Unsaturated ketones photochemistry

The quiaones have excellent redox properties and are thus important oxidants ia laboratory and biological synthons. The presence of an extensive array of conjugated systems, especially the a,P-unsaturated ketone arrangement, allows the quiaones to participate ia a variety of reactioas. Characteristics of quiaoae reactioas iaclude nucleophilic substitutioa electrophilic, radical, and cycloaddition reactions photochemistry and normal and unusual carbonyl chemistry. 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

Schuster, D. I. Krull, I. S. Photochemistry of unsaturated ketones in solution. XIX. Photochemistry of spiro[2.5]octa-4,7-dien-6-one. 2. Mechanistic aspects and the relationship to the photochemistry of quinone methides. Mol. Photochem. 1969, 1, 107-133. 

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... 

From the foregoing discussion, it is clear that DPM rearrangements are very general for a variety of 1,4-unsaturated systems, such as, 1,4-dienes, (3,7-unsaturated aldehydes and ketones, and different 1-aza-1,4-diene derivatives. Surprisingly, the literature was devoid of studies describing the photoreactivity of the closely related 2-aza-1,4-diene derivatives. For many years, the only studies in this area were carried out by Mariano and his co-workers [60] on the photochemistry of iminium salts derived from 2-aza-1,4-dienes. The results obtained demonstrated the synthetic utility of the photocyclizations of iminium salts to different heterocycles, in reactions that are initiated by intramolecular single electron transfer [60]. 

Various mechanisms have been proposed for these reactions, but little can be deduced yet from such a scanty supply of examples the light-induced addition of water to unsaturated ketones seems to represent a relatively unattended area of photochemistry. 

An excited state with charge separation has also been proposed by Chapman117 to help explain the photochemistry of unsaturated ketones this proposal has been further discussed by Kan.118... 

Acyclic a,jS-unsaturated ketones do not provide much photochemistry, except for facile geometric isomerization about the double bond. Although some also isomerize to /3,y-unsaturated ketones, others, such as mesityl oxide, apparently undergo no net reaction upon absorption of irradiation. Their low reactivity might result from a rapid relaxation to an appreciably more twisted geometry in their triplet states than can be obtained in the cyclic enones. Below are some examples of typical photochemical behavior.426,427... 

As we shall see, n —> tt singlet and triplet states of carbonyl compounds play an important role in photochemistry. Aldehydes and ketones display all the characteristics of absorption, fluorescence, phosphorescence, and intersystem crossing (5, —> T,) illustrated in Figure 28-1. Generally, they are more efficient at intersystem crossing than are unsaturated hydrocarbons, perhaps because the energies of the S and T states involved are not widely different. 

Mixed chromophores with both C=C and C=0 moieties such as ,/3-unsaturated enones are one of the most investigated chromophores in organic photochemistry. Conjugation of the two chromophoric moieties results in lowering the n —> ji and tt ji energy levels relative to the unconjugated chromophores. UV absorption spectra of selected alkenes, ketones and enones are presented in Table 1. 

Changing the anchoring atom from oxygen to carbon does not change the initial photochemistry of the unsaturated ketones. The rates of the subsequent thermal electrocyclic rearrangements are, however, changed substantially. The tri-fluoroacetyl substituent was chosen to maintain as strong a donor-acceptor interaction in the bicyclooctadienes as possible, as well as to maintain a lowest triplet. 

Wanting to study the photochemistry of a (3,y-unsaturated ketone that could not isomerise to a conjugated ketone, chemists5 chose 13. The obvious Wittig disconnection revealed the ketoaldehyde 14. Observing the symmetry of the carbon skeleton and the 1,3-relationship, they changed the ketone into an alcohol so that an aldol disconnection revealed two molecules of aldehyde 16. 

The photochemistry of o< -unsaturated ketones has attracted much attention and is still a field fiettd of current interest. /Numerous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition reactions and rearrangements. For example, cyclopentenones 1 and 2 readily rearrange to... 

Further studies on the photochemistry of friedelin have led to the isolation of the unsaturated aldehyde (130).105 Silver oxide oxidation of (130) gave the known putranjivic acid. Irradiation of friedelin in the presence of acetone afforded the hydroxy-ketone (131).106 Photochemically initiated reaction of 7/3-hydroxyfriedelane and 3/3,7/3-dihydroxyfriedelane with lead tetra-acetate-iodine... 

We have looked at the photochemistry of a-chloro-a-methylvalerophenone and find it to be almost hopelessly complex 198h At high conversion, the major photoproducts are the / ,y-unsaturated ketones plus ft and y-chloroketoqes formed by addition of HC1 to them. However, the reaction is quite complex, since many other products are formed only at low conversion. All of these, including some but not all of the / ,y-unsaturated ketones, are quenched by radical traps and triplet... 

Library of Congress Cataloging in Publication Data (Revised) Main entry under title Triplet states. (Topics in current chemistry 54-55, 66) Includes bibliographies and indexes. CONTENTS 1. Devaquet, A. Quantum-mechanical calculations of the potential energy surfaces of triplet states. Ipaktschi, J., Dauben, W. G., and Lodder, G. Photochemistry of B. y-unsaturated ketones. Maki, A. H. and Zuclich, J. A. Protein triplets states, [etc.] 1. Excited state chemistry-Addresses, essays, lectures. 2.Triplet state-Addrcsses, essays, lectures. I. Series Topics in current chemistry 54 fete.] QD1. F 58 vol. 54-55 [QD461.5] 540 8s [54P.24] 75-1466 ISBN 0-387-07107-5 (v. 1)... 

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