Photochemistry of N-Phenylbenzenecarbohydroxamic acid

N-Phenylbenzenecarbohydroxamic acid undergoes a photoreaction in cyclohexane and methanol to give benzanilide as a major product.

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Photochemie von N-Phenylbenzolcarbohydroxamsäure

Zusammenfassung N-Phenylbenzolcarbohydroxamsäure ergibt in einer Photoreaktion (in Cyclohexan und Methanol) Benzanilid als Hauptprodukt.

     

Photochemistry of condensed isoxazolines

Summary The condensed bridged isoxazolines4 are rearranged on irradiation with a low-pressure mercury lamp exclusively into condensed derivatives of tetrahydropyridine5. The selectivity of the rearrangement is due to a stabilization of the biradical8 by the overlap of the radical-electrons with -electrons of the C=C double bond and the heterocyclic ring. Quantum yields of the photorearrangement, established from the consumption of the starting materials4, were determined.

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Photochemie kondensierter Isoxazoline

Zusammenfassung Die kondensierten überbrückten Isoxazoline4 werden durch Bestrahlen mit einer Niederdruckquecksilberlampe ausschließlich zu kondensierten Tetrahydropyridinderivaten (5) umgelagert. Die Selektivität der Umlagerung beruht auf der Stabilisierung des Diradikals8 durch Überlappung der ungepaarten Elektronen mit -Elektronen der C=C-Doppelbindung und des Heterocyclus. Aus dem Verbrauch an Ausgangsmaterial (4) wurden Quantenausbeuten der Photoumlagerung bestimmt.

     

Mechanistic Organic Photochemistry

Organic photochemical reactions can be understood as transformations of the electronically excited states of the reactant molecules. By considering Lewis structure or molecular orbital representations of these excited states it is possible to outline the several possible reactions available in the case of a given reactant. A number of different types of photochemical transformations are now reasonably well understood. In these cases one finds the same common controlling feature, namely the tendency for an excited state species to follow mechanistic pathways of minimum energy and the requirement for continuous electron redistribution in following these pathways. These preferred transformations can often be selected by inspection of relative bond orders for different types of bonding, by comparison of the potential energy surfaces available to the excited state molecules, and by use of correlation diagrams. The reactions derive from both singlet and triplet states, and one of the more reliable methods now available for identifying excited states reacting is termed the “fingerprint method”. Examples of the author's mechanistic approach are given both for ketone and for hydrocarbon photochemistry.     

The Beginnings of Organic Photochemistry

Although sunlight induced photochemistry must have occurred on the planet Earth for billions of years, the chemical changes caused by light have attracted systematic scientific scrutiny only relatively recently. How did scientists first conceive the idea that the interaction of materials with light could not only cause physical phenomena, but could also alter their chemical nature? When sunlight began to be employed as a heat source for distillation, the eventual discovery of photochemical reactions was assured. One can envision three types of changes that would have aroused the curiosity of laboratory chemists: color changes; the evolution of gas bubbles (oxygen in photosynthesis); and the precipitation of a photoproduct less soluble than its precursor. Less predictable was the observation that sunlight caused crystalline santonin to burst because it is converted into a product with a different crystal lattice. In the course of the eighteenth and nineteenth centuries a variety of photochemical reactions, some observed by chance, others uncovered in carefully planned studies, ultimately led to a major systematic investigation that established photochemistry as a viable branch of chemistry.     

Photochemistry of p-Nitroacetophenone in 2-Propanol

Upon irradiation in 2-propanol, p-nitroacetophenone 1 was reduced via the triplet state to p-hydroxyaminoacetophenone 5 which was further reduced to p-aminoacelophenone 2 and 4,4′-diacetylazobenzene 4 . Similar irradiation of 5 also gave 2 and 4 , and its oxidation by oxygen gave 4,4′-diacetylazoxybenzene 3 . Photolysis of monomeric p-nitrosoacetophenone 6 afforded acetophenone and 3 that were not produced during the irradiation of 1 . Possible photoreaction pathways were discussed on the basis of published mechanisms.     

Memory of Chirality in Electron Transfer Mediated Benzylic Umpolung Reactions of Arene–Cr(CO)3 Complexes

As density functional calculations suggest , Cr(CO)₃-complexed benzylic radicals (such as 2 ) exhibit a significant degree of configurational stablility. This was exploited in an efficient method for the electron transfer mediated transformations of readily available 1-arylalkanol–Cr(CO)₃ derivatives 1 to afford alkylated products 3 in good yields and with a high degree of stereochemical retention.     

Liquid Crystals: A Tool for Studies on Chirality

Nematic liquid crystals are extremely sensitive to optically active molecules, which, at very low concentration, convert them into cholesteric liquid crystals. The purpose of this report is to review recent applications of liquid crystals to chirality studies. A brief discussion of the structure and properties of liquid crystals is followed by a survey of the methods currently being employed for the determination of the pitch and handedness of cholesteric liquid crystals. Of special interest in the formation of induced-cholestric phases are the twisting powers of optically active compounds and the structural relationships between dopants and nematics. Liquid crystals can be used, inter alia, for the detection of small optical activities and for the determination of thermal racemization barriers and absolute configurations.     

富勒烯[60]的光化学反应研究

从光物理出发,综述了近几年富勒烯「６０」的光化学反应研究进展。Ｃ６０能发生诸多的光化学反应：（１）光氧化;（２）光氢化还原;（３）「２＋２」光环化加成;（４）与叔胺的光加成;（５）与氨基酸的光加成;（６）与金属有机化合物的光加成。     

Photochemistry of 5-nitro-1,2-benzisothiazole derivatives: effects of substituents, solvents and excitation wavelength

3-R-5-Nitro-1,2-benzisothiazole derivatives (1, R = substituents) in solution, undergo photochemical isomerization to produce 2-R-5-nitro-1,2-benzothiazole derivatives. Here, generalizations and limitations by the substituent, solvent, and excitation wavelength for the photoisomerization reaction of 5-nitro-1,2-benzisothiazoles are reported.     

我国光化学研究的进展及展望

本文对我国开展光化学研究的历程和目前光化学研究的现状进行了简单的介绍,并对今后几年光化学的发展进行了展望.     

Construction of Supramolecular Chirality in Polymer Systems:Chiral Induction,Transfer and Application

Chirality, commonly found in organisms, biomolecules and nature such as L-amino acids and D-sugars, has been extensively studied in chemistry and biomedical science. Hence, the demand for simple and efficient construction of chiral structures, especially chiral polymers, has been rapidly growing due to their potential applications in chemosensors, asymmetric catalysis and biological materials. However, most chiral polymers reported are prepared directly from chiral monomers/chiral catalysts, the...     

Photochemistry in Interstellar Space and Reactions Generating Interstellar Molecules

Over 20 different molecules and radicals have so far been detected in interstellar space. The number of such molecules and radicals and their frequently complicated structure raise the question of the processes leading to their formation. Diatomic molecules and radicals could arise in gas-phase reactions. Formation of polyatomic molecules can be explained in terms of reactions proceeding on the surface dust particles and possibly involving interstellar radiation. The author demonstrates, with the aid of a model, that the molecular abundances and distributions within dust clouds in certain regions can be explained by photocatalytic reactions.     

Molecular Chirality Recognized by Achiral Compounds

Abstract

Three cases are described where chirality is recognized by achiral molecules, where chirality is induced into achiral compounds through interactions with chiral compounds, and lastly where induced chirality in the solid-state is utilized for an enantio-selective photoreaction. In the first instance, the thermodynamically and kinetically preferred diastereoisomer of an optically labile chromium complex depended on the nature of the achiral solvent. In the second case, for the first time 1,2-chloroethane was trapped and observed in a chiral near-eclipsed form and 1-chloropropane in the truly eclipsed form at room temperature in a 1:1 inclusion complex with an optically active host molecule. Finally, induced chirality in a prochiral compound in the solid-state was successfully employed in an enantio-selective photoreaction. In the two cases, solid-state CD provided valuable information.     

Photochemistry of Alkyltransition-Metal Complexes

Alkyltransition-metal complexes play an important role in catalytic processes (e.g. Ziegler-Natta low-pressure polymerization, hydroformylation, Fischer-Tropsch synthesis), in the biochemistry of humans and animals (e.g. coenzyme B₁₂), and in the classical theory of the chemical bond (e.g. multicenter bonding). The chemistry of alkyltransition-metal complexes is 30 years old, but only with the '70s came the understanding necessary for the systematic synthesis of thermally stable representatives of this class of compounds by blocking preferred decomposition pathways—e.g. β-hydrogen elimination—by the introduction of suitable ligands. In this article it is shown that thermally stable alkyltransition-metal complexes can be dealkylated under mild conditions (even at 12 K) with the help of UV light, thus producing highly reactive intermediates which can be trapped and used preparatively. Some alkyl complexes which do not react thermally with unsaturated compounds, e.g. olefins, under the influence of UV light display high activity as polymerization catalysts.     

Ultra-fast One-Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts

We report an ultra-fast helix induction and subsequent static helicity memory in poly(biphenylylacetylene) (PBPA- A ) assisted by a catalytic amount of nonracemic ammonium salts comprised of non-coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF⁻) as a counter anion. The remarkable acceleration of the helix-induction rate in PBPA- A accompanied by the significant amplification of the asymmetry relies on the two methoxymethoxy groups of the biphenyl pendants, which can gain access to enfold the chiral ammoniums in a crown-ether manner in specific aromatic solvents, leading to ultra-fast helicity induction, which is completed within 30 s. In aromatic solvents, helicity memory is lost rapidly, but is quite stable in long-chain hydrocarbons. The best use of specific solvents for helicity induction and static helicity memory, respectively, provides a highly sensitive chirality sensing system toward a small amount of chiral amines and amino acids when complexed with BArF⁻.     

The Photochemistry of Stilbenoid Compounds and Their Role in Materials Technology

Stilbenes and compounds containing stilbene units in their structures form the material basis for numerous research projects in photophysics and photochemistry. Moreover, because these compounds are easy to synthesize and are thermally and chemically stable, they are taking on an increasingly prominent role in the area of materials science investigations into optical, electrical, and optoelectronic properties. In accordance with the interdisciplinary nature of such studies, this article aims to provide a bridge extending from molecular theory and photophysical measurements, through preparative applications, to material effects and their potential technical applications.     

Specification of Molecular Chirality

The topological analysis of chiral molecular models has provided the framework of a general system for the specification of their chirality. The application, made in and before 1956, of this system to organic-chemical configurations is generally retained, but is redefined with respect to certain types of structure, largely in the light of experience gained since 1956 in the Beilstein Institute and elsewhere. The system is now extended to deal, on the one hand, with organic-chemical conformations, and, on the other, with inorganic-chemical configurations to ligancy six. Matters arising in connexion with the transference of chiral specifications from model to name are considered, notably that of the symbiosis in nomenclature of expressions of the general system and of systems of confined scope. For corrigendum see DOI: 10.1002/anie.196605111