Hydrogen abstraction, intramolecular allylic

We have demonstrated that intermolecularly, amidyl radicals preferentially abstract an allylic hydrogen rather than add to a TT bond of olefins such as cyclohexene and 1,3-pentadiene (33). This reactivity pattern is completely reversed in intramolecular reactions as shown in the following examples of alkenyl mitro-samide photolysis. In every case, the amidyl radicals generated from photolysis preferentially attack the ir bonds intramolecu-... 

Keywords cyclohexenone, photorearrangement, intramolecular allylic hydrogen abstraction... 

In addition to the well exploited radical additions to enamides, thiovinyl ethers have been shown to be suitable intramolecular acceptors for aryl radicals. By placing a thiophenyl substituent on terminal alkene of 33, the cyclization to 34 occurred exclusively in the 6-exo mode and was not complicated by internal hydrogen abstraction from the allylic positions of the alkene (Scheme 13) [83, 84]. 

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... 

In contrast to intramolecular cyclizations, the intermolecular addition of O-centered radicals to ti systems as initiating step in complex radical cyclization cascades has only recently attracted considerable attention. The reason for the low number of papers published on O radical addition to alkenes and alkynes could originate from the perception that O-centered radicals, such as alkoxyl radicals, RO, or acyloxyl radicals, RC(0)0", may not react with 7t systems through addition at rates that are competitive to other pathways, for example, allylic hydrogen abstraction and p-fragmentation in the case of RO" or decarboxylation in the case of RC(0)0" (Scheme 2.9). 

Combination of allyl radicals that result from hydrogen abstraction at methylene groups a to double bonds may give rise to dimeric compounds. Such radicals may also undergo disproportionation or inter- and intramolecular addition to C-C double bonds. Dimerization of unsaturated fatty acids can also occur via Diels-Alder reactions. In the case... 

Prolonged irradiation of the endo-Diels-Alder adduct formed between duroquinone and trnns,trans-hexa-2,4-diene yields the cage dione (930). Similar reaction of the exo-Diels-Alder adduct of quinone and diene, in which intramolecular allylic hydrogen abstraction by carbonyl oxygen or enone carbon is not impeded, results in the forma-... 

Scheme 7). The results outlined in Scheme 8 also clearly indicate that intramolecular hydrogen abstraction by the alkoxyl radical via a six-membered transition state predominates over fragmentation and intermolecular allylic hydrogen abstraction. For more details concerning this subject, readers should refer to the original literature and also to a review article."... 

Substitution in the y-position, led to poorly reactive allylstannanes, due to the decreasing rate of the radical additions to the double bond. It has been established that, generally, the competitive allylic hydrogen abstraction became predominant, leading to diene side-products [300]. There are very few successful examples using y-substituted allyltins in an intermolecular fashion [301], however, this limitation can be overcome for the intramolecular cyclization processes [302]. Recent advances showed that a-carbonyl radicals added efficiently to crotylstannanes at... 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

For Rh(I)/BINAP-catalyzed isomerizations of allylic amines, the mechanistic scheme outlined in Eq. (2) has been proposed. The available data are consistent with the notion that Rh(I)/PF-P(o-Tol)2-catalyzed isomerizations of allylic alcohols follow a related pathway [11]. For example, the only deuterium-containing product of the reaction depicted in Eq. (9) is the l,3-dideuterated aldehyde, which estabhshes that the isomerization involves a clean intramolecular 1,3-migration. The data illustrated in Eqs. (10) and (11) reveal that the catalyst selectively abstracts one of the enantiotopic hydrogens/ deuteriums alpha to the hydroxyl group. 

---