Synthesen durch Halbleiter‐Photokatalyse: Solare Chemie

Semiconducting zinc and cadmium sulfide powders are photocatalysts for novel organic syntheses. Due to their ability to generate reducing and oxidizing surface centers through light absorption, these powders can catalyze C‐C and C‐N coupling reactions via initial interfacial electron transfer with adsorbed substrates like olefins, imines, and 1,2‐diazenes. The thus obtained primary intermediates may be transformed to reduced and oxidized products, like in an electrochemical reaction, or combine to one unique addition product. In the latter case the addition of cyclic olefins to imines and 1,2‐diazenes affords novel homoallylamines and allylhydrazines. This is a good example for “green chemistry”, since no waste materials are produced and solar light is used as energy source. The eterogeneous sensitizer can be conveniently separated from the products by filtration.     

Integrating proton coupled electron transfer (PCET) and excited states

In many of the chemical steps in photosynthesis and artificial photosynthesis, proton coupled electron transfer (PCET) plays an essential role. An important issue is how excited state reactivity can be integrated with PCET to carry out solar fuel reactions such as water splitting into hydrogen and oxygen or water reduction of CO₂ to methanol or hydrocarbons. The principles behind PCET and concerted electron–proton transfer (EPT) pathways are reasonably well understood. In Photosystem II antenna light absorption is followed by sensitization of chlorophyll P₆₈₀ and electron transfer quenching to give P₆₈₀⁺. The oxidized chlorophyll activates the oxygen evolving complex (OEC), a CaMn₄ cluster, through an intervening tyrosine–histidine pair, Y_Z. EPT plays a major role in a series of four activation steps that ultimately result in loss of 4e^?/4H⁺ from the OEC with oxygen evolution. The key elements in photosynthesis and artificial photosynthesis – light absorption, excited state energy and electron transfer, electron transfer activation of multiple-electron, multiple-proton catalysis – can also be assembled in dye sensitized photoelectrochemical synthesis cells (DS-PEC). In this approach, molecular or nanoscale assemblies are incorporated at separate electrodes for coupled, light driven oxidation and reduction. Separate excited state electron transfer followed by proton transfer can be combined in single semi-concerted steps (photo-EPT) by photolysis of organic charge transfer excited states with H-bonded bases or in metal-to-ligand charge transfer (MLCT) excited states in pre-associated assemblies with H-bonded electron transfer donors or acceptors. In these assemblies, photochemically induced electron and proton transfer occur in a single, semi-concerted event to give high-energy, redox active intermediates.     

Efficiency of Electron Transfer Initiated Chemiluminescence

Although the mechanisms of many chemiluminescence (CL) reactions have been intensively studied, no general model has been suggested to rationalize the efficiency of these transformations. To contribute to this task, we report here quantum yields for some well‐characterized CL reactions, concentrating on recent reports of efficient transformations. Initially, a short review on the most important general CL mechanisms is given, including unimolecular peroxide decomposition, electrogenerated CL, as well as the intermolecular and intramolecular catalyzed decomposition of peroxides. Thereafter, quantum yield values for several CL transformations are compiled, including the unimolecular decomposition of 1,2‐dioxetanes and 1,2‐dioxetanones, the catalyzed decomposition of appropriate peroxides and the induced decomposition of properly substituted 1,2‐dioxetane derivatives. Finally, some representative examples of quantum yields for complex CL transformations, like luminol oxidation and the peroxyoxalate reaction, in different experimental conditions are given. This quantum yield compilation indicates that CL transformations involving electron transfer steps can occur with high efficiency in general only if the electron transfer is of intramolecular nature, with the intermolecular processes being commonly inefficient. A notable exception to this general rule is the peroxyoxalate reaction which, also constituting an example of an intermolecular electron transfer system, possesses very high quantum yields.     

Contributions of organic electrosynthesis to green chemistry

In organic electrosynthesis C–C bond formation and functional group interconversion proceed via reactive intermediates that are generated by electron transfer at the anode and cathode. Electron transfer combined with a chemical reaction provides conversions that are not available in non-electrochemical reactions. These are potential selectivity, redox-umpolung, and the substitution of a hydrogen atom for a nucleophile or the addition of two nucleophiles to a double bond in one-pot reactions. Furthermore electrolysis is well suited for oxidation and reduction of functional groups. Electrochemical syntheses need mostly fewer steps, produce less waste, provide a cheaper reagent, require less auxiliaries and allow often an easier scale-up than non-electrochemical syntheses. In addition, they can be conducted at ambient temperature and pressure. All these qualities agree well with the rules of green chemistry. This statement is substantiated with examples of C–C bond formation and functional group interconversion at the anode and cathode.     

DYE-SENSITIZED HOLE SATURATION CURRENTS IN AROMATIC HYDROCARBON CRYSTALS*

Abstract— Dye-sensitized hole currents have been investigated in aromatic single crystals of increasing ionisation energies. The influence of various parameters on these sensitized currents have been studied: voltage, intensity and wavelength of the incident light, concentration of the sensitizer and of an additional oxidant. Direct transfer of an electron occurs from the crystal to the excited singlet state of the sensitizer when the ionisation energy of the crystal is smaller than the electron affinity of the sensitizer. When this condition does not hold, a competitive population of the lowest triplet state in the system occurs. In this case sensitized hole generation involves a consecutive electron-transfer reaction with an additional oxidant.     

Electron transfer and structural change: distinguishing concerted and two-step processes

In electron-transfer reactions accompanied by structural changes, the structural change can be concerted with electron transfer or can occur in a separate reaction either preceding or following the electron-transfer step. In this paper we discuss ways of distinguishing concerted reactions from the latter two-step type. Included are recent examples in which no intermediates have been detected in the reactions, thus precluding the direct assignment to the two-step category. In these cases, other means are used to build support for the two-step mechanism with respect to the concerted process. These include an example of structural change preceding electron transfer, a demonstration that the current models of concerted reactions cannot fit the voltammetric data, and a case in which an independent measure of the inner reorganization energy was used to show that the reaction could not be a concerted electron transfer and structural change.     

Radical and Electron Recycling in Catalysis

An increasing number of enzymes are being discovered that contain radicals or catalyze reactions via radical intermediates. These radical enzymes are able to open reaction pathways that two‐electron steps cannot achieve. Recently, organic chemists started to apply related radical chemistry for synthetic purposes, whereby an electron energized by light is recycled in every turnover. This Minireview compares this new type of reaction with enzymes that use recycling radicals and single electrons as cofactors.     

New perspective of electron transfer chemistry

A new perspective of electron transfer chemistry is described for fine control of electron transfer reactions including back electron transfer in the charge separated state of artificial photosynthetic compounds and its synthetic application. Fundamental electron transfer properties of suitable components of efficient electron transfer systems are described in light of the Marcus theory of electron transfer, in particular focusing on the Marcus inverted region, and they are applied to design multi-step electron transfer systems which can well mimic the function of a photosynthetic reaction center. Both intermolecular and intramolecular electron transfer processes are finely controlled by complexation of radical anions, produced in the electron transfer, with metal ions which act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation to the promoting effects of metal ions on the electron transfer reactions. The mechanistic viability of metal ion catalysis in electron transfer reactions is demonstrated by a variety of examples of chemical transformations involving metal ion-promoted electron transfer processes as the rate-determining steps, which are made possible by complexation of radical anions with metal ions.     

PHOTOSYNTHESIS AS A RESOURCE FOR ENERGY AND MATERIALS*

Abstract— Photosynthesis, both natural and as a model process, is examined as a possible annually renewable resource for both material and energy. The conversion of carbohydrate from cane, beets and other sources through fermentation alcohol to hydrocarbon may again become economic in the light of improved fermentation technology. It may also be possible to produce material by direct fermentation of relatively labile carbohydrates in seaweed. Even the direct photosynthetic production of hydrocarbon from known sources (Hevea, etc.), or newly bred ones, seems possible in view of the large number of species and the new techniques of plant cell cloning which have already been successful on sugar cane. Finally, more distantly, synthetic systems constructed on the basis of our growing knowledge of the photosynthetic processes may produce fuel, fertilizer and power. Thus, from our current knowledge of the natural quantum conversion process in green plants we can envisage several photoelectron transfer processes. In a first one the excited sensitizer (chlorophyll) transfers its electron to an acceptor molecule such as an iron-sulfur complex which, in turn, could either reduce a carbon compound or pass that electron into a hydrogen-generating system leading to the evolution of molecular hydrogen. The remaining cation radical sensitizer would have to be neutralized through a chain of electron transfers which begins at another sensitized reaction (presumably by another kind of chlorophyll) through a quinone and other electron carriers. Finally, the last cation radical near the oxidation level of oxygen could be neutralized by electron transfer, ultimately from a water molecule (hydroxide or bicarbonate ion) involving a manganese catalyst. Some steps in this sequence of transfers have already been demonstrated in synthetic systems. However, the actual physical construction of such a complete system is a much more complex task.     

Metal phthalocyanine-sensitized photoreduction of nitro-compound

Metal phthalocyanine-sensitized photoreduction of dimethyl 4-nitrophthalate with ascorbic acid has been investigated. The primary photoreaction products are the corresponding amino-and hydroxylamino-compounds. The azoxy-compound is formed by coupling of the nitrosocompound with hydroxylamino-compound in the presence of air through secondary dark reaction. The redox potential and fluorescence quantum yield are also determined. The variation of the quantum yield of the sensitized photoreduction, the relative fluorescence quantum yield and their product with the concentration of nitro-compound has been examined. The efficiency of photoreduction sensitized by the excited singlet and triplet state of metal phthalocyanine has been also calculated. It is believed that electron transfer from the excited metal phthalocyanine to the nitro-compound is the initial process in the sensitized photoreduction. Quenching by electron transfer involves creation of an ion pair. Charge separation and back electron transfer is then a competitive process. Due to the spin selection rules, the efficiency of photoreduction sensitized by excited triplet state of metal phthalocyanine is higher than excited singlet state. Thus, a necessary requirement for a good sensitizer is that the triplet state is populated in high yield. An alternative way and also the aim of our work is to design a suitable phthalocyanine skeleton to overcome geminate recombination of the ion pair, in order to increase the efficiency of photoreduction sensitized by sir glet excited state of the sensitizer, so as to increase the quantum yield of the total sensitized photoreduction.     

氰基水解酶在有机合成中的应用

腈化物是有机合成中的重要中间体,因为其中的氰基可以通过多种方法引入,又能进一步转化成其它官能团。用氰基水解酶实现氰基降解不仅反应条件温和,少污染、易处理,更重要的是能实现一般化学转化所不能实现的优良的化学、区域及立体选择性。不管是从学术角度还是工程角度来看,氰基水解酶都是一种有巨大潜力的有机合成工具。     

Visible‐Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible‐light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross‐coupling reactions, α‐amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible‐light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible‐light‐absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo‐ or metal catalysis.     

PHOTOCHEMISTRY OF KETONES IN SOLUTION-49* A STUDY OF PHOTOSENSITIZED SPLITTING OF DIMETHYLTHYMINE DIMERS

Abstract— Several high energy ketone triplet sensitizers, e.g. carvone, camphor, 3-methylcyclohexanone, benzoin and 3-methylindanone, were studied as photosensitizers for the splitting of dimethylthymine dimers. The absence of splitting in all cases and the lack of quenching of benzoin and 3-methylindanone triplets by the trans-anti dimer of dimethylthymine strongly suggests that cleavage of dimethylthymine dimers cannot be achieved by a triplet mechanism on irradiation at Λ> 300 nm. The absence of optical rotation in the recovered chiral cis-anti dimethylthymine dimer after sensitized photolysis (12% splitting) in the presence of (—)-tryptophan suggests that. in highly polar solvents, such as methanol, where reaction probably takes place according to an electron transfer mechanism involving ion-pair intermediates, close approach of the sensitizer and substrate does not occur. To the extent that these results can be extrapolated to sensitized cleavage of cis-syn pyrimidine dimers in DNA brought about by action of photoreactivating enzyme or conventional photosensitizers, a mechanism involving dimer triplet states appears highly unlikely.     

Electron tunneling through sensitizer wires bound to proteins

We report a quantitative theoretical analysis of long-range electron transfer through sensitizer wires bound in the active-site channel of cytochrome P450cam. Each sensitizer wire consists of a substrate group with high binding affinity for the enzyme active site connected to a ruthenium-diimine through a bridging aliphatic or aromatic chain. Experiments have revealed a dramatic dependence of electron transfer rates on the chemical composition of both the bridging group and the substrate. Using combined molecular dynamics simulations and electronic coupling calculations, we show that electron tunneling through perfluorinated aromatic bridges is promoted by enhanced superexchange coupling through virtual reduced states. In contrast, electron flow through aliphatic bridges occurs by hole-mediated superexchange. We have found that a small number of wire conformations with strong donor–acceptor couplings can account for the observed electron tunneling rates for sensitizer wires terminated with either ethylbenzene or adamantane. In these instances, the rate is dependent not only on electronic coupling of the donor and acceptor but also on the nuclear motion of the sensitizer wire, necessitating the calculation of average rates over the course of a molecular dynamics simulation. These calculations along with related recent findings have made it possible to analyze the results of many other sensitizer-wire experiments that in turn point to new directions in our attempts to observe reactive intermediates in the catalytic cycles of P450 and other heme enzymes.     

Synthesis, isolation and characterization of cationic gold(I) N-heterocyclic carbene (NHC) complexes

A number of cationic gold(I) complexes have been synthesized and found to be stabilized by the use of N-heterocyclic carbene ligands. These species are often employed as in situ-generated reactive intermediates in gold catalyzed organic transformations. An isolated, well-defined species was tested in gold-mediated carbene transfer reactions from ethyl diazoacetate.     

Tosylhydrazones: new uses for classic reagents in palladium-catalyzed cross-coupling and metal-free reactions

Tosylhydrazones are useful synthetic intermediates that have been used in organic chemistry for almost 60 years. The recent discovery of a palladium-catalyzed cross-coupling reaction involving a tosylhydrazone coupling partner has triggered renewed interest in these reagents. This reaction shows nearly universal generality with regard to the hydrazone and can be employed for the preparation of polysubstituted alkenes. In the course of this research, novel metal-free C-C and C-O bond-forming reactions have been discovered. Since tosylhydrazones are readily prepared from carbonyl compounds, these transformations offer new synthetic opportunities for the unconventional modification of carbonyl compounds. This Minireview discusses all of these new reactions of a classic reagent.     

Visible‐Light‐Induced Organic Photochemical Reactions through Energy‐Transfer Pathways

Visible‐light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible‐light‐induced photochemical reactions occur through single‐electron transfer (SET) pathways. Recently, visible‐light‐induced energy‐transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible‐light‐induced EnT reactions.     

Cross- and Multi-Coupling Reactions Using Monofluoroalkanes

Carbon-fluorine bonds are stable and have demonstrated sluggishness against various chemical manipulations. However, selective transformations of C−F bonds can be achieved by developing appropriate conditions as useful synthetic methods in organic chemistry. This review focuses on C−C bond formation at monofluorinated sp³-hybridized carbons via C−F bond cleavage, including cross-coupling and multi-component coupling reactions. The C−F bond cleavage mechanisms on the sp³-hybridized carbon centers can be primarily categorized into three types: Lewis acids promoted F atom elimination to generate carbocation intermediates; nucleophilic substitution with metal or carbon nucleophiles supported by the activation of C−F bonds by coordination of Lewis acids; and the cleavage of C−F bonds via a single electron transfer. The characteristic features of alkyl fluorides, in comparison with other (pseudo)halides as promising electrophilic coupling counterparts, are also discussed.     

Light opens a new window for N-heterocyclic carbene catalysis

N-Heterocyclic carbenes (NHCs) are efficient Lewis basic catalysts for the umpolung of various polarized unsaturated compounds usually including aldehydes, imines, acyl chlorides and activated esters. NHC catalysis involving electron pair transfer steps has been extensively studied; however, NHC catalysis through single-electron transfer (SET) processes, despite having the potential to achieve chemical transformations of inert chemical bonds and using green reagents, has long been a challenging task in organic synthesis. In parallel, visible-light-induced photocatalysis and photoexcitation have been established as powerful tools to facilitate sustainable organic synthesis, as they enable the generation of various reactive radical intermediates under extremely mild conditions. Recently, a number of elegant visible-light-induced, NHC-catalyzed transformations were developed for accessing valuable organic compounds. As a result, this minireview will highlight the recent advances in this field.

This minireview summarized the recent advances on the photoinduced, NHC-catalyzed organic reactions according to the function of visible light.     

缺电子芳烃侧链基的光氧化

研究了9,10-二氰蒽(DCA)和四氯对苯二醌(TCBQ)敏化的甲苯、对氯甲苯、对氰基甲苯和对硝基甲苯的电子转移光氧化反应。DCA和TCBQ均可敏化甲苯和对氯甲苯的光氧化。产物为相应的取代苯甲酸和取代苯甲醛。DCA和TCBQ均不能有效敏化对氰基甲苯和对硝基甲苯的光氧化, 但在反应体系中加入与反应物等摩尔的联苯为共敏化剂后, 两者即可顺利氧化为相应的取代苯甲酸和取代苯甲醛。通过荧光淬灭和共敏化剂联苯、无水盐高氯酸镁、O2捕获剂对苯二醌以及电子给体对二甲氧基苯等外加试剂对光氧化的影响讨论了反应历程。