Hydrogen atom abstraction reactions photochemical

Suginome and coworkers have described the photochemical (2+2)-cycload-dition of alkenes to 2-acetoxynaphtho-l,4-quinone. The resultant adducts can be converted into the corresponding cyclobutanols which react with mer-cury(II) oxide/iodine to afford a cyclobutanoxyl radical. A laser-flash study has examined the photochemical behaviour of vitamin K3. This investigation sought to provide details for the hydrogen atom abstraction reactions in this system. ... 

In contrast to the photo physical processes just described, photochemical processes produce new chemical species. Such processes can be characterized by the type of chemistry induced by light absorption photodissociation, intramolecular rearrangements, photoisomerization, photodimerization, hydrogen atom abstraction, and photosensitized reactions. 

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

Photoreduction occurs by hydrogen atom abstraction or by electron transfer. The first process is a common photochemical reaction of carbonyl derivatives and other unsaturated molecules in the presence of suitable hydrogen atom donors (which can be alcohols, paraffins, ethers, etc., that is, almost any molecule with a not-too-strong C—H bond). 

Irreversible photochemical reactions can take place when one of the fragments of a photodissociation reaction reacts with a molecule of the glass matrix itself hydrogen atom abstraction is a common secondary reaction of this type. 

R CHORj. Such radicals have been formed by hydrogen atom abstraction from the ether by radicals produced from thermal decomposition of peroxides (67, 75, 76). Similar radicals may be produced in photochemical processes, either by direct irradiation (29, 54), or by the use of a photosensitizer or a photoinitiator, such as acetone or benzophenone (21, 64, 66). The ether radicals once produced, participate in a variety of chemical reactions. It might be noted that resonance forms as illustrated... 

The second type of photochemical reaction that has been used to form radicals with visible light, as well as with higher-energy light, is hydrogen atom abstraction by excited a-diketones or related groups (biacetyl, anthraquinone, benzoquinone) (42). 

Photolysis at lower temperatures and in the solid state decreases the extent of the hydrogen atom abstraction due to its higher activation energy compared to radical dimerization. Additionally, the lower mobility of the radicals in the solid state increases the cage reaction of the pairwise generated radicals. This way unsymmetrical diacylperoxides form unsymmetrical dimers with less than 1% of the symmetrical product [29]. Peroxides from carboxylic acids, being optically active at the a-carbon, couple photochemically in the solid with only a minor loss of optical activity [30]. 

Organic compounds can generate the initiators of free radical sequences through the primary photochemical processes homolytic dissociation into radicals, hydrogen-atom abstraction, photoionization, and electron transfer reactions. The homolytic dissociation reactions are limited to compounds containing relatively weak bonds (<98 kcal), such as sulfides, peroxides, and some halides and ethers. Representatives of all of these classes of compounds are certainly present in seawater, but the limited information on the qualitative and quantitative aspects of their occurrence does not allow for an estimate of their importance in the promotion of free radical reactions. The same is true for electron transfer reactions, which may be an important photochemical process for organic transition metal complexes. 

Photochemical reactions of t-BuOCl generally involve hydrogen atom abstraction by t-BuO, a radical which is considerably more selective than Cl . In the reaction of t-BuOCl with 2,3-dimethylbutane at 64 °C, there is a report of a selectivity identical to that of Cl upon photolysis but a higher selectivity with initiation by Bz202. Photolysis of t-BuOCl may lead to chlorine atoms, hydrogen chloride and molecular chlorine whereas BZ2O2 leads to the t-BuO radical chain. Reaction 29 will shift t-BuOCl... 

Photochemical intramolecular hydrogen-atom abstraction affords a method for determining the microviscosity of micelles, as the Barton reaction of (137)... 

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