Rearrangement and Hydrogen Abstraction

In the following reaction, 5-19 is a product resulting from migration of an aryl group, and 5-18 is a nonrearranged product. Thus, there is competition between rearrangement and hydrogen abstraction by a radical intermediate. 

Hammerum, S. Rearrangement and hydrogen abstraction reactions of amine cation radicals a gas-phase analogy to the Hofmann-Lbffler-Freytag reaction. Tetrahedron Lett. 1981,22,157-160. 

Photolysis of (204) in methanol gives rise to (208) and (209), products from Wolff rearrangement and hydrogen-abstraction, respectively. No traces of (212) were detected, implying that a symmetrical intermediate such as (210) could not have intervened (Scheme 45). It appears that fused-ring oxirenes are still elusive species and further research in this area is warranted. 

Weng, H. and Roth, H.D., Electron transfer photochemistry of norbornadiene and quadricyclane. Nucleophilic capture of radical cations, free-radical rearrangements, and hydrogen abstraction, . Org. Chem., 60, 4136-4145, 1995. 

Rearrangements.—Mechanistic and stereochemical aspects of the photochemical ring expansion of cyclic ketones via oxacarbenes have been discussed. Evidence for a quantum chain process in the photoreactions of benzvalene is reported, and hydrogen-abstraction reactions and subsequent rearrangements of 1,3-diene-p-benzoquinone Diels-Alder adducts have been extensively investigated. ... 

In subsequent experiments, it was found that rearrangement of 2 persisted in solid glasses at temperatures as low as 28 K, where the reaction rates became nearly temperature independent, as expected. The fact that similar kinetics were measured in solution and in low temperature solids indicated that matrix effects were minimal, at least in these intramolecular hydrogen abstractions. 

Excitation to produce a diradical-like intermediate (excited state) can result in either hydrogen abstraction or rearrangement and closure to form the cyclobutene ... 

The chemical reactions of sulphonyl nitrenes include hydrogen abstraction, insertion into aliphatic C—H bonds, aromatic substitution , addition to olefinic double bonds, trapping reactions with suitable nucleophiles, and Wolff-type rearrangement. Hydrogen-abstraction from saturated carbon atoms is usually considered to be a reaction typical of triplet... 

To explore the mechanism of allylic hydroxylation, three probe substrates, 3,3,6,6-tetradeuterocyclohexene, methylene cyclohexane, and /l-pinenc, were studied (113). Each substrate yielded a mixture of two allylic alcohols formed as a consequence of either retention or rearrangement of the double bond. The observation of a significant deuterium isotope effect (4-5) in the oxidation of 3,3,6,6-tetradeuterocyclohexene together with the formation of a mixture of un-rearranged and rearranged allylic alcohols from all three substrates is most consistent with a hydrogen abstraction-oxygen rebound mechanism (Fig. 4.48). 

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