Photochemical Reactions of Aromatic Compounds

Aromatic compoimds also undergo photocycloaddition reactions with alkenes, leading to 1,2-, 1,3-, and (less often) 1,4-adducts, as shown for the reaction of benzene with ethene in equation 12.70. Olefins with strongly electron-withdrawing or electron-donating substituents tend to give 1,2-photoaddition products, while olefins with alkyl substituents tend to give mostly 1,3-photoaddition. 

For a more detailed discussion of spectroscopic transitions in benzene, see Gilbert, A. Baggott, J. E. Essentials of Molecular Photochemistry CRC Press Boca Raton, FL, 1991 pp. 355-357. 

Rydberg states can be observed in benzene and its derivatives as well. The 3Rg state of benzene was reported to have a lifetime of 70 20 fs and to decay to a vibrationally excited ground state Wiesenfeld, J. M. Greene, B. 1. Phys. Rev. Lett. 1983,51,1745. 

Bryce-Smith, D. Longuet-Higgins, H. C. Chem. Commun. 1966, 593. 

See also the discussion by Cundall, R. B. in Burnett, G. M. North, A. M., Eds. Transfer and Storage of Energy by Molecules, Vol. 1 Wiley-Interscience London, 1969 pp. 1-66. 

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A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

The most characteristic photochemical reaction of aromatic compounds is their cydoaddition with alkenes. The intramolecular reaction is suitable for the synthesis of complex structures, such as those depicted in Scheme 9.49, where [3+2]-photocycloaddition leads to structures which resemble natural products (aphidico-line and stemoclinone). An interaction of the arene singlet excited state with the alkene ground state gives rise to the meta adduct [83, 84]. 

Elimination of NO and NO2 is observed in the photochemistry of a range of nitro and nitroso compounds, nitrites and nitrates. The photochemical reactions of aromatic compounds with tetranitromethane and other related reagents have been reviewed. These reactions proceed in the simplest cases by addition of the elements of tetranitromethane to the arene, e.g. to give nitro trinitromethyl adducts, but large numbers of other products can be formed by a diverse range of multistep sequences. 

A number of reviews and texts whose topics include photochemical reactions of aromatic compounds have been published during the year and will be mentioned here. A new volume of the Organic Photochemistry series has appeared and contains a chapter detailing the photochemical reactions of aromatic and heteroaromatic cations (e.g. cyclopropenlum ions, tropylium ions, pyrilium ions and... 

Several reviews whose topics include photochemical reactions of aromatic compounds have appeared during the year. The preparation of indole alkaloids by enamide photocyclisation reactions has been surveyed. A very useful four volume work dealing with the theory and experimental practicalities of light-induced electron transfer reactions has been published the third volume of this set is... 

A lot of photochemical reactions of aromatic compounds have been described in the literature. In this context, photocycloadditions are typical examples [6, 9]. In such reactions involving nn excitation of the chromophore, the aromatic character is not reestablished in the final products as is typical for ground state reactions. Very efficient methods based on this reactivity have been developed for the construction of polycyclic compounds. Thus, molecular complexity is generated from simple and easily available starting compounds in only one step. For these reasons, photochemical... 

Among the photochemical reactions of aromatic compounds, the photocycloadditions are most frequently applied to the synthesis of complex polycyclic compounds [6, 9]. The [2+3] or meta photocycloaddition of aromatic compounds and alkenes is the most prominent example [10]. This transformation also demonstrates complementarities between photochemical and ground state reactions since such reactions are almost impossible using conventional activation. A [2+2] ot ortho photocycloaddition between carbocyclic aromatic compounds and alkenes is observed as well. It is often competitive with other cycloaddition modes, in particular the [2+3] mode [11]. Many of these reactions are reversible, and photostationary equilibria are involved. This reaction was much less applied to organic synthesis. Recently, it was found that an acidic reaction medium may have an influence on the outeome of the reaction. The intramolecular photocycloaddition of resorcinol derivatives such as 1 is difficult due to its reversibility (Scheme 29.1). However, in an acidic reaction medium, the cycloadducts 2a,b are protonated at the oxygen atom of the tetrahydrofuran moiety... 

This chapter deals with the main photochemical reactions of aromatic compounds, including photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement and related photoreactions. 

Pac, C., Tosa, X, and Sakurai, H., Photochemical reactions of aromatic compounds. IX. Photochemical reactions of dimethylaniline with halobenzenes. Bull. Chem. Soc. Jpn., 45, 1169,1972. 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

The Photochemical Reactions of Azoxy Compounds, Nitrones, and Aromatic Amines N-Oxides G. G. Spence, E. C. Taylor and O. Buchardt, Chem. Rev., 1970, 70, 231-265. 

Three types of cycloaddition products are generally obtained from the photochemical reaction between aromatic compounds and alkenes (Scheme 31). While [2 + 2] (ortho) and [3 + 2] (meta) cycloaddition are frequently described, the [4 + 2] (para or photo-Diels-Alder reaction) pathway is rarely observed [81-83]. Starting from rather simple compounds, polycyclic products of high functionality are obtained in one step. With dissymmetric alkenes, several asymmetric carbons are created during the cycloaddition process. Since many of the resulting products are interesting intermediates for organic syntheses, it is particularly attractive to perform these reactions in a diastereoselective way. 

Such cyclic peroxides have been proposed as intermediates in photochemical and chemiluminescent reactions of aromatic compounds with oxygen (4, 5, 44) and in the biological hydroxylation of aromatic compounds (22, 23, 34). In dilute ferrous ion solution the cyclic peroxides could decompose ionically to give a predominantly electrophilic distribution of substituted phenols, while at higher concentrations the ferrous ion would cleave the peroxide bond homolytically this, followed by loss of water, would give a more random pattern of substituted phenols. 

Lewis structure models incorporating radical character are useful in the exposition of photochemical reactions of carbonyl compounds, alkenes, dienes, and aromatic compounds that are not substituted with polar groups. When heteroatoms are substituted onto an aromatic ring, considerable charge transfer character can be introduced into both the ground state and the excited state, and ionic resonance structures become more suitable as models for reactivity. 

Nevertheless, an increasing number of photochemical reactions of aromatic substrates exist in which the aromaticity in the products is built up again. These reactions are interesting for application to the preparation of various biologically active compounds. They are also considered systematically to be applied to the synthesis of organic materials where aromatic moieties are of particular interest. 

Scheme 9.1 Mechanism for fimnation of photochemical adducts from the reaction of aromatic compounds with alkenes...

Heterocyclic compounds are important targets in organic synthesis. They possess numerous biological activities and can be used as intermediates for further transformations. Starting from readily accessible starting materials, various photochemical cyclization reactions provide convenient methods to build up to a large variety of heterocycles several reviews deal with them. In this context, the cyclization of aromatic compounds plays an important role. In these reactions, as in many ground state reactions of aromatic compounds, the aromatic character is momentarily suppressed. In this chapter, some of these reactions are discussed. 

Electrocyclization provides an efficient way to synthesize a large variety of heterocyclic compounds. Among the many photocyclization reactions of aromatic compounds, electrocyclization is among the most frequently studied. An example of this is that illustrated in Scheme 1. This involves cyclization between the arene moieties of the protonated azobenzene 1, which reacts from the jtJt state reaction and is controlled by the Woodward-Hoffmann rules. Six rr-electrons are involved in the conrotatory cyclization. The reaction was not observed when unprotonated azobenzene was irradiated because in this case, the ntt state is populated. Since the photochemical electrocyclization is reversible, a consecutive trapping reaction is needed to obtain the final products in good yields. In the present case, as in many other reactions, a rearomatization step via oxidation took place to afford the final product 2. In the same way, the imine 3 can be cyclized to the phenanthridine derivative 4. In this case, the oxidation of the... 

Spence, G.G., Taylor, E.C., and Buchardt, O., The photochemical reactions of azoxy compounds, nitrones and aromatic amine N-oxides, Chem. Rev., 70, 231,1970. 

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

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