Abstract
A copper catalyst generates various reactive carbon species, which are useful reagents for organic synthesis. Among them, radical species are one of the most powerful tools to synthesize complex molecules. The reaction of α-bromocarbonyl compounds (Br-CRR’COR”) and Cu(I) salts possessing a multi-dentate nitrogen ligand generates alkyl radicals (·CRR’COR”). Although this reaction system has been well studied in radical additions and polymerizations, the development of diverse reaction modes by using the combination of olefins and α-bromocarbonyl compounds in the presence of a copper catalyst and excess base has not yet been explored. During the course of our study, we found that the resulting radical species show unique reactivities for olefins: 1) Mizoroki-Heck like olefinations of styrenes and α-bromocarbonyl compounds to give tertiary-alkyl substituted styrenes under Cu-PMDETA catalyst system, 2) distal-selective olefinations of α-alkylstyrenes and α-bromocarbonyl compounds under Cu-TPMA catalyst system, 3) formal [2+1] cycloadditions of styrenes and 2-bromomalonates to give donor-acceptor cyclopropanes, and 4) formal [3+2] cycloaddition of styrenes and 2-bromoketoesters to give dihydrofurans, which is very effective to synthesize all different aryl-substituted furans. Those reactions can be accomplished by the combination of a radical reaction and a polar reaction including an elimination with a base, and a nucleophilic reaction of an enolate.
