Photochemistry of Aromatic Compounds

The reactions of nitrenes produced in the photoelimination reactions of aryl azides. 

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 

Much of the photochemistry of N-iminopyridinium ylids such as (5) results from the initial formation of cyclised intermediates such as (6). This area has now been reviewed.  

Numerous systems have been examined for photochromic activity and many of these involve the isomerisation of aromatic ring systems a monograph has been published which surveys many of these rearrangements. A short review, written in Chinese, on the photo-dimerisation of aromatic compounds and the photoaddition of enones and alkynes to aromatic rings has been reported.  

Three monographs discussing chemical and theoretical aspects of photoinduced electron transfer have been published and one of these is devoted to photoinduced electron transfer reactions of organic compounds, including many examples of aromatic systems. 

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 

Fhotoisomerisation of a silyl substituted benzene has been observed in the case of (23) which rearranges to give a mixture of (24) and (25).  

The rearrangement of azulene to naphthalene induced by excitation into the S2 state has been examined under collision free conditions in a molecular beam and an approximate estimate of the rate constant for the rearrangement under these conditions has thereby been determined.  

Irradiation of benzene and certain of its derivatives results in valence isomerism, leading to nonaromatic products. Irradiation of liquid benzene with light of 254 nm wavelength results in the accumulation of a very small amount of tricyclo[3.1.0.0 ]hex-3-ene, also known as benzvalene  

Because of benzvalene s low photostationary concentration, photolysis of benzene is not an efficient way of accumulating it. The highly reactive molecule can be trapped, however, if it is generated in the presence of other molecules with which it reacts. Irradiation of benzene in acidic hydroxylic solvents gives products formally resulting from 1,3-bonding in the benzene ring and addition of a molecule of solvent  

These compounds are not photoproducts as such, however. The products of structure 6 arise by solvolysis of benzvalene, the initial photoproduct. Products of type 7 are secondary photoproducts derived from 6.  

The photoisomerization of benzene rings has also been studied, using 1,3,5-tri-t-butyl benzene. The composition of the photostationary state is shown below  

These various photoproducts possess structures that are alternative to the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the Nbutyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the r-butyl groups are coplanar in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent r-butyl groups. 

The maximum conversion to this product in liquid benzene is only 0.01%. A higher concentration ( 1 %) is achieved in solutions diluted with saturated hydrocarbons. 

The format of this Chapter follows that used previously in this series with the exception that the current literature concerning the photochemistry of furan, thiophen, and pyrrole is now considered by reaction type in the appropriate section. 

Interest in aromatic chemistry continues at the same high level as in recent years. Substitution and cyclization processes are two areas which have attracted considerable attention over the past year. Pertinent here, as elsewhere in this volume, are the reviews on photochemistry with circularly polarized light by Buchardt,1 and the photochemistry of carbonium ions by Cabell-Whiting and Hogeveen.2 Current literature describing light-induced reactions of pyrroles has also been briefly reviewed.8 

The formation of benzvalene from the irradiation of benzene was first reported in 1967,8 but the process continues to receive further study. The quantum yield (0.18) for formation of this valence-bond isomer from 254 nm radiation of oxygen-free solutions of benzene in hexane is reported to be independent of temperature within the range 9—50 °C. The benzvalene is estimated as 5,6-di-bromobicyclo[2,l,l]hex-2-ene and the results are discussed in terms of a kinetic model which incorporates the known step of triplet-benzene-sensitized disappearance of the benzvalene. Light-induced transposition of ring substituents in 

Chambers, R. Middleton and R. P. Corbally, ibid., p. 731). Comment on these reports must await next year s review but it is pertinent to note here that the former report describes an example involving pyrylium cations in which the fate of all the ring carbons is defined while the latter describes transposition of perfluoroalkyl-pyridazines and -pyridines. 

Similar results are reported for the perfluorodimethyl compound. The introduction of fluoro-substituents and particularly perfluoroalkyl groups thus imparts a notable increase in the ease of formation and stability of valence bond isomers of aromatic compounds, and the physical properties of those involved in the light-induced interconversion of perfluoro-1,3,5- and -1,2,4-trimethylbenzenes have also been discussed by the Manchester group.14 15 The energetics of the back reaction (i.e. valence-bond isomer to benzenoid compound) have been studied, and it is interesting to note that in the conversion of Dewar acetophenone into acetophenone, the location of the excitation starts off mr -like but ends as 

This year s report on the photochemistry of aromatic compounds follows the format of last year and is arranged in sections covering reactions involving isomerisation, addition, substitution, intramoiecuiar cyclisation, dimerisation. laterai nuclear rearrangements, and reactions of substituents on the peripherary of the aromatic ring whose reactivity is derived from the presence of the ring ( peripheral photochemistry ). 

Several reviews have appeared during the year. The photoaddition reactions of aromatic compounds have been discussed with special attention to the mechanistic and structural features governing the outcome of the interaction of photoexcited arenes with alkenes and other arenes. Morrison has published an account of the photoactivation of distal functional groups in polyfunctionai 

Also in the area of light-induced electron transfer processes, a paper from 

Farid s group describes the results of an investigation of the quantitative aspects of electron transfer sensitised arene photoreactions in this article the fate of the radical ion pair generated by irradiation of an electron acceptor in the presence of an arene donor was probed by measurement of the quantum yield of separation of the ion pair. Combination of this with the rate constant for separation of the ion pair allowed the determination of the rate constant for back electron transfer 

It is reported that the bridged Dewar benzene (7) upon irradiation in a low temperature matrix produces a product whose U. V./visible absorption spectra are consistent with the formation of the cyclophane (8). 2 Thus (8) is trapped to 

The interest in photoreactions that involve chemical change in an aromatic ring continues at a high level. Aromatic photosubstitution reactions have assumed a greater importance than they once had, but the classification of these reactions on a mechanistic basis is not easy because one of several different mechanisms may operate, and published information may not be sufficient to distinguish between the likely possibilities. However, it is clear that straightforward photochemical electrophilic substitution is as yet of very limited importance. 

Various thermal and photochemical rearrangements of the benzene ring have been reviewed including valence isomerization and ring transpositions, and, fairly brieffy, a range of other reactions that can be classed formally as rearrangements. 

Valence-bond isomers of aromatic compounds (both 6-membered and 5-mem-bered) that are stabilized by trifluoromethyl groups are reviewed, and it is concluded that both steric and electronic effects contribute to the stabilizing influence of the CF3 group. A fascinating example is provided of a substituted Dewar benzene (1) that is more stable thermodynamically than the isomeric 

Bryce-Smith and A. Gilbert, in Rearrangements in Ground and Excited States , ed. P. de Mayo, Academic Press, New York, 1980, Vol. 3, p. 349. 

A theoretical description (based on INDO/S calculations) of the benzene-to-Dewar benzene isomerization suggests that the photoreaction is more likely to occur through a singlet state (which is in keeping with an earlier report that reaction occurs via S2 benzene), although the thermal re-aromatization of Dewar benzene may give a small yield of triplet benzene. 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions, heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

The photoinduced cis-trans interconversions of 1,2-diarylethenes are reviewed in Chapter 3 of this Volume. It is, however, pertinent to note here the study into the geometric isomerization of cis 2,3-diphenylcyclopropane-l-earboxylic acid derivatives. Such compounds, contrary to what is known about 1,2-di-phenylcyclopropanes, are now reported to have triplet energies of approximately 311 kJ mol , to undergo the less common adiabatic isomerization to the Irons isomers, to display emission from the electronically excited 1,3-diradical intermediates, and not to undergo intersystem crossing on direct excitation. 

A new mechanism involving Dewar benzene intermediates has been shown to operate in the photoinduced electron-transfer promoted rearrangement of 2,2, 3,3 -tetraphenylbicyclopropenyls (1). Thus irradiation (A 410 nm) of 

Several previous reports have described various features of the reversible photoisomerization of l,8a-dihydroazulene dicarbonitriles to vinylheptafulvenes. The first time-resolved investigation of this ring-opening reaction has now been reported and, using sub-30 fs transient absorption spectroscopy, prominent workers in this area have determined that the ring opening of (13) to (14) occurs within 1.2 ps, which is followed by internal conversion of Si (14) to Sq (14) in 13 

Numerous reviews have appeared which are relevant to aromatic photochemistry and are also of general interest and application these include accounts of the chemistry of excited-state complexes by Davidson, the photochemical electron-transfer reactions of ethylenes and related compounds by Mattes and Farid, photosolvolyses and photoreactions involving carbo-cations by Cristol and Bindel, conformational flexibility and photochemistry by Wagner, asymmetric photochemistry in solution by Rau, and the photochemistry of iminium salts and related heteroaromatic systems by Mariano. The photochemistry of fragrance materials has been considered by Shibamoto and Mihara, and in their second article they deal with aromatic compounds and phototoxicity.  

In recent years there has been a sharp decline in the number of reports dealing with photoisomerizations of benzenes into fulvenes, benzvalenes, bicyclo[2.2.0]hexadienes (Dewar-benzenes), and prismanes. There is now 

The conversion of 9-t-butylanthracene into its Dewar isomer (1) and the back reaction were first reported in 1980, and have been further investigated 

Many photoreactions of aromatic compounds proceed by way of nonaromatic intermediates which are then either trapped inter- or intra-molecularly, or revert to starting materials. Such processes involving aromatic groups include the di-rr-methane rearrangement and photoenolization of aromatic carbonyl compounds. These particular reactions are considered fully elsewhere in this Volume, but examples of this type of photoisomerization from the current literature are given here for illustrative purposes. 

It is well known that benzocyclobutenes undergo thermal ring-opening to o-quinodimethane systems which readily undergo the Diels-Alder reaction with dienophilcs to yield tctrahydronaphthalene derivatives, and it has again 

Topics which have formed the subjects of reviews this year include photoinduced organic synthesis, photoisomerisations involving super-cyclophanes, regioselec-tive and stereoselective [2+2] photocycloadditions, position- and stereoselective photocyclisation, the photochemistry of indoles, five-membered heterocyclic compounds of the indigo group, pyrazoles and isothiazoles, and heterocyclic N-oxides, photochromic reactions of naphthopyran derivatives, photodegradation reactions of photochromic spirooxazines and 2H-chromenes, ° and chiral photo-chromic compounds.  

Fluorescent calix[4]arenes which respond to alkali metal ions have also been discussed.  

A new procedure has been described which enables the quantum yields of a reversible photoisomerisation to be determined.  

An intramolecular charge transfer excited state may be involved, and the inverse relationship between fluorescence and photoisomerisation suggests a singlet state mechanism. Irradiation of (Z)-urocanic acid, 3-(lH-imidazoI-4-yl)prop-2-enoic acid, in the presence of nitro blue tetrazolium and sodium azide promotes its photoisomerisation in a process which involves reversible addition of the azidyl radical to the double bond.  

4 -disubstituted with two P-cyclodextrins units has been described. Anomalously large increases have been observed in the quantum yields of cis-trans photoisomerisation of (6 n = 1, 2) when complexed by alkaline earth metal ions, and these have been attributed to interaction between the coordinated metal ions and the oxygen atoms on the 2,2 -positions and/or nitrogen atoms of the azobenzene moiety. The same authors also describe the photoisomerisation of (7) and report that the cw-isomer shows complexation selectivity towards Cs and Rb. The cationic double azobenzene-chain lipid (8) is reported to undergo an efficient trans-cis photoisomerisation in MeCN, but in co-aggregates with bis ether lipid (9) of certain compositions, or in pure liposomes, the degree of photoisomerisation is very sensitive to temperature.  

The photoisomerization of benzene into Dewar benzene, benzvalene and fulvene is a well-known process, and in 1995 the interconversion between the [4] paracyclophane (3) and the 1,4-bridged Dewar isomer (4) in a matrix at 77 K was described. The analogous Dewar isomers (5) and (6) have been synthesized by conventional means in order to investigate the properties of the derived substituted [4]paracyclophanes and the kinetic stabilization of this skeleton by substituents which sterically hinder reactions at the bridgehead sites. Irradiation under matrix isolation conditions of (5) and (6) yields the corresponding cyclophanes which have widely differing half-lives (1 min at — 90°C to 0.5h at — 20°C) for their cycloreversions the authors also discuss in detail the aromaticity of the extremely bent benzene ring in the [4]paracyclophanes. 

Phototransposition reactions of substituted benzenes and heteroarenes by way of valence-bond isomers and their diradical precursors, have been studied for a considerable number of years and are the subject of several reports in the review period. Such reactions within the six isomers of dimethylbenzotrifluoride are reported to be efficient, with each isomer giving rise to the others. The major product isomers observed in each case, however, allow the starting isomers to be divided into the two triads of 2,6-, 2,3- and 3,4-dimethyl- and 3,5-, 2,4- and 

2-iodo-phenoxide ion undergoes photoinduced ring contraction, giving the 

The reversible photoisomerization of l,8a-dihydroazulene dicarbonitriles (16) to vinyl heptafulvenes (17) has been the subject of several earlier reports, and prominent workers in this area now describe the photoreactions of 1,1-dicyano- 

2-diphenyl-l,2-dihydronaphthalene (IS). Elimination of CN from (18) to give the carbocation is a significant process on irradiation, but additionally a phenyl-vinylmethane rearrangement also occurs, yielding (19) irreversibly, while a more minor and reversible isomerization affords the benzoquinodimethane derivative (20), which is a new example of a photochromic 10-electron elec-trocyclization. The photochromism of 2H-l-chromenes such as (21) involves a 

Topics which have formed the subjects of reviews this year include theoretical studies of the photochemistry of thiiranes, photoaddition of amines to aryl olefins and arenes, the synthesis of heterocyclic compounds, photoamination directed towards the synthesis of heterocycles, selective addition of organic dichalcogenides to carbon-carbon unsaturated bonds, photocyclisation mechanisms of c/5-stilbene analogues, synthetic utility of the photocyclisation of aryl-and heteroarylpropenoic acids, photochromic diarylethenes, spiropyrans, cy-clophanes, and polycondensed aromatics, photochromic organic media, photophysics and photochemistry of P-carbolines, and the photochemical synthesis of macrocycles.Chirality switching by light has also been described.  

In the solid state photorearrangement of cw-l,2-dibenzoylalkenes, intramolecular carbon to oxygen phenyl migration is reported to be controlled by synlanti conformational constraints, and in the photolysis of tmm-2,3-diphenyloxirane, quantum yields for formation of the trans ylid, cw-2,3-diphenyloxirane, benzalde-hyde, and deoxybenzoin have been measured. This reaction gives both orbital symmetry-allowed and -forbidden products. Photolysis of 2,3-diaroylaziridines 

Considerable interest is currently being shown in photochromism. This phenomenon has been observed in the cyclised products (21), (22), (23), and 

Recent advances in organic photochromic storage materials, and photochromic switching, have also been described. 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured. (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate, and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange- 

Variously substituted tetrazolo[l,5-a]pyridines (19) and 2-azidopyridines (20) are photolysed to 2-alkoxy-lH-l,3-diazepines (21), 2-dialkylaraino-5H-1,3-diazepines (22), 2,3-dihydro-lH-l,3-diazepin-2-ones (23), and 2,4-diazabi-cyclo[3.2.0]-hepten-3-ones (24) the relative stabilities of the 2-alkoxy- and 

Irradiation of the P,ri-unsaturated ketones, bicyclo[2.2.1]hept-5-en-2-one and bicyclo[2.2.2]oct-5-en-2-one included within MY zeolites promotes an oxa-di-Ti-methane rearrangement. The transformation is thought to occur by a triplet state which has its origin in the heavy cations present within the supercage. 

Details of a range of new photochromic compounds have appeared in both the scientific and patent literature.  

It was reported last year that Dewar thiophene could be trapped as its furan adduct from room temperature irradiation 

Since 1973 several accounts have appeared which describe the phototransposition reactions of 4-hydroxy- to 2-hydroxy-pyrylium 

At 0°C the initial product of irradiation of the 2,3-dimethyl cation (3) is the 4,5-dihydroxycyclopent-2-enone (4) which can be isolated following neutralization. At higher temperatures a secondary acid catalyzed thermal process of (4) occurs to give acetylfur-ans (5). 

Each year there appear reports of photoreactions of aromatic 

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Chapter 2, by Miranda and Galinco, provides a critical survey of the photo-Fries reaction undergone by numerous aromatic esters, amides, and so forth. This chapter is a valuable companion to Chapter 5 by Fleming and Pincock, in Volume 3. Miranda and Galinco s chapter is the sixth chapter devoted to the photochemistry of aromatic compounds in this series. 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

Most of the fairly extensive photochemistry of aromatic compounds has not been studied in sufficient detail to permit disentanglement of singlet and triplet mechanisms. Theoretical calculations indicate that the pattern of substituent effects on side-chain reactions of excited disubstituted benzenes should be quite different from that observed in the ground states of the molecules. One problem associated with these predictions is the question of whether or not they are appropriate for triplets as well as for corresponding singlet excited states. Consider the following system ... 

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