Photochemistry Volume 35

A Review of the Literature Published between July 2002 and June 2003

By I. Dunkin

The Royal Society of Chemistry

Copyright © 2005 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-445-0

Contents

Introduction and Review of the Year By Ian R. Dunkin, ix,
Chapter 1 Photolysis of Carbonyl Compounds By William M. Horspool, 1,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool, 17,
Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool, 47,
Chapter 4 Photochemistry of Aromatic Compounds By Andrew Gilbert, 79,
Chapter 5 Photo-oxidation and Photo-reduction By Niall W.A. Geraghty, 116,
Chapter 6 Photoelimination By Ian R. Dunkin, 179,
Chapter 7 Polymer Photochemistry By Norman S. Allen, 206,

CHAPTER 1

Photolysis of Carbonyl Compounds

BY WILLIAM M. HORSPOOL

The focus of organic photochemistry continues to change. Over the years considerable research devoted to simple carbonyl compounds was published, however, this emphasis has been diminishing on an annual basis and continues to diminish in the period of this review.

Reviews of general interest in this area highlights microreactors that can be used for a variety of photochemical reactions such as the synthesis of large ring ketones. Interest in the control that can be exercised on the outcome of photochemical reactions in constrained environments continues to increase and reviews dealing with the enantioselective photoreactions of achiral compounds in chiral crystals and inclusion crystals have been published.

Other studies have been aimed at systems to protect a variety of functional groups with photolabile attachments. A mild CN bond-cleavage process has been described for the release of primary and secondary amines from a coumarin substrate. Fedoryak and Dore have reported the value of the quinoline derivatives (1) as photolabile protecting groups.A patent has been lodged dealing with the formation of photo releasable phenacyl carbonate protecting groups. Others have examined the photochemical deprotection of carboxylic acids from phenacyl and 2,5-dimethylphenacyl esters that can be carried out in a two-phase system. The results indicate, in benzene-water with added cetyltrimethylammonium bromide, that the yield of liberated acid is enhanced. Ashraf et al. have described the use of the hydroxyketone (2) as a further example of molecules that can be used as photoactivatable protecting groups for acids. The hydroxy group is readily esterified with a variety of acids to afford the esters (3). These, on excitation in methanol or ethanol with no need to exclude air, release the free acid in excellent yields and afford the furan (4) as the by-product. This furan is photochemically active under the reaction conditions and undergoes cis-stilbene type cyclization. Klan and Zabadal have reviewed the area of photoremovable protecting groups.

1 Norrish Type I Reactions

The photochemical decomposition of methanal in a solid Xe matrix has been studied. Work has also been reported dealing with the photodissociation dynamics of methanal, and ab initio calculations have been carried out on the photochemical decomposition of acetaldehyde into methane and CO. The photocatalytic decomposition of acetaldehyde to yield carbon dioxide has also been reported. The threshold for CC bond fission in propanal and the release of the CHO fragment has been shown to be at a wavelength of 326.26 nm. Chowdhury has reported the dissociation of propynal using multiphoton irradiation. Gas-phase photolysis of butyraldehyde in the 280-330 nm range has shown that the CHO radical is produced.

Laser-flash irradiated benzaldehyde in ethylene glycol has been examined using TRESR and CIDEP techniques. Benzoyl radicals and a-hydroxybenzyl radicals were detected. The photochemical dehalogenation and decarbonylation of 2-, 3- and 4-chlorobenzaldehydes has been studied.

Induced pre-dissociation is reported to be a photochemical path to ethane during the irradiation of acetone in the gas phase. Irradiation at 193 nm of ethyl vinyl ketone results in the formation of a variety of products such as n-butane, but-1-ene and buta-1,3-diene. The study was used to determine the rate of combination of ethyl radicals to yield butane and of vinyl radicals to afford buta-1,3-diene.

Supramolecular complexes of benzyl radicals are formed upon irradiation of the ketones (5) in supramolecules. Turro has reviewed some aspects of the decarbonylation of dibenzyl ketone derivatives in supercages.

A study of the benzoyl radicals obtained by irradiation of the ketones (6-11) has shown that the α-cleavage results from the excited triplet state. endo and exo-(2-Hydroxy-[2.2.2]bicyclo-5-en-1-yl)-phenylmethanones have been synthesized and studied as potential photoinitiators for radical polymerization. The photoinitiators (12) have been investigated in some detail.

The ketone (13) does not undergo loss of CO on irradiation in the crystalline phase. In benzene solution, however, decarbonylation does occur to give biradicals that disproportionate to yield (14) and (15). The more hindered ketone (16) behaves differently and decarbonylates in both the crystal and solution with different results. Thus (17) and (18) are formed in solution, while only the latter (18) is formed in the crystal. The initial report of the photodecarbonylation of (16) was made some time ago. A further study of this has indicated that it is possible to trap the biradical (19) formed on decarbonylation. In the absence of a trap, ring closure affords the cyclobutene derivative (18), but the adduct (20) is formed in 62% yield in the presence of alkenes such as dimethyl fumarate. Even better yields are obtained with dimethyl acetylene dicarboxylate as the trap, when (21) is produced in 89 % yield.

Irradiation of the ketones (22) brings about the fission of an α-bond to afford the biradicals (23). The fate of these is dependent upon the linking chain length and can afford the alkenals (24) or the cyclophanes (25). Magnetic field effects have been investigated for this system. The biradicals (26, n = 3, 4 or 5) are formed on decarbonylation of the cyclophane derivatives (27). When the linking chain is long enough, coupling leads to the formation of the products (28).

The photochemical ring expansion of the cyclobutanone (29) affords the usual carbene that is trapped by the bis alcohol (30) to afford (31).

2 Norrish Type II Reactions

2.1 1,5-Hydrogen Transfer. – Griesbeck has reported that spin-selectivity in carbonyl photochemistry is a useful tool for organic synthesis. He has suggested that spin-orbit coupling geometries are crucial for triplet to singlet intersystem crossing at the biradical stage of the Norrish Type II processes. The Norrish type II photocleavage of racemic leucine can be brought about using left- or right-circularly polarized light at 215 nm.

The three dialdehydes (32), (33) and (34) are photoreactive in the crystalline state. However the outcome of the reactions appears to be dependent upon the substitution pattern on the aryl ring. Irradiation of (32, X = H) and (34, X = H) gives dimers quantitatively. The structure of the dimers is illustrated by (35), which is formed from (32). The aldehyde (33, X = H) is unreactive. 1,5-Hydrogen abstraction to afford (36) is the quantitative reaction for (32, X = Br). The other derivatives (33, X = Br) and (34, X = Br) give mixtures of (36) and (37) in the ratios of 25:57 and 10:90, respectively. A mixture of (36) and (37) (43:57) is obtained from (33 X = Cl), while the chlorinated version of (36) is obtained from (34, X = Cl). Proton transfer within o-hydroxybenzaldehyde and o-hydroxyacetophenonehas been studied in a glass matrix. The proton transfers and the changes in the electronic properties are supported by ab initio calculations. Other studies on o-hydroxybenzaldehyde, have examined substituent effects on the fluorescence quantum yield.

Moorthy and Mal have reported that irradiation of the benzoyl ketones (38) results in photochemical conversion to the mixture of cyclobutanes (39) and (40). The yields are in the 31-43% range and, as can be seen from the ratios of products, there is a good degree of selectivity when the reactions are carried out in non-polar solvents. The ratios change when polar solvents are used. This change is more dramatic with the ketones (38, R = Ph), where the observed selectivity is reversed from non-polar to polar solvents. The keto derivatives (41, R = H or Ac, n = 1) undergo Norrish Type II hydrogen abstraction on irradiation at 300 nm in t-butanol as solvent. Cyclization results in the formation of the imidazolidinones (42 and 43) by cyclization within the resultant biradicals. Fission of the 1,4-biradical affords acetophenone in competition with cyclization. The products are obtained as racemic diastereoisomers. The other ether derivatives (41, n = 2) are also reactive and undergo formation of 1,5-biradicals on irradiation. The selectivity of the reactions was investigated in the presence of chiral hosts. The best yields were obtained for the series (41, R = H, n = 2) using the host molecule (44) in toluene at — 45°C. This gave a 70% yield of products with 60% ee. The ratio of exo:endo was 80:20. The reactivity of the ketone (45) in the crystalline phase was also studied. At low conversion (1%) the exo:endo ratio of the products (46) and (47) was 94:6 with an ee of 78%. This deteriorates on prolonged irradiation and at 36% conversion the ratio is 87:13 with an ee of 28%. The data collected suggest that the major product is the R isomer. Derivatives of 3-amino-1,6-anhydro-β-D-glucopyranose made by reaction with succinic, glutaric and tartaric anhydrides are photochemically reactive. Irradiation brings about a Norrish Type II γ-hydrogen-transfer process and radical ring closure to yield azetidinols.

The influence of chiral inductors on the photochemical cyclization of the adamantane-substituted ketones (48) in zeolites has been examined. Only the endo-products (49) are formed. The best ees are obtained for both derivatives (X = H or F) with ( — )-pseudoephedrine as the chiral auxiliary. The cyclobutanols undergo retro-Aldol ring opening to afford the ketones (50). The study was extended to the more heavily substituted derivatives (51).

Norrish Type II hydrogen abstraction occurs on irradiation of the enones (52). The hydrogen abstraction takes place from the proximate methylene, leading to a biradical. This brings about the migration of the double bond in the side chain to yield the products (53) in modest yields. The derivatives (54) do not undergo the double bond migration even though they undergo Norrish Type II hydrogen abstractions. The resultant biradical undergoes cyclization to a furan moiety to afford the two products (55) and (56). Norrish type II hydrogen abstraction has also been observed in the cyclization of related thiochromones to yield (57) and (58).

Laser flash photolysis of 2-methylbenzil shows that the triplet state is produced. However, irradiation in methanol involves a different intermediate that has been shown to lead to a mixture of photoenols. Irradiation in benzene affords 2-hydroxy-2-phenylindan-1-one as the principal product. The photoenol formed from o-methylbenzaldehyde reacts efficiently with derivatives of Meldrum’s acid. Norrish Type II hydrogen abstraction is at the centre of the route in a new method for the release of alcohols from esters. For example, the irradiation of (59) follows the Norrish type II path and the excited benzoyl chromophore abstracts hydrogen from the isopropyl group. Photoenols are formed, one of which undergoes intramolecular lactone formation (60) with the release of the alcohol (Scheme 1). The 1,4-biradical can be trapped as (61) in the presence of oxygen.

2.2 Other Hydrogen Transfer. – Irradiation of the arylketone derivative (62) brings about δ-hydrogen abstraction and cyclization within the resultant biradical, and affords the final product (63) in low yield (ca.10%). The reaction also takes place in the presence of triethylamine. In this case however, as well as the formation of (63), pinacolization yields (64), while trapping with the amine yields (65). The formation of these products suggests the involvement of an electron-transfer process. The hydrogen abstraction reactivity of the ketone (66) followed by cyclization within the resultant biradical affords the two products (67) and (68). When the reaction is carried out in methanol there is almost exclusive formation of the E-indanol (67): the ratio of the products (67):(68) is 99:1. This behaviour is different from that of the ketone (69) where cyclization within the biradical results in the Z-isomer predominantly. The authors propose that the difference in reactivity is due to the geometric differences within the initial biradicals, perhaps due to the constraints of the three membered ring substituent.

A further account of the intramolecular hydrogen abstraction processes within the cyclophanes of the type shown as (70) with a variety of linkers has been published. The irradiation brings about the conversion into the products such as (71) by a 1,6 hydrogen transfer. The yields are variable and are shown below the structures. Other studies by Park and his co-workers have reported other cyclizations using excitation at 350 nm in benzene. These results are shown in Scheme 2. As can be seen, excitation results in ô-hydrogen abstraction from the side chains, and the resultant 1,5-biradicals undergo ring closure to yield the diols. These products are readily dehydrated to afford the difuran derivatives in 40% overall yield. The latter compounds were used in reactions to synthesize novel cyclophanes.

Irradiation of ethyl 2-(8-oxo-5,6,7,8-tetrahydro-1-naphthyloxy)acetate in methanol affords the two products (72) and (73), where R = CO2Et. Similar hydrogen abstraction behaviour is observed with the corresponding nitrile. Nevertheless, while cyclization occurs, there is also incorporation of methanol to yield the isomeric adducts (74) and (75), where R = CN. A third product (76) is also observed in this reaction.

3 Oxetane Formation

Both oxetanes and β-hydroxy esters are formed following irradiation of aromatic carbonyl compounds in the presence of silyl ketene acetals. The products arise either by SET processes or by direct Paterno-Büchi additions. Griesbeck and Bondock have reported the influence of substrate concentration on the diastereoselectivity of the photochemical addition of aldehydes to (Z)- and (E)-cyclooctene. Miranda and co-workers have published physical evidence for the quenching of the triplet state of 2,4,6-triphenylpyrylium salts by 2,3-diaryloxetanes.

4 Miscellaneous Processes

4.1 Decarboxylation and Decarbonylation. – A study has demonstrated that conformational memory plays a major part in the photochemical dissociation of formic acid. The results of a photophysical study of the photochemical decomposition of formic acid in the vacuum-UV have been reported as has the photochemical decomposition of formic acid using 212.8 nm irradiation. The equilibrium geometries of N,N-dimethylformamide in the singlet and triplet excited states have been calculated.

The gas phase photochemistry of acetic acid has been studied by ab initio methods. The photochemical decomposition of aliphatic amino acids using circularly polarized light has been reported. Many examples were cited. A typical result is shown in Scheme 3 for the decomposition of valine in aqueous HCl solution. Pyrene has been established as the most effective polycyclic arene sensitizer for the photochemical decomposition (irradiation at 366 nm) of N-phenylglycine. The introduction of electron-donating groups into the aryl ring of the glycine also enhances the rate of decomposition.

Irradiation of propiolic acid at 193 nm results in the population of the S2 state as a result of ππ* excitation. Apparently, in this excited state decarboxylation is almost zero and the principal reaction is CO bond fission with the release of HO. Irradiation of but-3-enoic acid at 193 nm brings about excitation to an excited singlet state. Fission of CO and CC bonds results, affording hydroxy and COOH radicals.

The efficient photodecarboxylation of the keto acids (77) has been studied. The reactions involve the formation of the carbanions (78). Aqueous solutions of fenofibric acid (79) at pH 7.4 show the formation of two intermediates when subjected to laser excitation. The study has indicated that the triplet state of the acid in water is of a ππ* type. Photoionization is an important process in the aqueous medium. New photoreactive phenylalanine analogues (80) and (81) have been prepared. These were incorporated into position 5 of the pentapeptide, thymopentin. The resultant derivatives were photolabile and underwent decomposition on irradiation at 365 nm. Computational methods have been used to analyse the photoreactivity of the tryptophan derivative (82). The calculations were directed towards an understanding of the quenching of the fluorescence. The results indicate that hydrogen transfer alone does not quench the fluorescence, but that an aborted decarboxylation path is involved. Proton transfer within 3,7-dihydroxynaphthoic acid has been studied in protic and aprotic solvents. Bandichor and Reiser have commented on the photochemistry of 2-(1-naphthyl)ethyl benzoates reported by Pincock et al.

Efficient cleavage of the NO bond in the derivatives (83) and (84) occurs on irradiation in acetonitrile solution. The anthroyloxyl radicals produced by this fission were studied spectroscopically.

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Photochemistry Volume 34

A Review of the Literature Published between July 2001 and June 2002

By I. Dunkin

The Royal Society of Chemistry

Copyright © 2003 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-440-5

Contents

Introduction and Review of the Year By Ian R. Dunkin, 1,
Chapter 1 Photolysis of Carbonyl Compounds By William M. Horspool, 9,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool, 29,
Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool, 69,
Chapter 4 Photochemistry of Aromatic Compounds By Andrew Gilbert, 111,
Chapter 5 Photo-reduction and -oxidation By Andrew Gilbert, 143,
Chapter 6 Photoelimination By Ian R. Dunkin, 169,
Chapter 7 Polymer Photochemistry By Norman S. Allen, 197,

CHAPTER 1

Photolysis of Carbonyl Compounds

BY WILLIAM M. HORSPOOL

1 Norrish Type I Reactions

Formaldehyde undergoes photochemical decomposition in the 269 to 339 nm range in the gas phase. There are various dissociation paths for this molecule, affording hydrogen atoms and CHO radicals, CO and hydrogen atoms and hydrogen atoms and CO. The quantum yields for the processes were measured. The photochemical decomposition by Norrish Type I reactivity of propional-dehyde has been studied in the 280-330 nm range. Again the formation of CHO radicals was detected.

The multi-photon ionization processes arising within propanone in the irradiation range of 243-263 nm have been studied. The ionization processes that were detected arise from within the S1 and T1 states. Photodissociation (243 nm) of propanone, ethanal and ethanoic acid brings about release of hydrogen atoms. These were detected using two-photon absorption and induced fluorescence.Studies of propanone decomposition in air have been used to assess possible dissociation processes in the troposphere.

The stimulated nuclear polarization spectra from irradiation of the ketones (1) and (2) has been reported. A study of the Norrish Type I behaviour of the ketone (3) in supercritical CO2 has been reported, and an enhanced cage effect has been detected near the supercritical pressure. Turro and his co-workers have carried out a detailed EPR study of the persistent radicals formed on photolysis of the dibenzyl ketones (4) in zeolites. Some aspects of supramolecular chemistry have been reviewed. A short review has highlighted some of the research carried out in zeolites, focusing particularly on the exploitation of triplet-triplet energy transfer. A CIDNP study of the photochemical Norrish type I processes brought about by irradiation at 308 nm in of the two ketones (5) and (6) has been reported.

The study of some benzylbenzoin benzyl ethers has shown that they undergo Norrish Type I fission, affording benzoyl and benzyloxybenzyl radicals. The intermediates were characterized by laser flash photolysis. Previously the Norrish Type I fission reactions of ketones related to (7) had been reported; further work has shown that irradiation (305 nm in water-methanol) of (7) brings about its conversion into (8) in 94% yield. The reaction sequence was also demonstrated in oligonucleotides. Norrish Type I fission also occurs in systems like the cyclophane dione (9). This brings about sequential decarbonylation to yield the cyclophane (10) and the monoketone (11). Proof of the sequential nature of the reaction was demonstrated by the decarbonylation of (11) to yield (10). The time-dependency of the irradiations are shown below the structures. The α-fission of the ketone (12) affords the ring-opened ester (13) in 57% yield when the irradiations are carried out in methanol. The reaction is a conventional process and affords a ketene as a result of fission in the resultant 1,4-biradical produced by photochemical fission of the α-bond. Another facet of the Norrish Type I reaction is ring expansion of a cyclobutanone to a dihydrofuran. This process has been used by Lee-Ruff and co-workers in the photochemical ring expansion of ([+ or -])-3-[2′-(benzoyloxy)ethyl] -2,2-dimethylcyclobutanone. This has been used as a route for the synthesis of 2′,3′-dideoxynucleosides based on the apiose family.

Larger ring ketones undergo decarbonylation, as has been described following the irradiation of the cyclohexanone derivative (14) as a dilute solution in benzene with λ > 300 nm. The resultant biradical produced by the decarbonylation undergoes ring closure to give a mixture of the isomeric cyclopentanes (15) and (16) as well as the ring expanded product (17). Interestingly the compound (14) is unreactive in the crystalline phase. The authors reason that the failure to decarbonylate is a result of deactivation of the carbonyl excited state by interaction with the proximate benzyl group. Kadota and Ogasawara have described the photochemical decarbonylation of cyclic ketones containing the bicyclo[3.2.1]octane skeleton (Scheme 1). This process, carried out in methanol with Pyrex filtered light, provides reasonable yields of the ring-contracted compounds shown. These products can be readily converted in high yield into the pentose and hexose sugars illustrated.

The irradiation of the esters (18) results in a Norrish Type I fission, rupture of the ester carbonyl-O bond, with the formation of the xanthenyl radical and the corresponding formyl radical. The reactivity of these species was investigated, and some of the results obtained are shown in Scheme 2, where the principal process is shown to be cyclization of the unsaturated formyl radicals to yield a lactone or lactones. The yields obtained can be variable as indicated. Other products such as the alcohols (18) and formates (20) are also produced.

2 Norrish Type II Reactions

2.1 1,5-Hydrogen Transfer. – While solution phase photochemistry of o-alkyl-benzaldehydes affords a complex mixture of products, irradiation in the solid phase is much more specific. The aldehydes (21) are all photoreactive in the solid state and give the cyclobutenols (22). Even the liquid aldehyde (23) (Scheme 3), in a solid inclusion complex, is readily converted into the cyclobutenol (24), by a conventional [gamm]-hydrogen abstraction and cyclization within the resultant biradical. The conditions used are aerobic, and oxidation of the aldehyde to the acid (25) occurs in competition with the cyclization. The irradiation of the cyano-substituted aldehyde (26) (Scheme 4) in benzene affords the lactones (27) and (28) in a total yield of 25%. Interestingly, the related anisaldehyde derivatives (29) are all photochromic in the solid state. The reaction involves an intramolecular proton transfer with the formation of the photoenols (30). In the case of the derivative (29, X = CHO), the resultant enol is stated to be ‘remarkably stable’.Nicolaou et al. have studied the scope of the reaction shown in Scheme 5. Irradiation of (31) follows the Norrish Type II path with the formation of a photoenol (32). This then undergoes intramolecular addition to afford the tricyc-lic product (33) in high yield. Several examples were reported, such as the cyclization of (34) to afford (35) and of (36) to yield (37). In all cases the yields of products obtained are > 90%. They have extended the study to provide a path to some natural products of the hamigeran family. This was achieved using the cyclization of (38) into (39) as the key step. The photolysis of an adduct obtained from a thermal reaction of benzoquinone and a mixture of isopentafulvenes has been described. The reaction observed on irradiation is a Norrish Type II hydrogen abstraction process followed by cyclization within the resultant biradical. Irradiation of 3b-formyloxy-5a -bromo-6b-hydroxy-21-acetyloxypregnan-20-one has been used as a key step in a synthesis of a pregnenolone derivative.

Efficient decarboxylation of the keto acids (40) to the arylalkyl ketones (41) has been reported following their irradiation. The deuteriated compounds shown demonstrate that there is an intramolecular hydrogen transfer as the key step in the process. The m- and p-isomers are unreactive.

An account of the enantiospecific photocyclization of 2,5-diisopropyl-4′-car-boxybenzophenone in the solid phase has been described. The cyclization involves a Norrish Type II hydrogen abstraction and the outcome is controlled by the presence of (S)-phenylethylamine. The product obtained is the (R)(+)-cyclobutenol (42). The regioselectivity of the Norrish-Yang hydrogen abstraction process of the ketone (43) in the crystalline phase has been examined. The hydrogen abstractions occur at both positions ‘a’ and ‘b’ in the cyclohexane ring. Abstraction from ‘a’ affords (44) while (45) arises from the biradical afforded by abstraction from ‘b’. The selectivity observed depends on the nature of substitu-ents on the aryl ring.

Others have examined the influence of a variety of media on the Norrish Type II process. For example, the Norrish Type II reactivity of the arylketones (11-mercapto-1-phenylundecanone, 1-[4-dodecylphenyl]-11 -mercaptoundecanone, 1-[4-hexylphenyl] -11-mercaptoundecanone, 1-[4-(11-mercaptoundecyl)phenyl]hexanone and 1-[4[(11-mercaptoundecyl)-phenyl]-undecanone) has been studied with the ketones anchored as monolayers on gold nanoclusters. The photochemical efficiency is reasonable, with the cleavage process giving alkenes in 75% yield. The photoreactivity of pentan-2-one when it is include in zeolites has been examined with respect to the influence of the alkali metal cation. Changing the metal ion from Cs+ to Li+ brings about a decrease both in the Norrish Type II activity and in the photochemical reactivity. A study has highlighted the usefulness of irradiation in zeolites. The control that these substances exercise on the enantio- and diastereo-selectivity in some reactions was assessed. A short review has highlighted examples of time-related chirality memory in some photochemical reactions involving Norrish-Yang cyclizations. Norrish type II reactivity has been observed on photolysis at wavelengths > 300 nm of poly(4′-ethoxyacrylophenone). The reactivity arises from the triplet state of the carbonyl function, but the quantum yield for the process is lower than that observed in solution. The results from a study of temperature dependent photochemical reactions in a microwave field have been reported. Griesbeck and Heckroth have carried out a detailed study of the photochemical reactivity of a series of ketoamines (46). These undergo a variety of reactions dependent to some extent on the nature of the substitution. The two principal reactions are either Norrish Type II hydrogen abstraction or Norrish Type I fission. The fate of the former reaction is the formation of a biradical that either undergoes cyclization to yield (47) or fission of the 1,4-biradical to afford (48). The Norrish Type I process affords the amines (49). Norrish Type II reactivity is also shown in the propenamide derivatives (50). In this section, only the hydrogen abstraction process will be considered, and elsewhere (Part II, Chapter 2) the intramolecular cycloaddition reactions will be discussed. The three examples shown are induced by benzene sensitization and they all undergo Norrish type II hydrogen abstraction with the formation of the azetidines (51) in moderate to poor yields.

2.2 Other Hydrogen-Transfer Processes. – Hydrogen abstraction can also be brought about at sites other than the γ-position. Wessig and his co-workers described an example of this in last year’s account. This work involved a new route to cyclopropanes. A further report of this type of reactivity has been made. Succinimido and glutarimido substituted glycosans have been shown to undergo Norrish-Yang type cyclizations on irradiation at 254 nm. δ-Hydrogen abstraction occurs on excitation of the diketones (52). Cyclization within the resultant biradical provides a convenient route to the oxazinone derivatives (53).A further account of the intramolecular hydrogen abstraction processes within the cyclophanes (54), with a variety of linkers, has been published. The irradiation brings about the conversion into the products (55) by a 1,6-hydrogen transfer. The yields are variable and these are shown below the structures. Park and his co-workers have reported a further account of such cyclizations using excitation at 350 nm in benzene. These results are shown in Scheme 6. As can be seen, excitation results in δ-hydrogen abstraction from the side chains, and the resultant 1,5-biradicals undergo ring closure to yield the diols. These products are readily dehydrated to afford the difuran derivatives in 40% overall yield. These compounds were used to synthesize novel cyclophanes. A mechanistic study of the photochemical behaviour of a series of ring-substituted benzyl alkanoates has been reported.

Bochet has reviewed the area of photolabile protecting groups. Cano, Lad-low and Balasubramanian have described a polymer-linked system for the protection of amino acids. The systems are illustrated by (56): its irradiation affords good yields of the free acid (57).

3 Oxetane Formation

The photochemical addition of aldehydes and ketones (58) to the alkenols (59) has been described. The reactions show marked regio- and diastereo-selectivity (Scheme 7). A companion study (Scheme 8) has examined the results from the addition of benzophenone to the derivatives (60). Here the influence of the presence and absence of the hydroxy function on the outcome of the reaction was established. This is seen to best effect with the alkenols (60c,d), where acetylation of the hydroxy group (60c) virtually eliminates the regioselectivity. Others have demonstrated that pyrylium and thiapyrylium salts induce photochemical electron transfer from an oxetane to yield the resultant radical cation (61). This undergoes collapse to the radical cation of trans-stilbene.

Two oxetanes (62) and (63) are formed on photochemical addition of aryl aldehydes (p-CNC6H4, phenyl and 2-naphthyl) to the ketene acetals (64). There is a marked regioselectivity, with a ratio of (62):(63) of 95:5. Within the major product (62) the ratios of trans:cis are as shown below the structure. A short review of the above additions has also been published.

Kang and Scheffer have studied the photochemical behaviour of the ketone (65) in the solid state. Irradiation brings about the formation of the two oxetanes (66) and (67). Interestingly this behaviour is markedly different from that of (65) in solution, when the usual Norrish Type I reactivity is observed. This is also thought to be the case in the crystal. Thus irradiation essentially affords an aldehyde trapped close to the phenylcyclopentene. Photochemical addition affords the two products (66) and (67). Adam and his co-workers reported the photochemical addition of benzophenone to both cis- and trans-cyclooctene last year. Within this study they uncovered a remarkable temperature effect on the formation of the oxetane products. A further study has looked at this reaction again. Full details have been reported of the control observed in the oxetane forming reaction between the isomers of the cyclooctenes (68) and benzophenone and quinone. The detailed analysis of the results suggests that the outcome is the result of a variety of factors. The authors suggest these to be the syn or anti approach of the components, conformation changes in the triplet biradicals and competition between cyclization and cleavage of these biradicals.

Griesbeck et al. have examined the influence of solvent viscosity on the oxetane formation observed between aldehydes and dihydrofuran. Several solvents of varying viscosity were used in the study. The results shown in Scheme 9 are only a few of those recorded. The three solvents chosen in the scheme range from the lowest to the highest viscosity. It can be seen that there is an influence on the endo:exo ratio. The greatest effect is seen with the addition of propionaldehyde, where the ratio changes from 45.3:45.7 to 72.6:27.4 going from the lowest to the highest viscosity. This is bettered using glycerol, when a ratio of 80.2:19.8 is observed. A further study has reported on the oxetane formation between aromatic ketones and thiophenes and selenophenes.

The photoaddition of aldehydes (69) to the dihydropyridone (70) affords the oxetane derivatives (71), and Bach and his co-workers claim that this is a versatile building block and yields products with high regio- and diastereo-selectivity. The enantioselectivity of the system was assessed using the addition of the (+)-aldehyde (72). This affords the (-)-adduct (73) with a 95% ee. A review has described photochemical processes such as oxetane forming reactions involving Lewis acid-catalysis.

4 Miscellaneous Reactions

4.1 Decarboxylation. – A kinetic model for the photochemical decomposition of formic acid on a pilot-plant scale has been studied. The photodissociation dynamics at 193 nm of ethanoic acid have been studied. An FTIR examination of the photochemical decomposition of acetic acid on TiO2 has been reported. Results have been obtained demonstrating that matrix isolation of carboxylic acids provides a good method for the study of the various photochemical paths that are open to such molecules. The influence of UV irradiation in combination with ultrasound on the decomposition of trichloroacetic acid has been reported.

Petrenko, using a quantum mechanical simulation, has examined the possible paths for the formation of radicals following the irradiation of L-α-alanine in the crystalline state. The decarboxylation of 4-chloro-2-methylphenoxyacetic acid on TiO2 in aqueous suspension affords 4-chloro-2-methylphenol as the principal product. 2-Phenylpropionic acid undergoes photochemical decarboxylation on irradiation under a variety of conditions. The present work has demonstrated the influence of irradiation in a variety of cation-exchanged zeolites. Some of the results obtained are shown in Scheme 10.

(Continues…)Excerpted from Photochemistry Volume 34 by I. Dunkin. Copyright © 2003 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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