A-cleavage of ketones

When radical pairs are produced by a cleavage of ketones, and the loss of CO is prevented on energetic grounds, fragments of the reactions of the primary radical pair become important. Deoxybenzoins (15)30) or adamantyl ketones (20)25,30) provide examples of such systems. The photodecomposition of l,2-diphenyl-2-methyl-pro-panone (15)30) illustrates an example of a photofragmentation in reaction (Scheme VIII). The ratio of the major products are highly dependent on solvent environment. For example, the photochemistry of this compound in acetonitrile or benzene yields... 

The picture becomes somewhat more complex if the a cleavage of ketones is considered. 

The lowest excited state of many azo compounds, like that of ketones, is an (n, r ) state. Photolytic cleavage of a CN bond analogous to the a cleavage of ketones is therefore to be expected ... 

Giese et al. investigated the a-cleavage of ketone 18 (Chart 13) as a model of 4 -DNA radical strand cleavage. The observed polarization pattern can only accommodate a radical cation 20, so the initially formed radical 19 must eliminate (EtO)2P02 fast on the CIDNP timescale. This is evidence that anaerobic scission... 

With other unsaturated compounds, he considered the transient formation of radical or atoms, e.g., he rationalized the photochemical a-cleavage of ketones as indicated in Scheme 2.2. In the presence of water, the fragments were trapped (photochemical theory of valence). 

Table 3.10 Characteristic ions from the a-cleavage of ketones. ...

C. Cleavage of Bonds Attaching Atoms or Groups to the a-Carbon of Ketones... 

Photochemical autooxidation of ketones, 316 Photochemical a-cleavage, 296 Photochemical cleavage of bonds attaching atoms to the a-carbon of ketones, 307 Photochemical cleavage of exocyclic. S-bonds, 323... 

Another common type of C—C cleavage is a-cleavage of an a-hydroxy-ketone ... 

The Simmons-Smith cyclopropanation method has also found application for the a-methylation of ketones via an intermediate cyclopropane. The starting ketone—e.g. cyclohexanone 9—is first converted into an enol ether 10. Cyclopropanation of 10 leads to an alkoxynorcarane 11, which on regioselective hydrolytic cleavage of the three-membered ring leads to the semiketal 12 as intermediate, and finally yields the a-methylated ketone 13 ... 

As can be seen from Figure 3, the ratio of the isopropyl ether to isobutyrate is about 1 1. It is clear that after a-cleavage of the ketone the two radicals primarily formed are captured directly by nitroxide. This takes place without decarbonylation of the acyl radical (reaction (7)) ... 

The electrochemical reduction of various bicyclic a-amino ketones such as quinolizidinone (n = 2) (Scheme 159) can involve a cleavage of the carbon-nitrogen bond giving a ring enlargement [275, 276]. 

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