Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates

The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.     

Application of the dissociative electron transfer theory and its extension to the case of in-cage interactions in the electrochemical reduction of arene sulfonyl chlorides

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.     

Radical/Ion pair formation in the electrochemical reduction of arene sulfenyl chlorides

Important aspects of the electrochemical reduction of a series of substituted arene sulfenyl chlorides are investigated. A striking change is observed in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the arene sulfenyl chloride. With p-substituted phenyl 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 o-nitropheyl sulfenyl substituted chlorides a stepwise mechanism is observed where through space S...O interactions play an important role stabilizing both the neutral molecules and their reduced forms. Disulfides are generated through a nucleophilic reaction of the two-electron reduction produced anion (arenethiolate) on the parent molecule. The dissociative electron transfer theory, as well as its extension to the case of strong in-cage interactions between the produced fragments, along with the gas phase chemical quantum calculations results helped 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 gas phase. This study shows that despite the low magnitude of in-cage interactions in acetonitrile as compared to in the gas phase, their existence strongly affects the kinetics of the involved reactions. It also shows that, as expected, these interactions are reinforced by the existence of strong electron-withdrawing substituents.     

Model dialkyl peroxides of the fenton mechanistic probe 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH): kinetic probes for dissociative electron transfer

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.     

Dissociative electron transfer to organic chlorides: electrocatalysis at metal cathodes

The reductive cleavage of a series of organic chlorides, including chloroaromatics, benzyl chlorides, activated chloroalkanes and polychloromethanes, was investigated at Ag, Cu, Pd and glassy carbon (GC) electrodes in CH(3)CN + 0.1 M (C(2)H(5))(4)NClO(4). The silver cathode was either a 2-mm diameter disc, fabricated from Ag wire, or nanoclusters of average diameter d = 304 nm, prepared by electrodeposition on GC. Ag, Cu and Pd electrodes have shown remarkable electrocatalytic properties for the reduction of several compounds. The peak potentials recorded at these electrodes, for example, at upsilon = 0.1 V s(-1) are positively shifted by 0.3-0.8 V with respect to the reduction potentials measured at a non catalytic electrode such as GC. Electrocatalysis is strictly related to the concerted nature of the dissociative electron transfer to the carbon-chlorine bond. No catalysis is observed when the dissociative electron transfer to RCl occurs according to a stepwise mechanism involving the intermediate formation of a radical anion. The catalytic surfaces affect the reaction scheme, offering a more favourable route possibly through the formation of strongly adsorbed activated complexes.     

A unique autocatalytic process and evidence for a concerted-stepwise mechanism transition in the dissociative electron-transfer reduction of aryl thiocyanates

Electrochemical reduction of p-methyl-, p-methoxy-, and 3,5-dinitrophenyl thiocyanates as well as p-methyl- and p-methoxyphenyl disulfides was investigated in acetonitrile at an inert electrode. This series of compounds reveals a striking change in the reductive cleavage mechanism of the S-CN bond in thiocyanates as a function of the substituent on the aryl ring of the aryl thiocyanate. With nitro substituents, a stepwise mechanism, with an anion radical as the intermediate, takes place. When electron-donating groups (methyl and methoxy) are present, voltammetric as well as convolution analyses provide clear evidence for a transition between the concerted and stepwise mechanisms based on the magnitude of the transfer coefficient alpha. Moreover, a very interesting autocatalytic process is involved during the electrochemical reduction of these compounds. This process involves a nucleophilic substitution reaction on the initial aryl thiocyanate by the electrochemically generated arenethiolate ion. As a result of this unusual process, the electrochemical characteristics (peak potential and peak width) of the investigated series are concentration dependent.     

Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization: theoretical and experimental study

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.     

Elucidation of the electron transfer reduction mechanism of anthracene endoperoxides

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.     

A radical-anion chain mechanism initiated by dissociative electron transfer to a bicyclic endoperoxide: insight into the fragmentation chemistry of neutral biradicals and distonic radical anions

The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N-dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the O-O bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be -0.55+/-0.15 V and 20+/-3 kcal mol(-1), respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the O-O bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene-cis-1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane-cis-1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a beta-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals.     

Successive removal of chloride ions from organic polychloride pollutants. Mechanisms of reductive electrochemical elimination in aliphatic gem-polychlorides,alpha,beta-polychloroalkenes,and alpha,beta-polychloroalkanes in mildly protic medium

The factors that control the successive reductive expulsion of chloride ions from aliphatic gem-polychlorides are investigated, taking as examples the electrochemical reduction of polychloromethanes and polychloroacetonitriles in N,N-dimethylformamide. At each elimination stage, the reaction involves, as a rate-determining step, the transfer of one electron concerted with the cleavage of the carbon-chloride bond. The second step is an immediate electron transfer to the ensuing radical, taking place at a potential more positive than the potential at which the first electron transfer occurs. The carbanion thus formed is sufficiently basic to be protonated by any trace weak acid present in the reaction medium. The three successive elimination steps require increasingly negative potentials. Application of the "sticky" dissociative electron transfer model allows one to quantitatively unravel the factors that control the energetics of the successive reductive expulsion of chloride ions. The large potential gaps between each stage stem primarily from large differences in the dissociative standard potentials. They are also strongly affected by two cumulative intrinsic activation barrier factors, namely, the bond dissociation energy of the substrate that decreases with the number of chlorine atoms and the interaction between chloride ion and the radical that increases in the same direction. In the case of alpha,beta-polychloroethanes (Cl(3)C-CCl(3), Cl(2)HC-CCl(3), Cl(2)HC-CHCl(2), ClH(2)C-CHCl(2)) too, the first step is a dissociative electron transfer with sizable ion-radical interactions in the product cluster. Likewise, a second electron transfer immediately leads to the carbanion, which however prefers to expel a second chloride ion, leading to the corresponding olefin, than to be protonated to the hydrogenolysis product. The ion-radical interaction in the product cluster plays a major role in the control of the reduction potential. The reduction of the alpha,beta-polychloroethenes (Cl(2)C=CCl(2), ClHC=CCl(2), ClHC=CHCl) follows a similar 2e(-)-2Cl(-) reaction sequence, leading then to the corresponding alkynes. However, unlike the polychloroethane case, the expulsion of the first chloride ion follows a stepwise electron transfer/bond cleavage mechanism. The reduction potential is thus essentially governed by the thermodynamics of the anion radical formation.     

Electrochemical approach to the repair of oxetanes mimicking DNA (6-4) photoproducts

Electrochemical study of oxetanes mimicking DNA (6-4) photoproducts gives new insight into the repair mechanism by (6-4) photolyase. Both electrochemical oxidation and electrochemical reduction at carbon electrodes lead to the cleavage of the oxetanes in a retro-Paterno-Büchi sequence. Within the family of compounds investigated and the range of driving forces offered, transient formation of unstable radical ions is observed, for both oxidative and reductive cleavage. Taking advantage of the electrochemical signature of these mimics, enzymatic assay with Escherichia coli CPD photolyase coupled to electrochemical monitoring of the reaction brings evidence that enzymatic repair of (6-4) DNA photoproducts does involve a catalytic dissociative electron-transfer mechanism at the level of an oxetane intermediate.     

Theoretical and electrochemical analysis of dissociative electron transfers proceeding through formation of loose radical anion species: reduction of symmetrical and unsymmetrical disulfides

The dissociative reduction of a series of symmetrical (RSSR, R = H, Me, t-Bu, Ph) and unsymmetrical disulfides (RSSR', R = H, R' = Me and R = Ph, R' = Me, t-Bu) was studied theoretically, by MO ab initio calculations and, for five of them, also experimentally, by convolution voltammetry in N,N-dimethylformamide. The reduction is dissociative but proceeds by a stepwise mechanism entailing the formation of the radical anion species. The electrochemical data led to estimated large intrinsic barriers, in agreement with an unusually large structural modification undergone by the disulfide molecules upon electron transfer. The theoretical results refer to MP2/3-21G*//MP2/3-21G*, MP2/3-21*G*//MP2/3-21G*, CBS-4M, and G2(MP2), the latter approach being used only for the molecules of small molecular complexity. A loose radical-anion intermediate was localized and the dissociation pattern for the relevant bonds analyzed. For all compounds, the best fragmentation pathway in solution is cleavage of the S-S bond. In addition, S-S bond elongation is the major structural modification undergone by the disulfide molecule on its way to the radical anion and eventually to the fragmentation products. The calculated energy of activation for the initial electron transfer was estimated from the crossing of the energy profiles of the neutral molecule and its radical anion (in the form of Morse-like potentials) as a function of the S-S bond length coordinate. The inner intrinsic barrier obtained in this way is in good agreement with that determined by convolution voltammetry, once the solvent effect is taken into account.     

Sticky dissociative electron transfer to polychloroacetamides. In-cage ion-dipole interaction control through the dipole moment and intramolecular hydrogen bond

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.     

Theoretical studies to understand surface chemistry on carbon anodes for lithium-ion batteries: reduction mechanisms of ethylene carbonate.

Reductive decomposition mechanisms for ethylene carbonate (EC) molecule in electrolyte solutions for lithium-ion batteries are comprehensively investigated using density functional theory. In gas phase the reduction of EC is thermodynamically forbidden, whereas in bulk solvent it is likely to undergo one- as well as two-electron reduction processes. The presence of Li cation considerably stabilizes the EC reduction intermediates. The adiabatic electron affinities of the supermolecule Li(+)(EC)n (n = 1-4) successively decrease with the number of EC molecules, independently of EC or Li(+) being reduced. Regarding the reductive decomposition mechanism, Li(+)(EC)n is initially reduced to an ion-pair intermediate that will undergo homolytic C-O bond cleavage via an approximately 11.0 kcal/mol barrier, bringing up a radical anion coordinated with Li(+). Among the possible termination pathways of the radical anion, thermodynamically the most favorable is the formation of lithium butylene dicarbonate, (CH2CH2OCO2Li)2, followed by the formation of one O-Li bond compound containing an ester group, LiO(CH2)2CO2(CH2)2OCO2Li, then two very competitive reactions of the further reduction of the radical anion and the formation of lithium ethylene dicarbonate, (CH2OCO2Li)2, and the least favorable is the formation of a C-Li bond compound (Li carbides), Li(CH2)2OCO2Li. The products show a weak EC concentration dependence as has also been revealed for the reactions of LiCO3(-) with Li(+)(EC)n; that is, the formation of Li2CO3 is slightly more favorable at low EC concentrations, whereas (CH2OCO2Li)2 is favored at high EC concentrations. On the basis of the results presented here, in line with some experimental findings, we find that a two-electron reduction process indeed takes place by a stepwise path. Regarding the composition of the surface films resulting from solvent reduction, for which experiments usually indicate that (CH2OCO2Li)2 is a dominant component, we conclude that they comprise two leading lithium alkyl bicarbonates, (CH2CH2OCO2Li)2 and (CH2OCO2Li)2, together with LiO(CH2)2CO2(CH2)2OCO2Li, Li(CH2)2OCO2Li and Li2CO3.     

Kinetics of dissociative electron transfer to ascaridole and dihydroascaridole-model bicyclic endoperoxides of biological relevance

The homogeneous and heterogeneous electron transfer (ET) reduction of ascaridole (ASC) and dihydroascaridole (DASC), two bicyclic endoperoxides, chosen as convenient models of the bridged bicyclic endoperoxides found in biologically relevant systems, were studied in aprotic media by using electrochemical methods. ET is shown to follow a concerted dissociative mechanism that leads to the distonic radical anion, which is itself reduced in a second step by an overall two-electron process. The kinetics of homogeneous ET to these endoperoxides from an extensive series of radical anion electron donors were measured as a function of the driving force of electron transfer (deltaG(o)ET). The kinetics of heterogeneous ET were also studied by convolution analysis. Together, the heterogeneous and homogeneous ET kinetic data provide the best example of the parabolic nature of the activation-driving force relationship for a concerted dissociative ET described by Savéant; the data is particularly illustrative due to the low bond-dissociation enthalpy (BDE) of the O-O bond and hence small intrinsic barriers. Analysis of the data allowed the dissociative reduction potentials (E(o)diss) to be determined as -1.2 and -1.1 Vagainst SCE for ASC and DASC, respectively. Unusually low pre-exponential factors measured in temperature-dependent kinetic studies suggest that ET to these O-O bonded systems is nonadiabatic. Analysis of ET kinetics for ASC and DASC by the Savéant model with a modification for nonadiabaticity allowed the intrinsic free energy for ET to be determined. The use of this approach and estimates for the BDE provide approximations of the reorganization energies. We suggest the methodology described herein can be used to evaluate the extent of ET to other endoperoxides of biological relevance and to provide thermochemical data not otherwise available.     

Mechanisms of reductive eliminations in square planar Pd(II) complexes: nature of eliminated bonds and role of trans influence

The trans influence of various phosphine ligands (L) in direct as well as dissociative reductive elimination pathways yielding CH(3)CH(3) from Pd(CH(3))(2)L(2) and CH(3)Cl from Pd(CH(3))(Cl)L(2) has been quantified in terms of isodesmic reaction energy, E(trans), using the MPWB1K level of density functional theory. In the absence of a large steric effect, E(trans) correlated linearly with the activation barrier (E(act)) of both direct and dissociation pathways. The minimum of molecular electrostatic potential (V(min)) at the lone pair region of phosphine ligands has been used to assess their electron donating power. E(trans) increased linearly with an increase in the negative V(min) values. Further, the nature of bonds that are eliminated during reductive elimination have been analyzed in terms of AIM parameters, viz. electron density (ρ(r)), Laplacian of the electron density (?(2)ρ(r)), total electron energy density (H(r)), and ratio of potential and kinetic electron energy densities (k(r)). Interestingly, E(act) correlated inversely with the strength of the eliminated metal-ligand bonds measured in terms of the bond length or the ρ(r). Analysis of H(r) showed that elimination of the C-C/C-Cl bond becomes more facile when the covalent character of the Pd-C/Pd-Cl bond increases. Thus, AIM details clearly showed that the strength of the eliminated bond is not the deciding factor for the reductive elimination but the nature of the bond, covalent or ionic. Further, a unified picture showing the relationship between the nature of the eliminated chemical bond and the tendency of reductive elimination is obtained from the k(r) values: the E(act) of both direct and dissociative mechanisms for the elimination of CH(3)CH(3) and CH(3)Cl decreased linearly when the sum of k(r) at the cleaved bonds showed a more negative character. It means that the potential electron energy density dominates over the kinetic electron energy density when the bonds (Pd-C/Pd-Cl) become more covalent and the eliminated fragments attain more radical character leading to the easy formation of C-C/C-Cl bond.     

Reductive cleavage mechanism of methylcobalamin: elementary steps of Co-C bond breaking

Density functional theory has been applied to the investigation of the reductive cleavage mechanism of methylcobalamin (MeCbl). In the reductive cleavage of MeCbl, the Co-C bond is cleaved homolytically, and formation of the anion radical ([MeCbl]*-) reduces the dissociation energy by approximately 50%. Such dissociation energy lowering in [MeCbl]*- arises from the involvement of two electronic states: the initial state, which is formed upon electron addition, has dominant pi*corrin character, but when the Co-C bond is stretched the unpaired electron moves to the sigma*Co-C state, and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. The pi*corrin-sigma*Co-C states crossing does not take place at the equilibrium geometry of [MeCbl]*- but only when the Co-C bond is stretched to 2.3 A. In contrast to the neutral cofactor, the most energetically efficient cleavage of the Co-C bond is from the base-off form. The analysis of thermodynamic and kinetic data provides a rationale as to why Co-C cleavage in reduced form requires prior departure of the axial base. Finally, the possible connection of present work to B12 enzymatic catalysis and the involvement of anion-radical-like [MeCbl]*- species in relevant methyl transfer reactions is discussed.     

Theoretical study of the CH2 + O photodissociation of formaldehyde adsorbed on the Ag(111) surface

Configuration interaction calculations of the ground and excited states of the H2CO molecule adsorbed on the Ag(111) surface have been carried out to study the photoinduced dissociation process leading to polymerization of formaldehyde. The metal-adsorbate system has been described by the embedded cluster and multireference configuration interaction methods. The pi electron-attachment H2CO- and n-pi* internally excited H2CO* states have been considered as possible intermediates. The calculations have shown that H2CO* is only very weakly bound on Ag(111), and thus that the dissociation of adsorbed formaldehyde due to internal excitation is unlikely. By contrast, the H2CO- anion is strongly bound to Ag(111) and gains additional vibrational energy along the C-O stretch coordinate via Franck-Condon excitation from the neutral molecule. Computed energy variations of adsorbed H2CO and H2CO- at different key geometries along the pathway for C-O bond cleavage make evident, however, that complete dissociation is very difficult to attain on the potential energy surface of either of these states. Instead, reneutralization of the vibrationally excited anion by electron transfer back to the substrate is the most promising means of breaking the C-O bond, with subsequent formation of the coadsorbed O and CH2 fragments. Furthermore, it has been demonstrated that the most stable state for both dissociation fragments on Ag(111) is a closed-shell singlet, with binding energies relative to the gas-phase products of approximately 3.2 and approximately 1.3 eV for O and CH2, respectively. Further details of the reaction mechanism for the photoinduced C-O bond cleavage of H2CO on the Ag(111) surface are also given.     

Reactivite des β-cyclopropyl α-enones—II : Etude de l'addition des diorganocuprates

The lithium dimethylcuprate addition on six substituted bicyclo[3.1.0]hex-3-en-2-ones was studied. For five ketones, both expected 1,4-addition compound and 1,6-addition compounds are obtained. The last products result from a cyclopropane bond cleavage. There is no evidence for a correlation between the radical anion half-lives and the formation of ring opened compounds. In many case, the broken bond is different from that which is concerned in the reduction by solvated electrons in liquid ammonia. So the 1,6-addition products do nor probably arise through an electron transfer mechanism. However, a nucleophilic attack of the substrate by a copper atom followed by a reductive elimination inside the complex can be supposed; then both the nature and stereochemistry of reaction products can be explained.