Hydrogen abstraction, ketones

The sulfoxides 215 and 216 also show what is essentially alkene photochemistry [109], Photostationary states of E/Z isomeri2ation were obtained for the analogous sulfides and sulfones as well. Interestingly, if the sulfoxide is replaced by an ether, isomerization is followed by internal ketone hydrogen abstraction from R and five-membered ring formation. 

Lewis and Hirsch studied four a-cycloalkoxyacetophenones, in which the ring is even farther away from the carbonyl and the y-hydrogen is tertiary.Table 58.3 lists the results, which consist of oxas-piro[3,n]alkanol and cycloalkanone formation, via normal cyclization and cleavage of the 1,4-biradicals. Simple isopropoxyacetophenone is included for the sake of comparison to acyclic compounds. As with other a-alkoxy ketones, overall quantum yields are quite high and cyclization yields are reasonable, except for the cyclopropyloxy ketone. Hydrogen abstraction rate constants are very high and decrease as expected as the rings get smaller. 

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

In ketones having propyl or longer alkyl groups as a carbonyl substituent, intramolecular hydrogen abstraction can be followed by either cleavage of the bond between the a and P carbon atoms or by formation of a cyclobutanol ... 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

In order to avoid competitive bimolecular photoreactions such as ketone reduction by hydrogen abstraction, poor hydrogen donating solvents are recommended (acetonitrile, acetic acid, tertiary alcohols). In those cases where ketene trapping is desired, solvents must also be miscible with water or other protic nucleophiles. 

Photochemical substitution reactions of this type which involve selective hydrogen abstractions from intramolecular sites by the m.tt ketone oxygen, are reviewed in chapter 12. ... 

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... 

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

Figure 7.26. Photo-induced hydrogen abstraction from the y-carbon leads to biradical 72, which can (a) revert to the starting ketone, (b) cyclize, or (c) cleave the 2,3-CC bond. The structure for y-H abstraction for the starting ketone is also shown and the ideal parameters defined and listed.

These last two points are consistent with electron transfer from the tertiary amines to the triplet ketone in competition with hydrogen abstraction from benzhydrol ... 

INTRAMOLECULAR HYDROGEN ABSTRACTION BY KETONES (TYPE H CLEAVAGE)... 

With compounds containing C—H bonds y to the carbonyl moiety, one often observes intramolecular hydrogen abstraction, in contrast to the in-termolecular hydrogen abstraction by aryl ketones discussed earlier in this... 

Ring opening due to intramolecular hydrogen abstraction has been demonstrated for cyclopropyl ketone derivatives 1035 ... 

As mentioned in the introduction, there are conflicting views as to the contributions made to polymer degradation by various initiating species. Among these species, in addition to ketones, hydroperoxides are some of the more important chromophores. As it is known, the photolysis of hydroperoxides yields alkoxy and hydroxy radicals. In polymers, in the presence of oxygen, these radicals lead to the secondary formation of peroxy radicals. The latter in turn are converted by hydrogen abstraction into new hydroperoxides (Scheme I) ... 

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