The Photochemistry of Carbonyl Compounds

The photochemistry of carbonyl compounds has been extensively studied, both in solution and in the gas phase. It is not surprising that there are major differences between the photochemical reactions in the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collision, whereas in the liquid phase the excess energy is rapidly transferred to the solvent or to other components of the solution. Solution photochemistry will be emphasized here, since both mechanistic study and preparative applications of organic reactions usually involve solution processes. 

When considered as a part of the photochemistry of carbonyl compounds, irradiations of esters constitute a minor component. The more frequent photolyses of other carbonyl compounds, in particular ketones, is not surprising, as, even though parallels exist between ester and ketone photochemistry (for example, both experience a-cleavage and hydrogen abstraction-reactions), esters require radiation of higher energy for reaction, and typically produce more-complex mixtures of products. In addition to their similarity to other carbonyl compounds in their reactivity, esters also experience reactions that are uniquely their own. 

The photochemistry of carbonyl compounds plays an important part in the photochemical formation and breakdown of polymers. 

In summary, the above methodology proved successful in simulating the photochemistry of carbonylic compounds. Experimental evidences and validation using high-level ab initio methods support the results. Further applications are anticipated. 

The (n, it ) state plays a very important role in the photochemistry of carbonyl compounds and many heterocyclic systems. In conjugated hydrocarbons (it -> it ) transitions are most important and give intense characteristic absorption bands. Because of some overlap forbidden character, the (n - it ) transitions have low probability and hence weak absorption bands. 

Given this structural representation of the n-ir excited state, one has to determine the extent to which the photochemistry of carbonyl compounds is actually explicable as continuous electron redistribution processes of such a species. In actual fact, it is found (3-6) that n-ir photochemical reactions are amenable to mechanistic discussion in electron detail using this model of the excited state. 

Laser Techniques in the Study of the Photochemistry of Carbonyl Compounds Containing Ligninlike Moieties... 

Examples of photoreactions may be found among nearly all classes of organic compounds. From a synthetic point of view a classification by chromo-phore into the photochemistry of carbonyl compounds, enones, alkenes, aromatic compounds, etc., or by reaction type into photochemical oxidations and reductions, eliminations, additions, substitutions, etc., might be useful. However, photoreactions of quite different compounds can be based on a common reaction mechanism, and often the same theoretical model can be used to describe different reactions. Thus, theoretical arguments may imply a rather different classification, based, for instance, on the type of excited-state minimum responsible for the reaction, on the number and arrangement of centers in the reaction complex, or on the number of active orbitals per center. (Cf. Michl and BonaCid-Kouteck, 1990.)... 

The photochemistry of carbonyl compounds still continues to be a major general area of interest, and physical methods, especially e.s.r., C1DNP, and C1DEP (electron polarization), continue to be widely applied for detection of radical-like transient intermediates. Quinkert and Jacobs have provided further evidence that ring expansion of cyclobutanones to oxacarbenes, and thence to tetrahydrofurans, occurs in a concerted fashion without the intermediacy of biradicals formed by Norrish Type I fission of a C—CO bond. Medary et al. report that Norrish Type I cleavage of 2-ethylcyclopentanone is non-stereospecific, and gives the cis- and /ra/z.y-hept-4-enals previous reports (Srinivasan and Cremer, 1965) that reactions of this type are stereospecific appear to require revaluation. 

The photochemistry of carbonyl compounds has been extensively studied both in solution and in the gas phase. It is not surprising that there are major differences between the two phases. In the gas phase, the energy transferred by excitation... 

Oxetans.—Reviews of the photochemistry of carbonyl compounds and of their reactions with isocyanides have appeared. 

The photochemistry of carbonyl compounds may be studied under following categories ... 

Discuss the photochemistry of carbonyl compounds in P, y- unsaturated and (X, p-unsaturated compounds. 

The photochemistry of carbonyl compounds has been one of the main areas of research in organic photochemistry for many years. Among aU the different types of carbonyl compounds, P,y-unsaturated ketones have been the subject of extensive studies. The results obtained from these efforts, conducted over a 30-year period, show that, in general, direct irradiation of P,y-unsaturated ketones yields products resulting from 1,3-acyl migration, while triplet-sensitized reactions of these compounds affords cyclopropyl ketones by oxa-di-n-methane (ODPM) rearrangement pathways. Alternative reaction routes, such as decarbonylation, ketene formation, epimerization, [2+2]-intramolecular cycloadditions, Norrish Type I and Norrish Type II reactions, cis-trans isomerizations, and reductions of the C-C double bond, have also been described in some instances, depending on some particular structural features present in the P,y-unsaturated ketone. However, the photoreactivity of these compounds is dominated by the two main processes mentioned above. 

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