The breadth of scientific and technological interests in the general topic of photochemistry is truly enormous and includes for example, such diverse areas as microelectronics, atmospheric chemistry, organic synthesis, non-conventional photoimaging, photosynthesis, solar energy conversion, polymer technologies, and spectroscopy. Photochemistry reviews photo-induced processes that have relevance to the above wide-ranging academic and commercial disciplines, and interests in chemistry, physics, biology and technology. In order to provide easy access to this vast and varied literature, Photochemistry comprises sections sub-divided by chromophore and reaction type, and also a comprehensive section on polymer photochemistry. Throughout, emphasis is placed on useful applications of photochemistry. Volume 36 covers literature published from July 2004 to June 2005. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading authorities in the relevant subject areas, the series creates a unique service for the active research chemist, with regular, in-depth accounts of progress in particular fields of chemistry. Subject coverage within different volumes of a given title is similar and publication is on an annual or biennial basis. NOW AVAILABLE ELECTRONICALLY – chapters from volumes published 1998 onwards are now available online, fully searchable by key word, on a pay-to-view basis. Contents pages can be viewed free of charge. Visit www.rsc.org/spr for full details.

Photochemistry Volume 36

A Review of the Literature Published Between July 2003 and June 2004

By I. Dunkin

The Royal Society of Chemistry

Copyright © 2007 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-450-4

Contents

Introduction and Review of the Year By Ian R. Dunkin, 1,
Photolysis of Carbonyl Compounds By William M. Horspool, 9,
Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool, 23,
Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool, 55,
Photochemistry of Aromatic Compounds By Andrew Gilber, 91,
Photooxidation and Photoreduction By Niall W. A. Geraghty, 133,
Photoelimination By Ian R. Dunkin, 205,
Polymer Photochemistry By Norman S. Allen, 232,

CHAPTER 1

Photolysis of Carbonyl Compounds

BY WILLIAM M. HORSPOOL

Dundee, UK

There is little doubt that photochemically induced single electron-transfer (SET) processes are becoming of greater importance as the years pass and our understanding of them increases. As a consequence, the area has attracted exponents to compile reviews such as those by Hasegawa, who has reviewed the application of photochemically induced SET reactions to organic molecules, and Mattay and co-workers, who have reviewed the subject from preparative and mechanistic standpoints. Another area that attracts much attention is that of photochemical reactions within constrained media, and reviews of this directed at reactions within zeolites have been published. The behaviour of benzyl radials formed by the photochemical decomposition of dibenzylketone in NaY zeolites has been monitored using 4-(3-hydroxy -2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical as a probe. Cyclodextrins also provide a useful constraining medium for reactions, and β-cyclodextrin has been used in a study of the photochemical behaviour of variously substituted ketones (1) attached to peptide links. The results show that carbon-carbon bond formation is the outcome.

The use of the XeCl excimer 308 nm radiation system has been described in its application to scalable photochemical reactions of ketones.

1 Norrish Type I Reactions

α-Fission processes are commonly brought about by the irradiation of aldehydes and ketones. The simplest example of these, formaldehyde, is no exception, and studies have shown that the rate of photodissociation of formaldehyde in a xenon matrix is greater than that in other matrices at a variety of wavelengths. α-Fission into methyl and formyl radicals results on irradiation of acetaldehyde at 308 nm in the gas-phase. Calculations have been used to analyse the photofragmentation of crotonaldehyde.

The Rydberg state of acetone was studied by irradiation at 195 nm. An evaluation of the photodissociation of acetone at a series of wavelengths has been published. Wu and co-workers have reported the results of calculation on the low-lying pathways for the photodissociation of hydroxyacetone. The reactions encountered are Norrish Type I processes and involve the fission of either of the α-bonds. Albini and his co-workers have studied the Norrish Type I activity of the hydroxyalkyl ketone moiety of triamcinolone (2).

As mentioned earlier in this chapter, irradiations within zeolites continue to be of interest, and one study has shown that the irradiation of 1-naphthylacylates in NaY zeolites results in the formation of a single product. The selectivity in this reaction is in contrast to the reaction of the same ketones in solution when many products are formed. The authors suggest that the selectivity is due to restriction around the biradical formed initially. A review article has highlighted the control that can be exercised with cations on the photochemistry of ketones and other substrates contained within zeolites.

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nπ* triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state. Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5).

Irradiation of the ketone (6) in argon-degassed cyclohexane brings about Norrish Type I fission. In this case decarbonylation does not result and the triplet biradical formed by the fission affords the aldehyde (7). The enol (8) is also a product of this irradiation. An aldehyde is also the principal product on irradiation of (9) in benzene. The α-fission affords the aldehyde derivative (10) in 90% yield. Analogous behaviour is observed on irradiation of bicyclo-heptanone (11) to afford an aldehyde that was a key intermediate in the synthesis of dimethyl secologanoside.

2 Norrish Type II Reactions

2.1 1,5-Hydrogen Transfer. – Both experimental and theoretical approaches have been used to study the reactivity of n-butyrophenone included in alkali-metal-exchanged zeolites. The results indicate that with smaller cations the Norrish Type I process is enhanced over the Norrish Type II reaction. Others have reported that the photochemical decomposition of n-butyrophenone in a variety of solvents follows first-order kinetics.

α-Chlorovalerophenone undergoes a normal Norrish Type II cyclization path on irradiation. Interestingly, the corresponding α-bromovalerophenone undergoes only C–Br fission on irradiation. The ketone (12) undergoes Norrish Type II hydrogen abstraction to afford the usual biradicals, which can cyclize into cyclobutanols. Both the cis-(13) and the trans-isomeric forms are possible. This particular investigation has studied the influence of antibodies (12B4, 20F10 ad 21H9) on the cyclization reaction. The authors observed that the most reactive antibody, 20F10, catalyses the formation of the cis-product (13).

The benzaldehyde derivative (14) undergoes Norrish Type II hydrogen abstraction with the formation of a photoenol. This enol can be trapped efficiently (81% yield) using methyl 2-ethylacrylate as the dienophile, to afford the tetrahydronaphthalene derivative (15). A detailed analysis of intramolecular versions of the addition to photoenols has been described. The method provides a path to polycyclic carbon frameworks such as the conversion of (16) into (17). Examples are also reported using a four-carbon chain separating the dienophile from the photoenol. Bach and co-workers have demonstrated that irradiation of the aldehyde (18) affords the corresponding o -quinodi-methane derivative by a Norrish Type II process. 1,4-Biradicals are formed on irradiation of [4-(11-mercaptoundecyl)phenyl](2-methylphenyl)methanone as a monolayer. The biradicals collapse to yield photoenol intermediates that can be trapped in a Diels-Alder reaction. A study of the photochemical behaviour of the pyridyl aldehydes (19) has reported that irradiation brings about colour changes. Only the derivative (19e) undergoes Norrish Type II hydrogen abstraction with formation of the corresponding cyclobutene derivative.

The Norrish Type II reactivity of the acetophenone derivatives (20) has been exploited as a new photoremovable protecting system for carboxylic acids. The irradiation affords the usual 1,5-biradicals that then release the acids (21) in the yields shown in parenthesis. Irradiation times are short. Klan and co-workers have described the photochemical reactivity of 1,5-dimethylphenacyl phosphoric and sulfonic esters.

Chong and Scheffer have examined the photochemical and thermal reactions of the ketonic carboxylates (22). The photoreactions of the carboxylate salts are carried out in the crystalline phase, and the hydrogen abstraction reaction and ring opening of the cyclopropyl ring results in the formation of (23) with almost quantitative ee. The thermal reactions also afford the same product but with much lower ee. Scheffer et al. have also described the use of ionic chiral auxiliaries as a means of immobilizing ketoacids (24) within crystals. The irradiation of these gives excellent yields of either of the cyclized products (25) and (26). Thus, using (S)-(+)-1-phenylethylamine as the amine salt affords a quantitative yield of (25) with 98% ee. The corresponding (R)-amine again gives a quantitative yield of product but this time the products is (26) with an ee of 97%. The same enantiomer (26) is obtained using (1S, 2R)-(-)-1-amino-2-indanol with an ee of 96%, while (1S, 2S)-(+)-2-amino-3-methoxy-1-phenyl-1-propanol affords (25) with an ee of 95%.

Norrish Type II hydrogen abstraction is the predominant reaction on irradia- tion of the silylated ketones (27). This affords the dealkylated product (28). There is some Norrish Type I reactivity that results in the formation of the isomerized product (29) and the two ring-opened products (30) and (31). The ratio of the two reactions varies with the silyl group, with a 32:1 ratio of (28):(29) obtained from (27, R = Me) and a 13:1 ratio from (27, R = Ph or R3 = MePh2).

2.2 Other Hydrogen Transfers. – A full account of the photochemical reaction of ketones with leaving groups adjacent to the carbonyl function has been published. This study provides a route to a variety of di- and tri-substituted cyclopropyl ketones. Calculations have been carried out on the photobehaviour of α-substituted butyrophenones to establish a mechanism whereby cyclopropane systems can be formed. The photochemical behaviour inducing a hydrogen transfer reaction of 2-(o-tolyl)benzofuran-3-one has been studied.

3 Oxetane Formation

Griesbeck has published an account of how stereoselectivity in single and triplet addition reactions to afford oxetanes is linked to spin selectivity. A study of the stereo- and regioselectivity of oxetane formation by addition of aldehydes to substituted furans (Scheme 1) has been carried out. The total yield of adducts is high at 95%. The reactions are regio-random but stereoselective, affording an exo:endo ratio of 97:3 for the addition of benzaldehyde. The authors argue that the 1,4-biradicals formed in the addition reaction are the key to explaining the photochemical addition. D’Auria et al. have shown that the addition of benzaldehyde to furan affords the exo-adduct selectively. This selectivity is explained on the basis of adduct stability.

The cycloaddition of aldehydes to the 5-methoxyxoxazole derivatives shown in Scheme 2 has been developed as a path to esters of erythro-α-amino -β-hydroxy-carboxylic acids. The photoaddition occurs with excellent exo-diastereoselectivity. The dr obtained is >98:2 and chemical yields are >85%.

Intramolecular oxetane formation has been examined in the sclareolide system. Typical examples are the photocyclizations of the substituted keto derivatives (32) and (33). The yields obtained are high in this series.

The sensitizer dependency for the cycloreversion of trans,trans-2, 3-diphenyl-4-methyloxetane has been studied. When chloranil is used as the sensitizer, the reaction proceeds via the radical cation of trans-β -methylstyrene, while with pyrylium salts the trans-stilbene radical cation is involved. Other work in this area has examined the cycloreversion of the oxetanes (34) using (35) or chloranil as the sensitizers.

4 Miscellaneous Processes

A mixture of carbohydrates is formed on irradiation of formaldehyde at 77 K. The authors suggest that hydroxymethylene formation is the key to this and that addition of this intermediate to formaldehyde yields glyoxaldehyde.

The photodecomposition of 4-(6-methoxy-2-naphthyl)butan-2-one (nabume-tone) in water probably involves the formation of the nabumetone radical cation. This leads to the formation of 6-methoxy-2-naphthalene carboxaldehyde. Further study has examined the photodegradation of this ketone in n-butanol where it was shown that a first-order degradation took place. An excited singlet state is involved, and the author proposes that both concentration and hydrogen bonding are important in this solvent.

Irradiation of 4-methyl-5-p-anisyl substituted N-alkoxythiazolethiones brings about N–O bond homolysis with the formation of alkoxy radicals. This technique has been applied to the synthesis the compound (36). The aldehyde (37) undergoes addition to dimesitylsilene when it is irradiated at -57°C in hexane. The product, obtained in 76% yield, was identified as (38).

4.1 Decarboxylation and Decarbonylation. – The potential energy surfaces for the dissociation of formic acid have been determined by ab initio methods. The photochemical dissociation of simple amides such as formamide, acetamide and N-methylacetamide has been investigated using CASSCF/MRSDCI single point calculations.

Mori et al. have studied the decarboxylation of (39) under a variety of conditions and have found that the conversion to (40) occurs without the involvement of radicals. They suggest that the process is a concerted cheletropic extrusion via the s-cis conformation. A further study has examined the photodecarboxylation of the (S)-ester (39) in unstretched-polyethylene films. The decarboxylation affords (40) with complete retention of the stereochemistry. The yield of product is 98% and the ee is >98%. The photochemical behaviour of the ester in other confining media such as cyclodextrins indicates that cage-escape products are also formed. The irradiation of grandifloric acid (41) at 254 nm in acetonitrile brings about decarboxylation with the formation of epimers. In methanol a different reaction occurs that results in the conversion of the C-methyl group into a carbomethoxy substituent.

The photoreactivity of indoprofen is centred on the propionic acid moiety within the molecule. The salt of ketoprofen (42) is known to undergo decarboxylation to afford the anion (43). The present study has demonstrated that, under carefully controlled conditions in THF, the lifetime of the carbanion can be extended to many hours. Roberts and Pincock report that a carbocation is formed on irradiation of the acetate (44) in 2,2, 2-trifluoroethanol and methanol.

Pyruvic acid undergoes elimination of an OH radical on irradiation at 193 nm, a process involving the T1 excited state. Calculations have shown that photodissociation of formylcyanide is unlikely to occur from the excited singlet state.

4.2 Reactions of Miscellaneous Haloketones and Acid Chlorides. – UV irradiation of fluorocarbonyl iodide results in the formation of (OCIF) … I and (OCFI) … F complexes, while ab initio methods have been used to interpret the photoreactivity of bromoacetyl chloride. A further study on the latter system has shown that the C–Cl bond ruptures on irradiation at 248 nm.

α-Iodoketones (45) undergo facile conversion to the corresponding α-hydroxyketones (46) in the yields shown. Morrison and his co-workershave described evidence for the interaction between the ketone and the CBr moieties in 17α-bromo-3α-(triphenylsiloxy) -5α-androstan-6-one.

4.3 Other Processes. – The quantum yield for the release of benzoic acid from 2,5-dimethylphenacyl benzoate is temperature dependent in benzene solution. At room temperature φ = 0.22, while at 50°C the value rises to 0.28. A much greater effect is observed in methanol or ethanol, when there is a threefold increase in the quantum yield. The authors suggest that the reaction in heated methanol enhances the E-photoenol population. Givens and Lee have reviewed the use of the p-hydroxyphenacyl moiety as a photoprotecting group for biological substrates.

1- and 2-Naphthoyloxyl radicals can be formed by irradiation of the 2-pyridone derivatives (47) and (48), respectively. Apparently the presence of a methoxy group in the ring prevents decarboxylation.

The SET-induced ring opening of the cyclopropyl moiety in (49) results in its conversion to the mixture of products (40:3) shown in Scheme 3. The best results are obtained using a mixture of MeCN/5 equivalents Et3N/1 equivalent LiCIO4. These conditions were also applied to (50) that ring-opens to yield (51).

CHAPTER 2

Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones

BY WILLIAM M. HORSPOOL

Dundee, UK

1 Cycloaddition Reactions

Kakiuchi has reviewed some recent advances in enantioselective (2 + 2)- and (2 + 4)-photocycloaddition reactions in solution. Particular attention is given to the use of chiral host molecules and chiral auxiliary groups.

1.1 Intermolecular Cycloadditions

1.1.1 Open-Chain Systems. The influence of hydrogen bonding on the outcome of photochemical reactions between hydroxy derivatives of substituted chalcones has been studied in the crystalline phase. The reaction in Scheme 1 has been examined to establish the difference between exciplex and CT excitation. The results obtained from the two excitation paths are quite different, with different yields and different de values. For example, direct irradiation involving the exciplex in toluene at — 50°C yields the two products in 6% and 5% yields, respectively. The de values are -53 and +1. Charge-transfer excitation, however, at the same temperature affords yields of 4% and 10% with de values of +52 and -9.

The photochemical (2 + 2)-dimerization between trans-cinnamates contained within a photocrosslinkable dendrimer has been reported. The cycloaddition is accompanied by trans-cis isomerism. The β-form of trans-2,4-dichlorocinnamic acid dimerizes on irradiation to give the β-truxinic acid derivative in a first-order kinetic process. Others have studied the influence of substituents on the aryl ring of cinnamic acid, with a view to control crystal engineering. Ito et al. have reported the photodimerization in the solid state of cinnamic acids 1) as their ammonium salts. This treatment yields mainly the β-(2) and δ-(3) truxinic dimers. Imidazole salts are also reactive in this process. The control of the dimerization of cinnamates has been demonstrated using 5,5-dihexylbarbituric acid as a template. A marked efficiency of dimerization was observed.

(Continues…)Excerpted from Photochemistry Volume 36 by I. Dunkin. Copyright © 2007 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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