed (100) diamond surface as ‘anchor’ sites that enable shorter CH2 surface migration lengths and more step-edges for irreversible incorporation of such migrating groups on the growing diamond surface.

Reactions of 1,2-Azaborinine, a BN-Benzyne, with Organic π Systems.

Ortho-benzyne and 1,2-azaborinine are related by the formal exchange of the CC triple bond by the isoelectronic BN unit. The (2 + 2) and (2 + 4) cycloaddition reactions of 1,2-azaborinine with the different organic π systems (ethene, ethyne, 1,3-butadiene, 1,3-cyclopentadiene, furan, benzene) were examined computationally using density functional, second-order perturbation, and coupled-cluster methods. All reactions of 1,2-azaborinine with the studied substrates are highly exothermic and involve the formation of Lewis acid-base complexes of 1,2-azaborinine and respective π systems. The interaction between the π bond of the substrates and the empty p orbital of the boron atom in these complexes is remarkably strong, resulting in two-step mechanisms for the (2 + 2) and (2 + 4) cycloaddition reactions. Cycloaddition reactions have lower barriers than CH insertion reactions, and (2 + 4) reactions are favored over (2 + 2) cycloadditions.

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H.

The reaction of CH radicals with H2 has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k1 shows complex behavior: at low pressures, the main route for the CH3* formed from the insertion of CH into H2 is the formation of 3CH2 + H, and as the pressure is increased, CH3* is increasingly stabilized to CH3. The kinetic data on CH + H2 have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, 3CH2 + H → CH + H2. The resulting parametrization is kCH2+H(T) = (1.69 ± 0.11) × 10-10 × (T/298 K)(0.05±0.010) cm3 molecule-1 s-1, where the errors are 1σ.

Reaction of propionitrile with methylidyne: A theoretical study

AbstractThe results of a CCSD(T)‐F12/cc‐pVTZ‐f12//ωB97XD/cc‐pVTZ quantum‐chemical study of the potential energy surface (PES) for the reaction of propionitrile with methylidyne are combined with Rice‐Ramsperger‐Kassel‐Marcus (RRKM) calculations of the reaction rate constants and product branching ratios in the deep space conditions corresponding to the zero‐pressure limit at various collision energies. The most energetically favorable reaction pathways have been identified. The reaction outcome has been shown to strongly depend on the branching in the entrance reaction channel, between CH additions and insertions into various C‐H and C‐C bonds. For instance, CH addition to the N atom predominantly leads to 3H‐pyrrole + H (p9), with CH2NC + C2H4 (p2) also being a significant product. CH addition to the triple C≡N bond mostly results in 2‐methylene‐2H‐azirine + CH3 (p13), whereas CH insertions into C‐H bonds in the CH3 and CH2 groups of propionitrile form CH2CN + C2H4 (p1) and CH2CHCN + CH3 (p7) respectively. Less likely CH insertions into single C‐C bonds yield CH3CHCHCN + H (p5) and CH2CHCH2CN + H (p8). The results indicate that the methylidyne + propionitrile reaction may represent a critical step toward the formation of heterocyclic N‐containing molecules in the interstellar medium and in planetary atmospheres.

Oligomer formation from the gas-phase reactions of Criegee intermediates with hydroperoxide esters: mechanism and kinetics

Abstract. Hydroperoxide esters, formed in the reactions of carbonyl oxides (also called Criegee intermediates, CIs) with formic acid, play a crucial role in the formation of secondary organic aerosol (SOA) in the atmosphere. However, the transformation mechanism of hydroperoxide esters in the presence of stabilized Criegee intermediates (SCIs) is not well understood. Herein, the oligomerization reaction mechanisms and kinetics of distinct SCI (CH2OO, syn-CH3CHOO, anti-CH3CHOO, and (CH3)2COO) reactions, with their respective hydroperoxide esters and with hydroperoxymethyl formate (HPMF), are investigated in the gas phase using quantum chemical and kinetics modeling methods. The calculations show that the addition reactions of SCIs with hydroperoxide esters proceed through successive insertion of SCIs into hydroperoxide ester to form oligomers that involve SCIs as the repeated chain unit. The saturated vapor pressure and saturated concentration of the formed oligomers decrease monotonically as the number of SCIs is increased. The exothermicity of oligomerization reactions decreases significantly when the number of methyl substituents increases, and the exothermicity of anti-methyl substituted carbonyl oxides is obviously higher than that of syn-methyl substituted carbonyl oxides. The −OOH insertion reaction is energetically more feasible than the −CH insertion pathway in the SCI oligomerization reactions, and the barrier heights increase with increasing the number of SCIs added to the oligomer, except for syn-CH3CHOO. For the reactions of distinct SCIs with HPMF, the barrier of the −OOH insertion pathway shows a dramatic decrease when a methyl substituent occurs at the anti-position, while it reveals a significant increase when a methyl group is introduced at the syn-position and dimethyl substituent. Compared with the rate coefficients of the CH2OO + HPMF reaction, the rate coefficients increase by about 1 order of magnitude when a methyl substituent occurs at the anti-position, whereas the rate coefficients decrease by 1–2 orders of magnitude when a methyl group is introduced at the syn-position. These new findings advance our current understanding of the influence of Criegee chemistry on the formation and growth processes and the chemical compositions of SOA.

Pincer iridium(III)-catalyzed enantioselective C(sp3)-H functionalization via carbenoid C[sbnd]H insertion of 3-diazooxindoles with 1,4-cyclohexadiene

The asymmetric carbenoid CH insertion of 3-diazooxindoles into 1,4-cyclohexadiene has been accomplished in the presence of chiral bis(imidazoline) NCN pincer iridium(III) complexes as the catalysts. With a catalyst loading of 0.5 mol%, the reactions proceeded smoothly at 0 °C to afford a variety of chiral 3-substituted oxindoles in good yields with moderate to excellent enantioselectivities (up to 99% ee). The protocol exhibits good functional group tolerance with respect to 3-diazooxindoles and is readily scaled up to 2 mmol scale without any loss in activity and enantioselectivity. Density functional theory (DFT) calculations have been performed to better understand the reaction mechanism and to explain the stereochemical outcome of the reactions.

Facile, scalable, and universal modification strategy of polyolefin utilizing noncatalytic C[sbnd]H insertion capability of azide: Sulfonyl azide end-functionalized polystyrene to modify polyethylene

Despite a broad range of commercial uses, applications of polyolefins have often been limited because of a lack of functionality and structural diversity, which has sparked the motivation to modify polyolefins, especially postpolymerization modifications (PPMs). The CH insertion capability of nitrene intermediates has been of interest due to its universal applicability to organic macromolecules containing CH bonds. In this study, a sulfonyl azide (SA) end-functionalized polystyrene (SAPS) is prepared as a model compound by synthesizing a SA containing azo-type radical initiator. SAPS is then used to modify commercial grade high-density polyethylene (HDPE) through simple melt process, affording HDPE-graft-polystyrene. The material exhibits homogeneous fractured surface, decreased crystallinity value, increased complex viscosity value, and relatively high Young’s modulus value, suggesting the formation of the polymer hybrids. This study demonstrates the capability of SA end-functionalized polymers as a noncatalytic platform for the PPM of polyolefins.

DFT quest for mechanism and stereoselectivity in B(C6F5)3-catalyzed cyclopropanation of alkenes with aryldiazoacetates

In this study, the mechanism and stereoselectivity in the B(C6F5)3-catalyzed cyclopropanation of styrenes and aryldiazoacetates were studied by the density functional theory (DFT), revealing a number of important observations. (1) The whole reaction entails the coordination of B(C6F5)3 to the Lewis basic ester site on diazo compound, generating an alkene diazonium species. Loss of dinitrogen affords a resonance stabilized Lewis acid-activated carbene intermediate, which then undergoes a concerted [2 + 1] cycloaddition with styrene to yield a S-isomeric product and regenerate the catalyst. (2) Additional calculation results for E/Z-β-methyl styrene and allyl 4-bromophenyldiazoacetate also support the concerted pathway. (3) For tosylvinylindole substrate, the reaction tends to produce a cyclopropane of the olefin rather than the CH insertion product or cyclopropanation of the 2,3-bond of the indole unit. (4) The calculation results for three ineffective diazo substrates indicate that the activation free energy for the formation of Lewis acid-activated carbene is too high to conduct further reaction. In addition, the distortion/interaction analysis showed that the chemoselectivity of cyclopropanation (C1 or C2) and CH functionalization were entirely determined by the distortion energy, while the stereoselectivity is influenced by the interaction and distortion energies. The obtained results can be of particular significance in homogeneous catalysis toward the design and prediction of organocatalytic molecular systems and [2 + 1] cycloaddition transformations involving alkenes and diazo derivatives.

Theoretical Study of the Reaction of the Methylidyne Radical (CH; X2Π) with 1-Butyne (CH3CH2CCH; X1A').

Ab initio CCSD(T)-F12/cc-pVTZ-f12//ωB97X-D/6-311G(d,p) + ZPE[ωB97X-D/6-311G(d,p)] calculations were carried out to unravel the area of the C5H7 potential energy surface accessed by the reaction of the methylidyne radical with 1-butyne. The results were utilized in Rice-Ramsperger-Kassel-Marcus calculations of the product branching ratios at the zero pressure limit. The preferable reaction mechanism has been shown to involve (nearly) instantaneous decomposition of the initial reaction adducts, whose structures are controlled by the isomeric form of the C4H6 reactant. If CH adds to the triple C≡C bond in the entrance reaction channel, the reaction is predicted to predominantly form the methylenecyclopropene + methyl (CH3) and cyclopropenylidene + ethyl (C2H5) products roughly in a 2:1 ratio. CH insertion into a C-H bond in the methyl group of 1-butyne is anticipated to preferentially form ethylene + propargyl (C3H3) by the C-C bond β-scission in the initial complex, whereas CH insertion into C-H of the CH2 group would predominantly produce vinylacetylene + methyl (CH3) also by the C-C bond β-scission in the adduct. The barrierless and highly exoergic CH + 1-butyne reaction, facile in cold molecular clouds, is not likely to lead to the carbon skeleton molecular growth but generates C4H4 isomers methylenecyclopropene, vinylacetylene, and 1,2,3-butatriene and smaller C2 and C3 hydrocarbons such as methyl, ethyl, and propargyl radicals, ethylene, and cyclopropenylidene.

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X2Π)) with cyclopentadiene (c-C5H6) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C6H6 + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C6H6 isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C6H6 isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C6H7 potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Novel synthesis of benzotriazolyl alkyl esters: an unprecedented CH2 insertion.

We have developed a novel method for the synthesis of benzotriazolyl alkyl esters (BAEs) from N-acylbenzotriazoles and dichloromethane (DCM) under mild conditions. This reaction is one of few examples to show the use of DCM as a C-1 surrogate in carbon–heteroatom bond formation and to highlight the versatility of using DCM as a methylene building block.

Tunneling Enhancement of the Gas-Phase CH + CO2 Reaction at Low Temperature.

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO2 → HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO2 reaction accelerates at low temperature as zero-point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO2 reaction, while the equivalent effect for the CD + CO2 reaction results instead from a submerged barrier with respect to reactants.

A one-pot process for synthesis of eight-membered cyclopalladated amidines via cascade C[sbnd]H activation and insertion

A one-pot process involving cascade CH activation and insertion between amidines and alkynes in the presence of palladium salt is described. With this methodology, various novel eight-membered cyclopalladated amidines were efficiently constructed. Notably, a series of functionalities with potential application were readily accommodated under the reaction conditions. Isotope effects and H/D exchange suggested that this cascade is initiated by the cleavage of the ortho CH bond in the N-phenyl ring of amidines.

Aromatic substituent effects in palladium-catalyzed intramolecular olefin oxyarylation reactions

The effect of electron-donating groups on the palladium-catalyzed intramolecular oxyarylation reaction was studied. In the case of activation at the ortho-position, the reaction favors the formation of a tricyclic lactone via CH insertion. However, when the ipso-position is activated, the major product is instead a novel α,β-unsaturated lactone formed by way of a stabilized phenonium intermediate followed by elimination.

A Proposal for the Mechanism of the CH + CO2 Reaction

Few experimental studies on the CH + CO2 global reaction propose H, CO, and HCO as major products. However, the reaction mechanisms behind this process have not yet been elucidated. Moreover, some intriguing kinetic particularities were noticed in these previous investigations. The advanced theoretical study performed here shows that a CH insertion mechanism is capable of explaining all the experimental data available. Hence, the strong deviations from a traditional Arrhenius behavior ascribed to the rate-determining elementary reaction (the CH insertion step) account for the kinetic particularities observed experimentally. A change in the preferred product channel as temperatures increase (from HCO + CO to H + 2CO) is also predicted to occur due to the HCO decomposition, although the CH depletion rates in typical conditions are not affected by this additional step.

Stereochemistry of oxidative addition reactions of cycloneophyl complexes of Platinum(II): A methylene insertion reaction from dichloromethane

Unusual features are observed in studies of oxidative addition to cycloneophylplatinum(II) complexes [Pt(CH2CMe2C6H4)(NN)], 2, NN = 3,4,7,8-tetramethyl-1,10-phenanthroline (phen*) or 3, NN = 4,4′-di-t-butyl-2,2′-bipyridine (bubipy). The oxidative addition of 4-nitrobenzyl bromide to 2 or of HgCl2 to 3 occurs with cis stereochemistry to give [PtBr(CH2-4-C6H4NO2)(CH2CMe2C6H4)(phen*)] or [PtCl(HgCl)(CH2CMe2C6H4)(bubipy)], whereas similar reactions with dimethylplatinum(II) complexes occur with trans stereochemistry. The reaction of 2 with CH2Cl2 occurs, with formal insertion of methylene into the arylplatinum bond, to give [PtCl2(CH2CMe2C6H4CH2)(phen*)], the first time such a reaction has been observed with the reagent CH2Cl2. Hydrogen peroxide reacts with 2 to give oxidative addition with trans stereochemistry to give [Pt(OH)2(CH2CMe2C6H4)(phen*)] and reaction with chlorinated solvent then gives [PtCl(OH)(CH2CMe2C6H4)(phen*)]. Depending on the reaction conditions, the peroxyacid m-ClC6H4CO3H reacts with 2 to give [Pt(OH2)2(CH2CMe2C6H4)(phen*)](m-ClC6H4CO2)2, [{Pt(OH2)2(CH2CMe2C6H4)(phen*)}.{Pt(OH)(OH2)(CH2CMe2C6H4)(phen*)}](m-ClC6H4CO2)3, or [PtCl{OOC(=O)C6H4Cl-m}(CH2CMe2C6H4)(phen*)]. Computational studies indicate that the stereochemistry of oxidative addition may be determined by kinetic or thermodynamic factors in different cases, and that the unique CH2 insertion reaction with CH2Cl2 occurs by a radical mechanism. X-ray structure determinations are reported for eight of the platinum(IV) complexes.

Temperature-dependent interactions in the chitosan/cyclosporine A system at liquid\u2013air interface

Chitosan (Ch) is a polysaccharide mainly used in cosmetics, biotechnology and medicine owing to its biocompatibility, biodegradability and low cytotoxicity. One of the challenges is the development of mechanisms responsible for its action in biomedical applications. In this aspect, it is important to get more profound insight into the chitosan interactions with peptide drugs at the molecular level. The drug of interests was a cyclosporine A (CsA) known for its effective immunosuppressive action against transplant rejection. In this study, the Langmuir technique was used to determine the interactions between Ch (dissolved in the subphase) and the monomolecular films of CsA spread at the air–chitosan solution interface. The surface pressure versus the area per molecule (π–A) measurements of the CsA monolayers on the subphase with or without Ch was conducted at 20 °C and 37 °C. The Ch insertion into the CsA monolayers was monitored by relative changes in the frac{Delta A}{A} area and the compression modulus. The findings demonstrate that the presence of Ch in the subphase provides the CsA monolayer expansion and affects the molecular packing and ordering in the temperature-dependent way. The temperature increase induces conformational changes in the peptide monolayers being a decisive factor which influences the kind and strength of interactions between Ch and CsA.

Arene C[sbnd]H insertion catalyzed by ferrocene covalently heterogenized on graphene acid

The heterogenation of molecular catalysts on solid supports is a viable route for the preparation of hybrid materials that combine the high selectivity and activity of homogeneous active species with the enhanced stability and recyclability imparted by the heterogeneous nature of the support. In this work we describe the covalent functionalization with ferrocene (Fc) of two graphene derivatives: graphene acid (GA), a graphene layer whose basal plane is modified with COOH groups, and graphene oxide (GO). The surface modification is performed exploiting the carbodiimide chemistry, which allows introducing up to 3.6% at. of iron in the GA-based material. Compared to GO, GA owns a superior functionalization degree, which is attributed to its controlled surface chemistry. Both Fc-modified materials are tested as catalysts in the CH insertion of diazonium salts employing arene substrates. The materials are active, versatile and recyclable catalysts that show a catalytic performance comparable to or even better than molecular Fc, together with a 100% recyclability which does not alter the catalytic performance. The GA-based hybrid catalyst results more active than that based on GO due to the presence of more extended aromatic domains that facilitate the adsorption of the reagents close to the active sites.

Study of Methylidyne Radical (CH and CD) Reaction with 2,5-Dimethylfuran Using Multiplexed Synchrotron Photoionization Mass Spectrometry.

At 298 K the reactions of 2,5-dimethlyfuran + CH(X2Π) and + CD radicals were investigated using synchrotron radiation coupled with multiplexed photoionization mass spectrometry at the Lawrence Berkeley National Laboratory. Reaction products were characterized based on their photoionization spectra and kinetic time traces. The CBS-QB3 level of theory was used for all energy calculations, and potential energy surface scans were used to determine thermodynamically favorable reaction mechanisms. The two entrance pathways observed in the reactions are CH insertion within the C-O bond and CH addition to the π-bond system. Both yield initial 6-membered ring radical intermediates. Primary products from the CH addition pathway were observed at m/ z = 108, 66, and 42. The two C7H8O isomers at m/ z = 108 formed are 1,2,4-heptatrien-6-one and 3-hepten-5-yne-2-one. At m/ z = 66, the three C5H6 isomers observed are 1,3-cyclopentadiene, 3-penten-1-yne (E), and 1-pent-4-yne. Ketene ( m/ z = 42) is also observed. From CH insertion entrance channel, the three C6H8 isomers produced are 1,2,4-hexatriene (Z), 2-hexen-4-yne (E), and 1,3,4-hexatriene. Patterns of H-loss, CHO-loss, and CO-loss observed were also in agreement with trends observed in other similar studies. H-assisted isomerization pathways have been considered as well for the formation of m/ z = 66, 80, and 108 isomers.