Bromine radical-mediated cyclopropylcarbinyl-homoallyl rearrangement of alkylidenecyclopropanes was effectively accomplished by C-C bond formation with allylic bromides, which led to the syntheses of 2-bromo-1,6-dienes. A three-component coupling reaction comprising alkylidenecyclopropanes, allylic bromides, and carbon monoxide also proceeded well to give 2-bromo-1,7-dien-5-ones in good yield.
The C(sp3)–H allylation of alkanes is investigated by using allyl bromides under radical reaction conditions. In many cases, methine C–H allylation preceded methylene and methyl C–H allylation with complete or a high degree of site selectivity. The C–H allylation of allylic compounds, such as allylbenzene, gives 1,5-dienes with the SH2′ reactions of the allyl radicals occurring at the less hindered carbon.
A radical-chain addition of allyl bromides to aryl alkenes, vinyl ester, and vinyl phthalimide was studied in which elusive β-bromoalkyl radicals were trapped efficiently to give 5-bromo-1-pentenes in good to high yields (16 examples). A subsequent carbonylative radical cyclization with AIBN/BuSnH/CO was successful in giving the corresponding 3,5-disubstituted cyclohexanone derivatives in moderate yields. Synthesis of a piperidine ring was also successful by subsequent reaction with primary amine.
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