Reactivity ketones, hydrogen abstraction

For aliphatic or benzylic hydrogen abstraction, ketones with a lowest tt-tt triplet state are much less reactive than those with the lowest n-jr triplet state. The lifetime of the triplet is generally on the order of 10-100 because of the small spin-orbit... 

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. 

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. 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

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. 

Since intramolecular hydrogen abstraction could not compete with decay of the two ketones having it,it lowest triplets, the reaction must have a rate constant of < 103 sec-1, the rate of their decay in solution.155,337 Consequently a factor of at least 10 exists between the reactivities of pure n,ir and -it,it ketone triplets in this particular reaction. Since the phenyl ketone triplets are about one-tenth as reactive as the triplets of aliphatic ketones, we have suggested that... 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

Inasmuch as naphthyl ketones are not totally unreactive at hydrogen abstractions 54,60), there must be a small amount of reactivity intrinsic to n,n states. Such reactivity is actually known for 71,71 olefin triplets 81> and for enone triplets 6 2). In the case of phenyl ketones the carbonyl and aryl jr-systems are mixed, such that a locally excited carbonyl n,n triplet contributes a small fraction to the overall wave function for the triplet. Given this intrinsic reactivity, which is some 10-4—10-6 that of an n,n triplet, the observed reactivity of 71,71 triplets with nearby n,7t triplets can be expressed as follows, where k% is the rate constant... 

The only studies specifically designed to differentiate between vibronic mixing and equilibration as the source of ,jr -reactivity in ketones with 71,71 lowest triplets have been performed here at Michigan State University. Perhaps the best evidence that equilibration is the major mechanism involves a comparison of intramolecular triplet state hydrogen abstraction by a series of phenyl ketones hi,71 lowest) and an analogous series of p-methoxyphenyl ketones (37i,n lowest). 

A careful study of polymethyl substituted butyrophenones has revealed that their observed rates of triplet state y-hydrogen abstraction are no greater than what would be predicted from estimates of AEy, if equilibration of triplets is the only source of reactivity 63). In fact, meta- and >ara-methyl ketones are less reactive than would have been expected. In these cases, the n,7i and 71,71 triplets are so close that they presumably undergo maximum vibronic mixing, moving the... 

Structure-reactivity relationships are now well understood for hydrogen abstraction and a-cleavage reactions of monofunctional excited ketones. The generality of CT quenching is recognized but many aspects are poorly understood. Most aspects of /9-cleavage reactions are poorly understood. 

The two ketones (28) and (29) are known to undergo the Norrish Type II hydrogen abstraction process, and their photochemical reactivities have now been studied in chirally modified zeolites. The zeolites were modified by stirring them with known amounts of ( —)-ephedrine. Irradiation of the ketones in the zeolites brought about some enantiomeric enhancement. However, the various zeolites studied behaved differently and the NaX zeolite favoured the (+)-isomer of the product (30) while the NaY favoured the ( —)-isomer. The other ketone (29) showed only low enantiomeric enhancement and gave both the cis and the trans cyclobutanols (31) and (32) in a ratio of 4 1. ... 

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