共查询到20条相似文献,搜索用时 15 毫秒
1.
Costentin C Louault C Robert M Teillout AL 《The journal of physical chemistry. A》2005,109(12):2984-2990
The reductive cleavage of chloro- and polychloroacetamides in N,N-dimethylformamide gives new insights into the nature of the in-cage ion radical cluster formed upon dissociative electron transfer. Within the family of compounds investigated, the electrochemical reduction leads to the successive expulsion of chloride ions. At each stage the electron transfer is concerted with the breaking of the C-Cl bond and acts as the rate-determining step. The reduction further leads to the formation of the corresponding carbanion with the injection of a second electron, which is in turn protonated by a weak acid added to the solution. From the joint use of cyclic voltammetric data, the sticky dissociative electron-transfer model and quantum ab initio calculations, the interaction energies within the cluster fragments (*R, Cl-) resulting from the first electron transfer to the parent RCl molecule are obtained. It is shown that the stability of these adducts, which should be viewed as an essentially electrostatic radical-ion pair, is mainly controlled by the intensity of the dipole moment of the remaining radical part and may eventually be strengthened by the formation of an intramolecular hydrogen bond, as is the case with 2-chloroacetamide. 相似文献
2.
Cyrille Costentin Prof. Dr. Ludovic Donati Marc Robert Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(3):785-792
Reductive cleavage of the three cyanobenzyl chloride isomers in N,N‐dimethylformamide gives new insights into the factors that control the mechanism during dissociative electron transfer. Within the family of investigated compounds, electrochemical reduction leads to expulsion of the chloride ion. While electron transfer is concerted with breaking of the C? Cl bond and acts as the rate‐determining step in the case of both the ortho and para isomers, an intermediate anion radical is formed before rapid fragmentation in the case of the meta isomer. Such an unexpected mechanistic shift (all key thermodynamic parameters are very similar for the three chlorides) is interpreted in the framework of a modified version of the dissociative electron‐transfer model that includes electronic coupling effects between the diabatic states of the products. These effects appear to control the very existence of a transient species along the reaction pathway. 相似文献
3.
Lin CY Coote ML Gennaro A Matyjaszewski K 《Journal of the American Chemical Society》2008,130(38):12762-12774
High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions. 相似文献
4.
Hamed EM Doai H McLaughlin CK Houmam A 《Journal of the American Chemical Society》2006,128(20):6595-6604
Important aspects of the electrochemical reduction of a series of substituted benzyl thiocyanates were investigated. A striking change in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the benzyl thiocyanate was observed, and more importantly, a regioselective bond cleavage was encountered. A reductive alpha-cleavage (CH(2)-S bond) was seen for cyano and nitro-substituted benzyl thiocyanates leading to the formation of the corresponding nitro-substituted dibenzyls. With other substituents (CH(3)O, CH(3), H, Cl, and F), both the alpha (CH(2)-S) and the beta (S-CN) bonds could be cleaved as a result of an electrochemical reduction leading to the formation of the corresponding substituted monosulfides, disulfides, and toluenes. These final products are generated through either a protonation or a nucleophilic reaction of the two-electron reduction-produced anion on the parent molecule. The dissociative electron transfer theory and its extension to the formation/dissociation of radical anions, as well as its extension to the case of strong in-cage interactions between the produced fragments ("sticky" dissociative electron transfer (ET)), along with the theoretical calculation results helped rationalize (i) the observed change in the ET mechanism, (ii) the dissociation of the radical anion intermediates formed during the electrochemical reduction of the nitro-substituted benzyl thiocyanates, and more importantly (iii) the regioselective reductive bond cleavage. 相似文献
5.
Important aspects of the electrochemical reduction of a series of substituted arene sulfonyl chlorides are investigated. An interesting autocatalytic mechanism is encountered where the starting material is reduced both at the electrode and through homogeneous electron transfer from the resulting sulfinate anion. This is due to the homogenous electron transfer from the two-electron reduction produced anion (arene sulfinate) to the parent arene sulfonyl chloride. As a result, the reduction process and hence the generated final products depend on both the concentration of the substrate and the scan rate. A change is also observed in the reductive cleavage mechanism as a function of the substituent on the phenyl ring of the arene sulfonyl chloride. With 4-cyano and 4-nitrophenyl sulfonyl chlorides a "sticky" dissociative ET mechanism takes place where a concerted ET mechanism leads to the formation of a radical/anion cluster before decomposition. With other substituents (MeO, Me, H, Cl, and F) a "classical" dissociative ET is followed, where the ET and bond cleavage are simultaneous. The dissociative electron transfer theory, as well as its extension to the case of strong in-cage interactions between the produced fragments, along with gas phase chemical quantum calculations results helped us to rationalize both the observed change in the ET mechanism and the occurrence of the "sticky" dissociative ET mechanism. The radical/anion pair interactions have been determined both in solution as well as in the gas phase. The study also shows that despite the low magnitude of in-cage interactions in acetonitrile compared to the gas phase their existence strongly affects the dynamics of the involved reactions. It also shows that, as expected, these interactions are reinforced by the existence of strong electron-withdrawing substituents. The occurrence of an autocatalytic process and the existence of the radical/anion interaction may explain the differences previously observed in the reduction of these compounds in different media. 相似文献
6.
Saeki A Yamamoto N Yoshida Y Kozawa T 《The journal of physical chemistry. A》2011,115(36):10166-10173
Dynamics of radical cations and electrons in an admixture of a linear saturated hydrocarbon (n-dodecane) and halocarbon (carbon tetrachloride, CCl(4)) were investigated by picosecond electron beam pulse radiolysis. The decay of thermalized electrons (e(th)(-)) observed in infrared transient photoabsorption were simply accelerated by the addition of CCl(4), giving a high rate constant of 2.3 × 10(11) mol(-1) dm(3) s(-1). The decrease of the initial yield of e(th)(-) was quantified by C(37) (50 mmol), which is linked to the reaction of epithermal electrons (e(-)) with CCl(4). In contrast, the n-dodecane radical cation (RH(2)(?+)) monitored in the near-infrared indicated a convex-type dependence of the decay rate on CCl(4) concentration, although the initial yield of RH(2)(?+) remained almost constant up to a much higher CCl(4) concentration. The decay of RH(2)(?+) was analyzed by Monte Carlo simulations of geminate ion recombination with e(th)(-), chlorine anion (Cl(-)) formed via dissociative electron attachment, and CCl(4) radical anion. The results showed a good agreement with the experiments by considering two assumptions: (1) CCl(4) radical anion formed via e(th)(-) attachment and (2) narrowing of the initial distribution of Cl(-). The decrease in the initial yield of RH(2)(?+) at high CCl(4) concentration was well explained by immediate decomposition of CCl(4)(?+) to CCl(3)(+) and hole transfer from CCl(4)(?+) to adjacent RH(2) without diffusive motion of the reactants. Time-dependent density functional theory supported the spectroscopic assignment of intermediate species in the n-dodecane/CCl(4) system. The present results would be of help in understanding the electron capture reaction in multicomponent systems such as a chemically amplified resist in lithography. 相似文献
7.
Kéki S Török J Nagy L Zsuga M 《Journal of the American Society for Mass Spectrometry》2008,19(5):656-665
Atmospheric pressure photoionization mass spectrometric (APPI-MS) study on three types of polyisobutylene derivatives is reported. Two of the polyisobutylenes investigated were polyisobutylene with dihydroxy and diolefinic end-groups derived from aromatic moieties [dicumyl chloride, 1,4-bis(2-chloro-2-propyl)benzene], and the third contained no aromatic moieties with a monohydroxy end-group. All three polyisobutylene derivatives (PIBs) had an average molecular weight (M(n)) of approximately 2000 g/mol, with a polydispersity lower than 1.2. In the positive ion APPI mode, protonated PIB molecules were formed, but the molecular weights obtained were considerably lower than those expected, indicating fragmentation of the PIB chains. In the negative APPI mode, using solvents such as tetrahydrofuran and toluene as dopants, no signal was obtained. However, in chlorinated solvents, such as CCl(4), CHCl(3), and CH(2)Cl(2), in the presence of toluene dopant, PIB adducts with chloride ions were formed with relatively high signal intensity. In the case of CH(2)Cl(2), no dopant (toluene) was necessary to generate chlorinated adduct ions, albeit increasing the toluene concentration in the flow increased the PIB signal intensity. The effect of the toluene concentration on PIB signal intensity was studied and models that include (1) photoionization of toluene, (2) formation of chloride ions from the chlorinated solvents by dissociative electron capture, (3) formation of chlorinated adduct ions and charge recombination reactions between the toluene radical cation, (4) chloride ions, and (5) chlorinated adduct ions are proposed based on the experimental results. 相似文献
8.
The homogeneous and heterogeneous reductions of the endoperoxides 9,10-diphenyl-9,10-epidioxyanthracene (DPA-O2) and 9,10-dimethyl-9,10-epidioxyanthracene (DMA-O2) were investigated, and they were found to undergo a dissociative electron-transfer reduction of the O-O bond to yield a distonic radical anion, with no evidence for C-O bond dissociation. A number of thermochemical parameters for each were determined using Savéant's model for dissociative electron transfer (ET), including E degrees, DeltaG(o)++, and bond dissociation energies. The products of the ET are dependent on the mode of reduction, namely heterogeneous or homogeneous, and on the electrode potential or standard potential of the homogeneous donor, respectively. The dissociative reduction of DMA-O2 under heterogeneous and homogeneous conditions yields the corresponding 9,10-dihydroxyanthracene DMA-(OH)2, quantitatively, in an overall two-electron process. In the case of DPA-O2, ET reduction also yields the corresponding 9,10-dihydroxyanthracene DPA-(OH)2 from reduction of the distonic radical anion, but in competition with this reduction, an O-neophyl-type rearrangement occurs that generates a carbon radical with a minimum rate constant of 5.9 x 10(10) s(-1). In the presence of a sufficiently reducing medium, the carbon-centered radical is reduced (E degrees = -0.85 V vs SCE) and ultimately yields 9-phenoxy-10-phenyl anthracene (PPA). The observation of this product is remarkable. In the heterogeneous ET, the yield of DPA-(OH)2/PPA is 97:3 and allows an estimate of the rate constant for ET to the distonic radical anion. In homogeneous reductions, the O-neophyl rearrangement is quantitative, but the yield of PPA depends on the redox properties of the donor. A unified mechanism of reduction of DPA-O2 is presented to account for these observations. 相似文献
9.
主要研究2,2,2-三氟卤乙烷(碘、溴、氯、氟)和碳负离子的反应,在一定的温度下,以DMF作溶剂,除了2,2,2-三氟氟乙烷外,反应都得到了相应的2,2,2-三氟卤乙烷的衍生物以及碳负离子的偶联产物.该反应能被紫外光加速,能被对二硝基苯和对二苯酚阻止.因此,该反应是按SRN1即自由基亲核取代反应机理进行.由于此反应能在黑暗中进行,所以它可能是通过热引发或自发引发来完成的. 相似文献
10.
Collisions between Cl(2+) and CO have been investigated using time-of-flight mass spectrometry over a collision energy range between 2.2 eV and 7.1 eV in the centre-of-mass frame. The formation of Cl(+), CO(+) and C(+) in electron transfer reactions has been detected and an unusual bond-forming reaction which generates CCl(2+) has also been observed. The reactive cross-sections, in arbitrary units, for the electron transfer reactions have been evaluated. To extract these cross sections we employ a new method of analysing mass spectral intensities for crossed-beam experiments, an algorithm which allows inter-comparison of the fluxes of all the ionic products from the electron transfer reactions. The observed electron transfer reactivity has been rationalized by calculations based on Landau-Zener theory. To account for the observation of CCl(2+), we have calculated the relevant energetics showing that the lowest lying doublet state of this dication is bound and is energetically accessible at our collision energies. These energetic arguments indicate that electron transfer in the exit channel between the separating CCl(2+) and O atom probably forms C(+) ions via the dissociation of CCl(+). Additionally, collisions between HCl(2+) and CO have been studied at collision energies from 2.2 to 7.0 eV in the centre-of-mass frame. In this collision system, proton transfer to form HCO(+) is observed to compete efficiently with dissociative and non-dissociative electron transfer. 相似文献
11.
The idea that significant ion/radical interactions should vary with solvent if they do exist in the liquid phase was pursued by an investigation of the dissociative electron-transfer reactivity of carbon tetrachloride and 4-cyanobenzyl chloride in four different solvents, 1,2-dichloroethane, N,N-dimethylformamide, ethanol, and formamide, by means of their cyclic voltammetric responses. Modification of the conventional dissociative electron transfer theory to take account of an interaction between fragments in the ion/radical pair resulting from the dissociative electron reaction allows a satisfactory fitting of the experimental data leading to the determination of the interaction energy. There is an approximate correlation between the interaction energies in the ion/radical pair and the solvation free energies of the leaving anion, Cl(-). The interaction is maximal in 1,2-dichloroethane, which is both the least polar and the least able to solvate Cl(-). The interaction is smaller in the polar solvents, albeit distinctly measurable. The two protic solvents, ethanol and formamide, which are the most able to solvate Cl(-), give rise to similar interaction energies. The interaction is definitely stronger in N,N-dimethylformamide, which has a lesser ability to solvate Cl(-) than the two other polar solvents. The existence of significant ion/radical interactions in polar media is thus confirmed and a route to their determination opened. 相似文献
12.
The primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: a dissociative and a metastable one. In analogy with formerly studied donor systems, this result seems to reflect femtosecond oscillations in the ground state of the sulfides such as torsion motions around the Ar-S bond. This motion is accompanied by a marked electron fluctuation within the HOMO (or the n) orbitals. The FET products observed in the nanosecond time scale such as the metastable sulfide radical cations (Ar-S-CR3*+), the dissociation products R3C+; and R3C*, and their (experimentally) nondetectable counterparts Ar-S* as well as Ar-S+ can be understood with the simplified assumption of two extreme conformations, namely a planar and a twisted donor molecule. Using mediator radical cations (benzene, butylbenzene, biphenyl), the stepwise reduction of the free energy of the electron transfer from -DeltaH = 2.5 to 相似文献
13.
The molecular ions of the title compounds appear to lose a benzyl radical, which must be due to the presence of two benzyloxy groups, as benzylalkyl ethers do not exhibit such an expulsion upon electron impact. The results of the partition of the labels deuterium and 18O in the ions m/e 107 (protonated benzaldehyde) and [M-benzyl-benzaldehyde]+ put forward evidence that this process is initiated by a successive migration of a benzylic H atom to the opposite ether function and transfer of the benzyl cation from this protonated O atom to the uncharged O atom in an SNi-type reaction (cf Scheme 5). 相似文献
14.
Isse AA Gennaro A Lin CY Hodgson JL Coote ML Guliashvili T 《Journal of the American Chemical Society》2011,133(16):6254-6264
The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(?-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(?-), which is shown here to be nonexistent. 相似文献
15.
Reaction of hydrated electrons with aromatics substituted with both bromine and chlorine results in the production of significant yields of chloride ion and the complementary bromine substituted phenyl radical. The total yields show that in all cases the reduction is essentially quantitative. For the dihalogenated benzenes and phenols the relative yields for C-Br and C-Cl bond rupture (0.86:0.14) reflect the relative rates for electron attachment at the Br and Cl positions, suggesting that there is little intramolecular charge transfer on the time scale of dissociation of the initial anion. In the case of dihalogenated benzoates about 40% of the reduction results in C-Cl bond rupture. In this case the added charge initially localized on the carboxyl group is transferred preferentially to the chlorine atom indicating that solvation of the intermediate radical anion must play an extremely important role in controlling the overall reduction process. 相似文献
16.
Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting methoxy radical from an added alcohol generates an alpha-hydroxy radical that reduces another pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium) of approximately 15-20, at reactant concentrations of approximately 0.02-0.04 M. The reaction can also be sensitized with triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain propagation are imposed by the relationship between the oxidation potential of the alpha-hydroxy radical and the reduction potential of the pyridinium salt. The chain reactions proceed despite approximately 0.25 eV endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a combination of steady state and transient kinetics. The bimolecular rate constants for the chain propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from the diphenylketyl radical to P1 are approximately 6 x 10(6) and 1.1 x 10(6) M(-1) s(-1), respectively. The kinetic data indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d(3), is a dominant chain-terminating process. Because of a large deuterium isotope effect, approximately 7, the quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile. 相似文献
17.
Two dialkyl peroxides, devised as kinetic probes for the heterogeneous electron transfer (ET), are studied using heterogeneous and homogeneous electrochemical techniques. The peroxides react by concerted dissociative ET reduction of the O-O bond. Under heterogeneous conditions, the only products isolated are the corresponding alcohols from a two-electron reduction as has been observed with other dialkyl peroxides studied to date. However, under homogeneous conditions, a generated alkoxyl radical undergoes a rapid beta-scission fragmentation in competition with the second ET resulting in formation of acetone and a benzyl radical. With knowledge of the rate constant for fragmentation and accounting for the diffuse double layer at the electrode interface, the heterogeneous ET rate constant to the alkoxyl radicals is estimated to be 1500 cm s(-1). The heterogeneous and homogeneous ET kinetics of the O-O bond cleavage have also been measured and examined as a function of the driving force for ET, deltaG(ET), using dissociative electron transfer theory. From both sets of kinetics, besides the evaluation of thermochemical parameters, it is demonstrated that the heterogeneous and homogeneous reduction of the O-O bond appears to be non-adiabatic. 相似文献
18.
Karsten Donabauer Dr. Kathiravan Murugesan Urša Rozman Dr. Stefano Crespi Prof. Dr. Burkhard König 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(57):12945-12950
A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey–Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C−H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical–radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths. 相似文献
19.
PhenothiazinederivativeshaveattractedmuchinterestforalongtimeduetotheirdiversechemicalandbioIogicalproperties,aswellasmedicalaPplicationsl.Theradicalcationofphenothiazinesisbelievedtobeinvolvedinthebiotransformationofphenothiazinedrugs2andnumerousstudieshavethusfocusedonthegenerationandreactionsofphenothiazineradicalcations3-6.WereporthereinanovelphotochemicalreactionofN-methylphenothiazineincarbontetrachloridewhichcoupIestwoparentmoleculeswitheliminationofonemethylgroup(Schemel).Resultsandd… 相似文献
20.
In fluprednisolone and chloroprednisone acetate, the polarographic reduction of the carbon—halogen bond in position 6 occurs first. The carbanion—enolate formed is reduced at the dropping mercury electrode at more negative potentials than the conjugate acid. Controlled potential electrolysis at a mercury pool electrode where the carbanion—enolate can be protonated, yields the unsaturated ketone. Polarographic reduction of clobetasol-17-propionate and of clobetasone-17-butyrate results in cleavage of the C—Cl bond in the side-chain. This process is followed by reduction of the α,β-unsaturated ketone in the A-ring. Analytical methods for the determination of these compounds in ointments, creams and eye/ear drops gave results with standard deviations of 1–2%. 相似文献