Carbonyl photoreactions

The photochemistry of acid derivatives is not well understood. Since these compounds absorb in the far ultraviolet and rarely react with great quantum efficiency, they have not been studied at all systematically. Consequently, this review will emphasize aldehyde and especially ketone photochemistry. So much is known about carbonyl photochemistry and so much is added each year (e. g., over 200 pages in the Chemical Society s 1973 survey of photochemistry D), that this review had to be limited to but a few aspects of carbonyl photochemistry. Although the main kinds of carbonyl photoreactions have been adequately reviewed often 2>, several important generalizations have resulted from the past few years work. This review will attempt to summarize these new generalizations which may not yet be familiar to the entire chemical community. Several unresolved problems will also be outlined. 

It is clear from the preceding discussion that organometallic photoinitiators (metal carbonyl or chelate derivatives) can provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end group or photoreactive pendant groups or side chains through the polymer chain. 

To this category belong, e.g., homogeneous photocatalytic systems based on soluble metal complexes or organic dyes as photocatalysts. Instructive examples are photoreactions assisted by heteropolyacids (HPAs), transition meal complexes with carbonyl, phosphine or some other ligands, and metal porphyrins. 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

Henriksen, U., Buchardt, O., and Nielsen, P. (1991) Azidobenzoyl-, azidoacridinyl-, diazocyclopentadi-enyl-carbonyl-, and 8-propyloxypsoralen photobiotinylation reagents. Syntheses and photoreactions with DNA and protein. Photochem. Photobiol. A Chem. 57, 331-342. 

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

Earlier reviews on the photochemistry of unsatured ketones and amines are available39,40. The photoreactions of a,/i-unsaturated carbonyl compounds in the presence of amines have been reported to yield 1 1 amine adducts32,33 as well as photoreduction... 

Scheme 54 Photoreaction of aromatic carbonyl compounds with p,p-dimethyl ketene silyl acetals.

Sakamoto et al. reported an interesting example involving the solid-state photoreaction of AT-(a,p-unsaturated carbonyl)thiobenzamides 1 leading to thietane-fused p-lactam 2 (Scheme 2) [29-31]. 

Sarel and co-workers have examined some reactions of alkynylcyclopropanes with iron carbonyl compounds [1]. Treatment of cyclopropylacetylene (5) with iron pentacarbonyl under photolytic conditions gives, after cerium(IV) oxidation, isomeric quinones 6 and 7, derived from two molecules of 5 and two carbonyls with both cyclopropane rings intact [6]. Furthermore, the photoreaction of dicyclopropylacetylene (8) with iron carbonyl gives some ten different products depending on the reagents and the reaction conditions, and some of them have the cyclopentenone skeleton formed by the opening of cyclopropane ring coupled with carbonyl insertion [7] (Scheme 2). 

The photoreactions of a number of a-diketones in which the carbonyl groups are held in a somewhat rigid m-configuration have been reported. These compounds undergo a variety of reactions depending upon the strain of the system and the solvent in which the reaction is carried out. 

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