Alkene Cyclopropanation with <i>gem</i>-Dichloroalkanes Catalyzed by (PNN)Co Complexes: Scope and Mechanism
Bin Cao, Guixia Liu, Zheng Huang
Abstract
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 ( t Bu PNN)CoCl 2 of a phosphinobipyridine (PNN) pincer with t Bu 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 Co I species ( t Bu PNN)CoCl to keep the free alkyl radical at low concentration, as elucidated by the relationship between catalytic activity and reduction potentials of four ( t Bu PNN)Co complexes with various electronic properties.