Alkenes, homoallylic radicals

Radical iodine atom transfer [3 + 2]-cycloaddition with alkene (118) using dimethyl 2-(iodomethyl)cyclopropane-l,l-dicarboxylate (117) forms cyclopentane derivative (119), through the formation of an electron-deficient homoallyl radical, followed by the addition to alkene, and cyclization via 5-exo-trig manner as shown in eq. 4.41. 

The synthesis of carbocyclic compounds from acyclic precursors can be accomplished by a sequence of radical reactions, where thiyl radicals act as catalysts. As an example, the generation of homoallyl radicals through the addition of phenyl-thiyl radical to the double bonds of vinyl cyclopropanes results in the multi-step synthesis of cyclopentanoids [62]. The mechanism is shown in Scheme 13. Feldman and coworkers have demonstrated that the success of this strategy is based on the coupling of vinyl cyclopropanes with the electronically complementary functionalized alkenes. A judicious choice of substituents R and X accelerates the rate limiting step (c). 

Feldman s group was one of the most active in this field, and published several applications of the synthesis of cyclopentanoid compounds via [3 + 2] annulation [60]. Thus, the PhS radical-catalyzed reaction of substituted vinylcyclopropanes with functionalized alkenes affords vinylcyclopentane derivatives (equation (24)). The reaction mechanism is shown in Scheme 9 [60, 61]. Initiation occurs by PhS radical addition to the vinylcyclopropane (step a), followed by three additional steps prior to termination via ejection of the PhS radical to afford the vinylcyclopentane product (step e). The intermediate steps include ring opening to afford the homoallylic radical (step b), bimolecular addition of the alkene to produce the 5-hexenyl radical (step c), and cyclization to cyclopentanyl carbinyl radical (step d). 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

Lewis acids, such as SnCU, also catalyze the reaction, in which case the species that adds to the alkenes is H2C O SnCl4. " Montmorillonite KIO clay containing zinc (IV) has been used to promote the reaction. " The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form homoallylic alcohols. A typical example is the reaction of methylenecyclohexane with an aryl aldehyde to give 89. 

Various types of [3 + 0] cyclizations via y-functionalized metal alkyls (either as stable compounds or reactive intermediates) are known. The y-carbon can act as an acceptor, a donor or radical center. Homoallyllic systems 1 can cyclize via alkene insertion to form (metallamethyl)cyclo-propanes 2. 

The mode of cyclization of a-silyl radicals generated from homoallylic (bromomethyl)dimethylsilyl ethers is highly dependent upon the substitution pattern on the distal alkenic carbon. Thus while those with unsubstituted distal alkene carbons provide seven-membered siloxanes (J-endo) (eq 1) those having substituted carbons undergo a 6-exo-mode of radical cyclization in a highly regio- and stereoselective manner (eq 8). These six-membered siloxanes provide, upon Tamao oxidation, branched chain 1,4-diols (eq 8). ... 

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