The realization of a phosphine‐stabilized diborene, Et3P?(Mes)B?B(Mes)?PEt3 ( 4 ), by KC8 reduction of Et3P?B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC‐stabilized diborene IMe?(Dur)B?B(Dur)?IMe ( 1 ). Importantly, both species feature unusual electron‐rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the B? B double bond system. Thus, the stronger σ‐donor strength and larger electronegativity of the NHC ligand results in an energetically higher‐lying HOMO, making 1 a stronger neutral reductant as 4 ( 1 : E1/2=?1.55 V; 4 : ?1.05 V), and a smaller HOMO–LUMO gap of 1 accompanied by a noticeable red‐shift of its lowest‐energy absorption band with respect to 4 . Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron‐centered radical cations ( 5 and 6 ). 相似文献
The transfer hydrogenation of N‐heterocyclic carbene (NHC)‐supported diborenes with dimethylamine borane proceeds with high selectivity for the trans‐1,2‐dihydrodiboranes. DFT calculations, supported by kinetic studies and deuteration experiments, suggest a stepwise proton‐first‐hydride‐second reaction mechanism via an intermediate μ‐hydrodiboronium dimethylaminoborate ion pair. 相似文献
The two‐electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na2S2O4 gave a corresponding isophlorin ( Iph ) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph . Further reduction of Iph proceeded to form an unprecedented four‐electron‐reduced porphyrin ( IphH2 ), which was fully characterized by spectroscopic and X‐ray crystallographic analysis. IphH2 , with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton‐coupled four‐electron reversible redox system. 相似文献
One‐electron reduction is commonly used in organic chemistry for the formation of radicals by the stepwise transfer of one or two electrons from a donor to an organic substrate. Besides metallic reagents, single‐electron reducers based on neutral organic molecules have emerged as an attractive novel source of reducing electrons. The past 20 years have seen the blossoming of a particular class of organic reducing agents, the electron‐rich olefins, and their application in organic synthesis. This Review gives an overview of the different types of organic donors and their specific characteristics in organic transformations. 相似文献
Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH2) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ .? produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH2. DDQ is recovered by the reaction of DDQH2 with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen. 相似文献
Summary: The possibility of transforming a living anionic polymerization into a stable radical‐mediated radical polymerization (SFRP) was demonstrated. For this purpose, 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) alcoholate, formed by a one‐electron redox reaction between potassium naphthalene and TEMPO, was used to initiate the living anionic polymerization of ethylene oxide (EO). Poly(ethylene oxide) obtained in this way possessed TEMPO terminal units and was subsequently used as an initiator for the SFRP of styrene to give block copolymers.
A one‐electron redox reaction gives rise to TEMPO alcoholate, which is able to initiate the living anionic polymerization of ethylene oxide (EO). 相似文献
Rate constants khom and khet are reported for the homogeneous electron‐self‐exchange and the heterogeneous electrochemical electron‐transfer reactions, respectively, of the cyclooctatetraene/cyclooctatetraene? (COT/COT.?) redox couple. In acetonitrile, the values khom(298 K)=(5±3)×105M ?1s?1 and khet(295 K)=8×10?3cm s?1 are found, whereas slightly faster rates are obtained in dimethylformamide, namely, khom(298 K)=(1.6±0.6)×106M ?1s?1 and khet(295 K)=2×10?2cm s?1. The khom rates are obtained from electron spin resonance (ESR) line broadening whereas the khet rates are measured at a mercurized Pt electrode by using Nicolson’s method. The slowness of both electron‐transfer reactions is caused by the high inner‐sphere reorganization energy that results from the inevitable conformational change that takes place upon going from the tub‐like COT molecule to the planar COT.? anion. The rates are well‐understood in terms of Marcus theory, including an additional medium inner‐sphere mode which is responsible for the flattening of COT. 相似文献
To explore the nature of electron attachment to guanine‐centered DNA single strands in the presence of a polarizable medium, a theoretical investigation of the DNA oligomer dinucleoside phosphate deoxyguanylyl‐3′,5′‐deoxyguanosine (dGpdG) was performed by using density functional theory. Four different electron‐distribution patterns for the radical anions of dGpdG in aqueous solution have been located as local minima on the potential energy surface. The excess electron is found to reside on the proton of the phosphate group (dGpH?dG), or on the phosphate group (dGp.?dG), or on the nucleobase at the 5′ position (dG.?pdG), or on the nucleobase at the 3′ position (dGpdG.?), respectively. These four radical anions are all expected to be electronically viable species under the influence of the polarizable medium. The predicted energetics of the radical anions follows the order dGp.?dG>dG.?pdG>dGpdG.?>dGpH?dG. The base–base stacking pattern in DNA single strands seems unaffected by electron attachment. On the contrary, intrastrand H‐bonding is greatly influenced by electron attachment, especially in the formation of base‐centered radical anions. The intrastrand H‐bonding patterns revealed in this study also suggest that intrastrand proton transfer might be possible between successive guanines due to electron attachment to DNA single strands. 相似文献
A novel fluorescent molecular tweezer (FMT), built on the pyridine‐2,6‐bis‐carboxamide framework, has been developed that, in presence of a red emitter, gives rise to white light emission in response to the addition of H2PO4? anions. The FMT incorporates two pyrene moieties as fluorescent reporter units and a strategically placed amine residue that imparts pH sensitivity to the fluorescence and offers additional electrostatic/hydrogen‐bonding interactions to the anions. As a result, this FMT selectively binds monoanionic tetrahedral oxyanions such as H2PO4? and HSO4? that contain hydrogen bond donors and acceptors, and can sense their presence in aqueous acetonitrile through changes in fluorescence. Anion binding results in excimer formation by the pyrenes and a bluish‐green emission from the FMT. Both amide and amine residues of the FMT interact with these anions. The binding stoichiometry with H2PO4? and HSO4? was found to be 1:1 and affinity of the FMT for these anions is of the order of 104 m ?1. The limit of detection for H2PO4? was found to be 13 nm . Addition of a perylene monoimide‐based red emitter gives rise to panchromatic emission perceived as white light. 相似文献
A hydrogen‐bonded complex was successfully isolated as crystals from the anthranol/anthroxyl pair in the self‐exchange proton‐coupled electron transfer (PCET) reaction. The anthroxyl radical was stabilized by the introduction of a 9‐anthryl group at the carbon atom at the 10‐position. The hydrogen‐bonded complex with anthranol self‐assembled by π–π stacking to form a one‐dimensional chain in the crystal. The conformation around the hydrogen bond was similar to that of the theoretically predicted PCET activated complex of the phenol/phenoxyl pair. X‐ray crystal analyses revealed the self‐exchange of a hydrogen atom via the hydrogen bond, indicating the activation of the self‐exchange PCET reaction between anthranol and anthroxyl. Magnetic measurements revealed that magnetic ordering inside the one‐dimensional chain caused the inactivation of the self‐exchange reaction. 相似文献
A cobalt–porphyrin catalyst encapsulated in a cubic M8L6 cage allows cyclopropanation reactions in aqueous media. The caged‐catalyst shows enhanced activities in acetone/water as compared to pure acetone. Interestingly, the M8L6 encapsulated catalyst reveals size‐selectivity. Smaller substrates more easily penetrate through the pores of the “molecular ship‐in‐a‐bottle catalysts” and are hence converted faster than bigger substrates. In addition, N‐tosylhydrazone sodium salts are easy to handle reagents for cyclopropanation reactions under these conditions. 相似文献
The development of new bifunctional linchpins that permit the union of diverse building blocks is essential for the synthetic utility of anion relay chemistry (ARC). The design, synthesis, and validation of three vinylepoxide linchpins for through‐bond/through‐space ARC are now reported. For negative charge migration, this class of bifunctional linchpins employs initial through‐bond ARC by an SN2′ reaction, followed by through‐space ARC exploiting a 1,4‐Brook rearrangement. The trans‐disubstituted vinylepoxide linchpin yields a mixture of E/Z isomers, whereas the cis‐disubstituted and the trans‐trisubstituted vinylepoxide linchpins proceed to deliver three‐component adducts with excellent E selectivity. 相似文献
The direct fusion of a diphenylmethane segment to a NiII 5,10,15‐triarylporphyrin with three linkages furnished an air‐ and moisture‐stable neutral radical through unexpected and spontaneous oxidation. This radical was demetalated by treatment with H2SO4 and CF3CO2H to provide the corresponding free‐base radical. These porphyrin radicals are very stable owing to spin delocalization and have been fully characterized through UV/Vis/NIR absorption spectroscopy, X‐ray crystallographic analysis, magnetic susceptibility measurements, electrochemical studies, laser‐based ultrafast spectroscopic studies, and theoretical calculations. They were chemically oxidized and reduced to the corresponding cation and anion but did not react with hydrogen‐atom donors. 相似文献
The successful chain‐growth copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) polymerization employing Cu(0)/pentamethyldiethylenetriamine (PMDETA) and alkyl halide as catalyst is first investigated by a combination of nuclear magnetic resonance, gel‐permeation chromatography, and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. In addition, the electron transfer mediated “click‐radical” concurrent polymerization utilizing Cu(0)/PMDETA as catalyst is successfully employed to generate well‐defined copolymers, where controlled CuAAC polymerization of clickable ester monomer is progressed in the main chain acting as the polymer backbone, the controlled radical polymerization (CRP) of acrylic monomer is carried out in the side chain. Furthermore, it is found that there is strong collaborative effect and compatibility between CRP and CuAAC polymerization to improve the controllability.