Energetics of Electron‐Transfer Reactions of Photoinitiated Polymerization: Dye‐Sensitized Fragmentation of N‐Alkoxypyridinium Salts

Electron transfer from excited dyes to N‐alkoxypyridinium salts leads to reductive cleavage of the N−O bond to give an alkoxy radical that can be used to initiate polymerization. Bond‐dissociation energies obtained from calculations based on density‐functional theory are in agreement with predictions from a thermochemical cycle. These data show a difference of ca. 290 – 315 kJ/mol between the BDE of the pyridinium and that of the pyridyl radical and indicate that the fragmentation of the radical is highly exothermic. The energetic requirements for the photochemical electron transfer are discussed in terms of a simplified model that shows that the initiation efficiency of the radical polymerization can be correlated with a single parameter, the reduction potential of the sensitizing dye. Dyes from many classes and with absorption bands spanning the entire visible region were found to be effective in initiating photopolymerization of acrylate monomers in this system. Doubling of the photoresponse can be achieved through coupling of the reductive cleavage of the N‐alkoxypyridinium with an oxidative cleavage of a C−B bond of an alkyltriarylborate, a process that utilizes the chemical potential stored in the oxidized dye following electron transfer to the pyridinium salt.     

Cob(I)alamin als Katalysator. Retention der Konfiguration bei der reduktiv-protonenübertragenden Spaltung der Co,C-Bindung eines Alkylcobalamins. 7. Mitteilung [1]

Cob(I)alamin as Catalyst. 7. Communication [1]. Retention of Configuration during the Reductive Cleavage of the Co, C-Bond of an Alkylcobalamin Using catalytic amounts of cob(I)alamin (see Scheme 1) in aqueous acetic acid (?)-α-pinen ( 1 ) and (?)-β-pinen ( 2 ; s. Scheme 3) have been reduced. A large excess of metallic zinc served as electron source. The saturated products 5–8 (see Scheme 3) and the mechanistic aspects of their generation are discussed. The relative amounts of cis- ( 5 ) and trans-pinane ( 6 ) lead to the conclusion that the reductive cleavage of the Co, C-bond accompanied by H⁺ transfer in an alkylcobalamin occurs with retention of configuration. This result is in agreement with the corresponding cleavage of the Co,C-bond of an alkyl[hydroxy-diazaoctahydroporphinato]cobalt complex [9].     

Excess electron transfer from an internally conjugated aromatic amine to 5-bromo-2'-deoxyuridine in DNA

DNA duplexes containing an N,N,N',N'-tetramethyl-1,5-diaminonaphthalene analogue and 5-bromo-2'-deoxyuridine (BrdU) provide a readily accessible system for investigating excess electron transfer in DNA. Photoexcitation of the aromatic amine (lambda > 335 nm) induces reductive electron transfer as observed by strand cleavage adjacent to the BrdU residue. The weak exponential distance dependence (0.3 A-1) of electron transfer determined for this system of mixed dA-T and dG-dC base pairs suggests that thermally activated electron hopping is competitive with proton transfer within the dG.dC radical anion. The UV-dependent transfer of excess electrons and subsequent strand cleavage proceeds equivalently under anaerobic and aerobic conditions and is not sensitive to e-(aq) or hydroxyl radical trapping agents.     

Electron‐Transfer and Hydride‐Transfer Pathways in the Stoltz–Grubbs Reducing System (KOtBu/Et3SiH)

Recent studies by Stoltz, Grubbs et al. have shown that triethylsilane and potassium tert ‐butoxide react to form a highly attractive and versatile system that shows (reversible) silylation of arenes and heteroarenes as well as reductive cleavage of C−O bonds in aryl ethers and C−S bonds in aryl thioethers. Their extensive mechanistic studies indicate a complex network of reactions with a number of possible intermediates and mechanisms, but their reactions likely feature silyl radicals undergoing addition reactions and S_H2 reactions. This paper focuses on the same system, but through computational and experimental studies, reports complementary facets of its chemistry based on a) single‐electron transfer (SET), and b) hydride delivery reactions to arenes.     

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.     

Azo-pyrene–based fluorescent sensor of reductive cleavage of isomeric azo functional group

In this study we investigated the reductive azo cleavage of an azo compound presenting a pyrene fluorophore (Azo-py). Because of dramatic changes in its fluorescence, Azo-py could be used as a monitoring system for the reductive azo cleavage. Electron transfer from the pyrene unit to the azo moiety induced fluorescence quenching; this quenched fluorescence was recovered after the reductive azo cleavage. IR and NMR spectroscopy were used to study the various structural states. The rate of reductive cleavage of the azo compound, determined through fluorescence monitoring, depended on its structural state: the cleavage of trans-Azo-py was much faster than that of the cis-Azo-py. Furthermore, the Azo-py fluorophore was highly sensitive to the presence of zinc, but not other metal compounds, and the pH.     

Pushing the Limits of Neutral Organic Electron Donors: A Tetra(iminophosphorano)‐Substituted Bispyridinylidene

A new ground‐state organic electron donor has been prepared that features four strongly π‐donating iminophosphorano substituents on a bispyridinylidene skeleton. Cyclic voltammetry reveals a record redox potential of ?1.70 V vs. saturated calomel electrode (SCE) for the couple involving the neutral organic donor and its dication. This highly reducing organic compound can be isolated (44 %) or more conveniently generated in situ by a deprotonation reaction involving its readily prepared pyridinium ion precursor. This donor is able to reduce a variety of aryl halides, and, owing to its redox potential, was found to be the first organic donor to be effective in the thermally induced reductive S? N bond cleavage of N,N‐dialkylsulfonamides, and reductive hydrodecyanation of malonitriles.     

Photosensitized Electron Transfer Promoted Reductive Activation of Carbon-Selenium Bonds To Generate Carbon-Centered Radicals: Application for Unimolecular Group Transfer Radical Reactions

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of photosensitized electron transfer (PET) promoted reductive activation of organoselenium substrates. PET activation of substrates 1-5 is achieved through a photosystem comprised of light-absorbing 1,5-dimethoxynaphthalene (DMN) as electron donor and ascorbic acid as co-oxidant. The fluorescence quenching of (1)DMN by organoselenium compounds 1-5, correlation of fluorescence quenching rate constant with the reduction potentials of 1-5, and the dependence of photodissociation quantum yields of 1-5 on their concentration suggests the occurrence of electron-transfer (ET) processes between (1)DMN and 1-5. Steady state photolysis of organoselenium substrates (R(2)CHSePh) in the presence of (1)DMN and ascorbic acid leads to the cleavage of the -C-Se- bond to produce a carbon-centered radical and PhSe(-) species via the intermediacy of R(2)CH-SePh&uprhbr;(-)(*). The mechanistic interpretation for the reductive activation of -C-Se- bonds and the synthetic utility of observed cleavage pattern is extended for the unimolecular group transfer radical sequences.     

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates: the role of radical/ion pair formation

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.     

Graphene‐Functionalized 3D‐Carbon Fiber Electrodes – Preparation and Electrochemical Characterization

Graphene nanosheets were produced on the surface of carbon fibers by in situ electrochemical procedure including oxidative and reductive steps to yield first graphene oxide, later converted to graphene. The electrode material composed of graphene‐functionalized carbon fibers was characterized by scanning electron microscopy (SEM) and cyclic voltammery demonstrating superior electrochemical kinetics comparing with the original carbon paper. The interfacial electron transfer rate for the reversible redox process of [Fe(CN)₆]^3?/4? was found ca. 4.5‐fold higher after the electrode modification with the graphene nanosheets. The novel electrode material is suggested as a promising conducting interface for bioelectrocatalytic electrodes used in various electrochemical biosensors and biofuel cells, particularly operating in vivo.     

A DFT Study on the Conversion of Aryl Iodides to Alkyl Iodides: Reductive Elimination of R−I from Alkylpalladium Iodide Complexes with Accessible β‐Hydrogens

DFT calculations have been performed on the palladium‐catalyzed carboiodination reaction. The reaction involves oxidative addition, alkyne insertion, C?N bond cleavage, and reductive elimination. For the alkylpalladium iodide intermediate, LiOtBu stabilizes the intermediate in non‐polar solvents, thus promoting reductive elimination and preventing β‐hydride elimination. The C?N bond cleavage process was explored and the computations show that PPh₃ is not bound to the Pd center during this step. Experimentally, it was demonstrated that LiOtBu is not necessary for the oxidative addition, alkyne insertion, or C?N bond cleavage steps, lending support to the conclusions from the DFT calculations. The turnover‐limiting steps were found to be C?N bond cleavage and reductive elimination, whereas oxidative addition, alkyne insertion, and formation of the indole ring provide the driving force for the reaction.     

Quantum chemical estimation of the possibility of the formation of chloro- and bromomethane radical anions and their fragmentation paths in the gas phase and in solution by MNDO and AM1 techniques

By semiempirical MNDO and AM1 methods it was shown that electron transfer on the chloro-and bromomethane molecules of the general formula CH_nHal_4–n (n=1–3) results either in a kinetically independent particle, i.e., a radical anion, or in C-Hal-bond cleavage with the formation of Hal^– and the respective radical. The enthalpy, activation energy of the reactions, and data on the geometry of the radical anion obtained show that the increasing the number of halogen atoms in the initial molecule and decreasing the solvent polarity favor radical anion stabilization. It was established that the cleavage of the C-H-bond in the radical anion is not favored energetically. Fragmentation at the C-H-bond can proceed according to the mechanism of dissociative electron capture by halomethane molecule only with additional factors favoring this reaction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1886–1892, November, 1993.     

Reductive Asymmetric Aza-Mislow-Evans Rearrangement by 1,3,2-Diazaphospholene Catalysis**

1,3,2-diazaphospholene hydrides (DAP−H) enable smooth conjugate reduction of polarized double bonds. The transiently formed phosphorus-enolate provides a potential platform for reductive α-functionalizations. In this respect, asymmetric C-heteroatom bond forming processes are synthetically appealing but remain elusive. We report a 1,3,2-diazaphospholene-catalyzed three-step cascade reaction of N-sulfinyl acrylamides comprised of conjugate reduction, [2,3]-sigmatropic aza-Mislow-Evans rearrangement and subsequent S−O bond cleavage. The obtained enantio-enriched α-hydroxy amides are formed in good yields and excellent enantiospecificity. The stereo-defined P-bound N,O-ketene aminal ensures an excellent transfer of chirality from the sulfur stereocenter to α-carbon. The transformation operates under mild conditions at ambient temperature. Moreover, DAP−H is a competent reductant for the cleavage of formed sulfenate ester, eliminating the extra step in traditional Mislow-Evans processes.     

Reductive electron transfer in phenothiazine-modified DNA is dependent on the base sequence

A new DNA assay has been designed, prepared and applied for the chemical investigation of reductive electron transfer through the DNA. It consists of 5-(10-methyl-phenothiazin-3-yl)-2'-deoxyuridine (Ptz-dU, 1) as the photoexcitable electron injector and 5-bromo-2'-deoxyuridine (Br-dU) as the electron trap. The Ptz-dU-modified oligonucleotides were synthesised by means of a Suzuki-Miyaura cross-coupling protocol and subsequent automated phosphoramidite chemistry. Br-dU represents a kinetic electron trap, since it undergoes a chemical modification after its one-electron reduction that can be analysed by piperidine-induced strand cleavage. The quantification of the strand cleavage yields from irradiation experiments reveals important information about the electron-transfer efficiency. The performed DNA studies focused on the base sequence dependence of the electron-transfer efficiency with respect to the proposal that C*- and T*- act as intermediate electron carriers during electron hopping. From our observations it became evident that excess-electron transfer is highly sequence dependent and occurs more efficiently over T-A base pairs than over C-G base pairs.     

Synthesis of 8-amino-4-methylthio-6-methyl-2-(β-D-ribofuranosyl)-2,6-dihydro-1,2,3,5,6,7-hexaazaacenaphthylene and an unusual reductive ring-opening of the 1,2,3,5,6,7-hexaazaacenaphthylene ring system

The tricyclic nucleoside 8-amino-4-methylthio-6-methyl-2-(β-D-ribofuranosyl)-1,2,3,5,6,7-hexaazaacenaphthylene ( 3 ) was synthesized from 3-cyano-4,6-bis(methylthio)-1-(β-D-ribofuranosyl)pyrazolo[3,4-d]pyrimidine ( 1 ). Attempts to synthesize 8-amino-6-methyl-2-(β-D-ribofuranosyl)-1H-2,6-dihydro-1,2,3,5,6,7-hexaazaacenaphthylene ( 5 ) ([an aza analog of 6-amino-4-methyl-8-(β-D-ribofuranosyl)-1,3,4,5,8-pentaazaacenaphthylene (TCN)], which is a potent antitumor agent), by the treatment of 3 with Raney nickel did not afford the desired aza analog of TCN. Instead, it was established that a reductive cleavage of the pyridazine moiety of 3 had occurred to give 4-methylamino-6-methylthio-1-(β-D-ribofuranosyl)-1H-pyrazolo[3,4-d]pyrimidine-3-carboxamidine ( 6 ). Assuming that solubility was a problem in the reductive step, the isopropylidene derivative of 3 , 8-amino-6-methyl-4-methylthio-2-(2,3-O-isopropylidene-β-D-ribofuranosyl)-2,6-dihydro-1,2,3,5,6,7-hexaazaacenaphthylene ( 8 ), was treated with Raney nickel, only to observe that a similar reductive ring cleavage of 8 had occurred to afford 4-methylamino-6-methylthio-1-(2,3-O-isopropylidene-β-D-ribofuranosyl)-1H-pyrazolo[3,4-d]pyrimidine-3-carboxamidine ( 10 ) and 4-methylamino-1-(2,3-O-isopropylidene-β-D-ribofuranosyl)-1H-pyrazolo[3,4-d]pyrimidine-3-carboxamidine ( 11 ). Structural assignments for all products were established by physico-chemical procedures.     

Unraveling the Mechanism and Regioselectivity of the B12‐Dependent Reductive Dehalogenase PceA

PceA is a cobalamin‐dependent reductive dehalogenase that catalyzes the dechlorination of perchloroethylene to trichloroethylene and then to cis‐dichloroethylene as the sole final product. The reaction mechanism and the regioselectivity of this enzyme are investigated by using density functional calculations. Four different substrates, namely, perchloroethylene, trichloroethylene, cis‐dichloroethylene, and chlorotheylene, have been considered and were found to follow the same reaction mechanism pattern. The reaction starts with the reduction of Co^II to Co^I through a proton‐coupled electron transfer process, with the proton delivered to a Tyr246 anion. This is followed by concerted C?Cl bond heterolytic cleavage and proton transfer from Tyr246 to the substrate carbon atom, generating a Co^III?Cl intermediate. Subsequently, a one‐electron transfer leads to the formation of the Co^II?Cl product, from which the chloride and the dehalogenated product can be released from the active site. The substrate reactivity follows the trend perchloroethylene>trichloroethylene?cis‐dichloroethylene?chlorotheylene. The barriers for the latter two substrates are significantly higher compared with those for perchloroethylene and trichloroethylene, implying that PceA does not catalyze their degradation. In addition, the formation of cis‐dichloroethylene has a lower barrier by 3.8 kcal mol^?1 than the formation of trans‐dichloroethylene and 1,1‐dichloroethylene, reproducing the regioselectivity. These results agree quite well with the experimental findings, which show cis‐dichloroethylene as the sole product in the PceA‐catalyzed dechlorination of perchloethylene and trichloroethylene.     

Electrochemical reduction of some methanofullerenes. On the mechanism of the retro-Bingel reaction

The reactions of electrochemical reduction of methanofullerenes bearing phosphonate and alkoxycarbonyl groups at theexo-carbon atom were studied. The mechanism of the retro-Bingel reaction as the cleavage of two C−C bonds between the C(61) atom and the fullerene shell accompanied by electrochemical electron transfer was proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 426–429, March. 2000.     

Distinction between stepwise and concerted mechanisms in reductive cleavage reactions—use of voltammetric current function in the analysis of non-linear kinetic laws

A systematic way of distinguishing stepwise and concerted mechanisms in reductive cleavage reactions has been formulated involving current function analysis of the voltammetric data. The electrochemical reductive cleavage of the carbon-iodine bond in 1,3-dichloro-2-iodobenzene has been analyzed from a mechanistic point of view to illustrate the methodology. 1,3-Dichloro-2-iodobenzene undergoes an initial stepwise electron transfer obeying quadratic activation-driving force relationship. The current function analysis yields the reorganization energy for the reduction of 1,3-dichloro-2-iodobenzene and the results have been verified independently using convolution potential sweep voltammetry.     

Intramolecular Acetyl Transfer to Olefins by Catalytic C−C Bond Activation of Unstrained Ketones

A rhodium‐catalyzed intramolecular acetyl‐group transfer has been achieved through a “cut and sew” process. The challenge arises from the existence of different competitive pathways. Preliminary success has been achieved with unstrained enones that contain a biaryl linker. The use of an electron‐rich N‐heterocycilc carbene (NHC) ligand is effective to inhibit undesired β‐hydrogen elimination. Various 9,10‐dihydrophenanthrene derivatives can be prepared with excellent functional‐group compatibility. The ¹³C‐labelling study suggests that the reaction begins with cleavage of the unstrained C?C bond, followed by migratory insertion and reductive elimination.