Type II cleavage of ketones

Type I cleavage of ketones, 75 Type II cleavage of ketones effect of 7-substituents, 122-124 effect of ring substituents, 124 of 5 -(+)-4-methyl-l-phenyl-1- hexanone, 118... 

One possible explanation for the lack of correspondence between emitting and reacting states is reaction of the singlet. In the case of benzophenone, there is little question that the reaction involves only the triplet state, since triplet quenchers can effectively inhibit the reaction. This need not be the case with all carbonyl compounds. For example, it has been shown that both the n,ir singlet and triplet states can be involved in the Norrish type II cleavage of alkyl ketones (25-27). At high concentrations, piperylene quenches only that part of the 2-hexanone cleavage which arises from the triplet. The rate constants for... 

Before the concerted vs. two-step question was further elucidated, another basic mechanistic puzzle was raised. One research group found that type II cleavage of 2-pentanone was quenched by biacetyl [6], which was known to quench excited triplets rapidly. Another group found that the reaction of 2-hexanone was not quenched under the same conditions [7]. The two groups obviously differed as to which excited state undergoes the reaction. The apparent conflict was neatly solved by the revelation that each of the two ketones reacts from both states, with 2-hexanone undergoing more unquenchable singlet reaction than 2-pentanone [8,9]. 

P. J. Wagner, P. Kian, Chapter 52 Norrish Type II Photoelimination of Ketones Cleavage of 1,4-Biradicals Formed by y-Hydrogcn Abstraction, in CRC Handbook of Organic Photochemistry and Photobiology, W. M. Horspool, F. Lengi, (Eds.), CRC Press, Boca Raton, 2003, p. 1-31. 

Nonish Type II Photoelimination of Ketones Cleavage of 1,4-Biradicals Formed by y-Hydrogen Abstraction... 

Hwu, J. R., Chen, B. L., Huang, L. W., and Yang, T. H., Influence of beta-silyl groups in cycloal-kanones on the Norrish type I and type II cleavages, /. Chem. Soc., Chem. Commun., 299, 1995. Ramamurthy, V., Organic-photochemistry in organized media. Tetrahedron, 42, 5753, 1986. Turro, N. J., Liu, K.-C., and Chow, M.-R, Solvent sensitivity of type II photoreaction of ketones as a device to probe solute location in micelles, Photochem. Photobiol., 26, 413, 1977. 

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (58)]. Of these, Norrish Type II cleavage and the related... 

Ddla, quantum yield for disappearance of ketone j, quantum yields for type I cleavage n, u, quantum yields for type II cleavage cb. cb> quantum yields for cydobutane formation. 

The preceding discussion applied to aromatic ketone triplet reactions. With aliphatic ketones the situation is quite dilferent. As stated previously, aliphatic ketones undergo type II cleavage from both the excited singlet and the triplet state. By studying the reaction with and without added quencher, one can determine the characteristics of the reaction for each state, that is, the singlet reaction can be studied in the presence of a strong triplet quencher while the triplet reaction characteristics can be obtained by the difference between the reaction without quencher and that when quencher is added. For example, for the reaction... 

Further mechanistic information concerning type 1 cleavage reactions has been obtained by Yang in a study of alkyl-t-butyl ketones.(13) Irradiation of these ketones in hexane solution results in a mixture of products corresponding to both type I and type II cleavage, for example,... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

It is interesting to note that although in principle ketone 48 can undergo secondary Norrish type II cleavage to give a mixture of methyl 4-acetyIbenzoate... 

Intramolecular hydrogen abstraction from a g carbon occurs when certain ketones are irradiated. For e.g., vapours of Hexan-2-one on photolysis gives an alkene and a ketone (via the enol form). It is referred to as Norrish type II cleavage. 

This reaction, called Norrish Type II cleavage 1 involves intramolecular abstraction of the y hydrogen followed by cleavage of the resulting diradical38 (a secondary reaction) to give an enol that tautomerizes to the aldehyde or ketone product.39... 

Calvert has studied the competing type I and II processes in various -propyl alkyl ketones in the gas phase, and finds that increases and decreases as the alkyl group is changed from methyl through r-butyl,359 as might be expected. Perhaps the most dramatic illustration of the inherent inefficiency of type II photoelimination is provided by the fact that at 100°, where the quantum yield for type I cleavage of diethyl ketone is 1.0, the total quantum yield for disappearance of di- -propyl ketone is only 0.58.359... 

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