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 =...