Rearrangements ketones, hydrogen abstraction

Recent evidence seems to indicate, however, that radical reactions inside a zeolite do not necessarily follow the same pathways as classical radical reactions. The oxidation of the hydrocarbon ethylcyclopropane with H202 catalyzed by TS-1, which should proceed through radical intermediates, gives only the alcohol (27%) and the ketone (73%) resulting from hydrogen abstraction at the carbon linked to the cyclopropane ring, and not the products that would be expected from the rearrangement of the radical intermediate ... 

Nitro compounds, in particular aromatic and heterocyclic derivatives, absorb strongly in the near UV. They have properties similar to ketones in their excited state. These compounds are characterized by an unpaired electron in the n0 orbital and thus by a radical character. A typical example of this radical character is the easy intramolecular hydrogen abstraction in nifedipine and related vasodilators (Sch. 7) (18). Another manifestation of the radical character of the nitro group is the rearrangement often observed with nitrated five-membered heterocycles, as in the case of metronidazole (Sch. 8) (19). 

Attempts to trap other di-alkyl ketyl radicals from methyl-ethyl, di-ethyl, di-n-propyl and di-isopropyl ketones were unsuccessful. The observed spectrum was not that expected from the ketyl radical but in each case it could be identified as that of a radical formed by a hydrogen abstraction from the parent ketone. The structurally similar radicals, CH3—CH—CO—CHg and CH3—CH—CO—CHa—CHg, were formed from methyl-ethyl and di-ethyl ketone respectively and the radical (0113)2—C—CO—CH—(0113)2 was formed by abstraction of the tertiary hydrogen from di-isopropyl ketone. The reaction of di-n-propyl ketone was somewhat unexpected as the spectrum obtained was typical of that for an allyl radical rather than an alkyl radical. It is possible that the allyl radical was formed by abstraction of a hydrogen adjacent to the carboxyl group in a parent molecule followed by an intramolecular rearrangement. Thus... 

A special case of allylic oxidation with rearrangement occurs in the action of chromium(VI) agents on 3,4-unsaturated ketones (9), and this is shown in Scheme 6. Hydrogen abstraction by the oxidant takes... 

The Oxa-Di-n-methane reaction and Related Processes. Interestingly, some P,y-unsaturated ketones do not undergo 1,3-acyl shifts or the oxa-di-Ti-methane rearrangement. An example of this is compound (61), which on irradiation undergoes only a Norrish Type II hydrogen abstraction. ... 

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. ... 

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