Alkene Cyclopropanation with <i>gem</i>-Dichloroalkanes Catalyzed by (PNN)Co Complexes: Scope and Mechanism

Highly regioselective and diastereoselective cis-arylcyclopropanation (ACP) and trans-vinylcyclopropanation (VCP) of 1-alkenes with gem-dichloroalkanes are described. By employing stable and readily available donor carbene precursors and zinc powder as the additive, the cobalt dichloride complex (tBuPNN)CoCl2 of a phosphinobipyridine (PNN) pincer with tBu substituents on the phosphorus atom catalyzes mild and efficient cyclopropanation with high functional-group compatibility and wide substrate scope, furnishing high diastereoselectivity in the case of trans-VCP that remains underdeveloped. The synthetic potential was demonstrated by late-stage modification of bioactive molecules. Stoichiometric experiments and cyclic voltammetry experiments provide insights into the reaction mechanism and identify key factors to achieve this reaction; of particular importance is relatively slow reduction of gem-dichloroalkane by a formal CoI species (tBuPNN)CoCl to keep the free alkyl radical at low concentration, as elucidated by the relationship between catalytic activity and reduction potentials of four (tBuPNN)Co complexes with various electronic properties.