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...