Hydrogen abstraction, ketones study

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. 

Bredt s rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts [281]. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like y-hydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=0-Ha distance is 2.67 A. Other abstractable hydrogens (Hb) are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If i-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a. 

No attention is given to the mechanistic importance of a reaction rather, an attempt has been made to concentrate on reactions that have an actual (potential) synthetic role. This is not always an obvious selection, because photochemistry has not been sufficiently used in such syntheses, and mechanistic studies are not necessarily a reliable guide towards this aim. As an example, hydrogen abstraction by ketones (Scheme 1.3), which probably is the most thoroughly studied photochemical reaction, is not mechanistically discussed. Neither is presented the resultant photoreduction of ketones (Scheme 1.3, path a), because this will hardly ever become a sensible synthetic alternative for the reduction. However, other reactions arising from the same primary photoprocess, namely bimolecular reduction (path b) and... 

These have been extensively studied by Neckers and coworkers [21]. Those which undergo y-hydrogen abstraction cleave cleanly to aldehyde or ketone, and thus provide a possible methodology for environmentally friendly oxidation of alcohols. The secondary photolysis of the hydroxyketene forms benzaldehyde and carbon monoxide, both of which could be considered nuisances, although it might be possible to trap the carbene intermediate in order to make a 1-phenylcyclopropanol. 

Substituents in a-Position. To rationalize the ratio between Norrish type II cleavage and cyclization (see preceding chapter) the influence of a-substituents on cyclization were studied exclusively on straight-chain aryl ketones with preferential y-hydrogen abstraction. 

Hydrogen abstraction from alkyl benzenes occurs efficiently by using aromatic ketones The mechanism of the reaction has been extensively studied, with ketones having both a mi and a jiji state as the lowest triplet, and found to involve some degree of electron transfer, which grows with more easily reduced ketones [32,33]. The same reaction occurs intramolecularly, e.g., in the photoinduced hydrogen transfer in 2-methyl-benzophenone to give the (trappable) enol [34-36]. 

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... 

Stiver and Yates have studied the photochemical reactions of some hydroxy-keto steroids (28, 29). Irradiation of the isomeric compounds (28a, 29a) showed that the products obtained, (30) and (31) respectively, had retained the configuration of the carbon to which the hydroxy group is attached. The use of deuteriated derivatives (28b, 29b) has identified the hydrogen abstraction processes involved in the conversion of these ketones into the lactones (30b) and (31b) respectively. The authors " propose that there are two major factors which control the stereospecificity of the reactions. These are the shape of the hydroxy-bearing C-atom and the hydrogen transfer within the biradical formed on Norrish Type I fission. The stability of the biradical intermediate clearly plays an important part in determining the outcome of the reactions. 

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. ... 

Other Hydrogen Transfers - Weigel and Wagner S have studied the 1,4-hydrogen abstraction reactions within the ketones (37) leading to the cyclopropanols (38). The study used wavelengths > 300 nm with benzene or methanol as the solvents. The yields of products are shown below the structures. Polar solvents appeared to have no effect on the outcome of the reaction. 

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