Felkin cyclization, also called the Felkinene magnesium cyclization, is a nickel-catalyzed organomagnesium sequence that converts butadiene and an alkyl Grignard reagent into substituted vinylcyclopentylmethylmagnesium species. In the defining reaction, propylmagnesium bromide and two equivalents of butadiene combine in the presence of a nickel complex to generate an octa-2,7-dienylmagnesium intermediate, which cyclizes intramolecularly to cis-(2-vinylcyclopentyl)methylmagnesium bromide ^[1]. Thermal equilibration can furnish the corresponding trans organomagnesium isomer, and protonation or electrophilic trapping converts the carbon–magnesium bond into a neutral substituted vinylcyclopentane product ^[2]. The concept therefore concerns a stereoselective five-membered-ring-forming diene/Grignard process. It is not the Felkin–Anh model for facial selectivity in nucleophilic carbonyl addition, despite the shared eponym, and it is not a general cyclopropanation of alkenes. Nickel-mediated assembly and cyclization of the unsaturated organomagnesium intermediate are its essential mechanistic features.