Hydrogen abstraction, ketones mechanism

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. 

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. 

Predict the products of the intermolecular hydrogen abstraction reactions of ketones and discuss the mechanisms of these reactions. 

For example, 2-hexanone in MeCN gave 5-acetamino-2-hexanone (44) in 40% yield (Scheme 17) [82], and 2-pentanone was oxidized in trifluoroacetic acid to give a mixture of 4-trifluoroacetoxy-2-pentanone (24%) and 3- and 4-penten-2-one (6%) [83]. A mechanism involving intramolecular hydrogen abstraction by a ketone cation radical that forms a car-benium ion via a [l,5]-hydrogen shift was proposed. 

For the reactions described so far in this section, the ketone substrates have lowest excited states that are (n.ii ) in character aliphatic ketones may react by way of the singlet or the triplet state, and aryl ketones normally through the triplet because intersystem crossing is very efficient. The efficiency of photochemical hydrogen abstraction from compounds such as alcohols or ethers is very much lower if the ketone has a lowest (Ji,n triplet state, as does I - or 2-acetylnaphthalene (CmH-COMe). However, all aryl ketones, regardless of whether their lowest triplet state is fn,Jt l or (Jt.Ji ), react photochemically with amines to give photoreduction or photoaddition products. A different mechanism operates (4.38), that begins... 

Methyl ethyl ketone is unique, in that long and irreproducible induction periods were observed on occasion, reaction ensued only after 7 hours and then was completed within 10 minutes. During the long induction period the only detectable product was methanol. No convincing reason can be advanced to account for this anomalous behavior. The virtual absence of ethylene from the products of the low temperature slow combustion of methyl ethyl ketone strongly suggests that the low-temperature mechanism proceeds almost exclusively by further oxidation of the radicals produced by hydrogen abstraction from the parent ketone. 

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. 

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). 

The experimental evidence [22,89,90,110-115] suggests that ketones react with OH radicals via a hydrogen abstraction mechanism, leading to a water molecule and a new radical. Nevertheless, there is a peculiarity in the ketones -I- OH reactions hydrogen atoms attached to carbon atoms in beta positions to the carbonyl group are the most likely to be abstracted [110-113,115]. However, if the beta carbon is a primary carbon, its contribution to the total reaction is much less important. The contribution is about 66% [116] to 67% [117] for secondary beta carbons, while it is only about 11% [116] to 17% [117] for primary ones. To explain the large contribution of the beta abstractions, Wallington and Kurylo [90] have proposed a complex... 

Further evidence to support for this mechanism comes from the work of Salem [118]. Salem examined a number of photochemical reactions with particular attention to potential energy surface symmetry for both ground and excited states. One such reaction considered was hydrogen abstraction by ketones ... 

A wide variety of photoadditions to unsaturated oxygen and sulfur heterocycles has been reported. It has, however, proved difficult to classify these processes, especially as the reaction mechanisms are not fully understood in all cases. Most additions of solvent to oxygen heterocycles arise via hydrogen abstraction pathwyas, often initiated by added ketone. Polar addition is relatively rare in these compounds the addition of methanol to... 

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