Alkene Cyclopropanation\nwith <i>gem</i>-Dichloroalkanes\nCatalyzed by (PNN)Co Complexes: Scope and Mechanism

Highly regioselective and diastereoselective <i>cis</i>-arylcyclopropanation (ACP) and <i>trans</i>-vinylcyclopropanation\n(VCP) of 1-alkenes with <i>gem</i>-dichloroalkanes are described.\nBy employing stable and readily available donor carbene precursors\nand zinc powder as the additive, the cobalt dichloride complex (<sup><i>t</i>Bu</sup>PNN)CoCl<sub>2</sub> of a phosphinobipyridine\n(PNN) pincer with <sup><i>t</i></sup>Bu substituents on\nthe phosphorus atom catalyzes mild and efficient cyclopropanation\nwith high functional-group compatibility and wide substrate scope,\nfurnishing high diastereoselectivity in the case of <i>trans</i>-VCP that remains underdeveloped. The synthetic potential was demonstrated\nby late-stage modification of bioactive molecules. Stoichiometric\nexperiments and cyclic voltammetry experiments provide insights into\nthe reaction mechanism and identify key factors to achieve this reaction;\nof particular importance is relatively slow reduction of <i>gem</i>-dichloroalkane by a formal Co<sup>I</sup> species (<sup><i>t</i>Bu</sup>PNN)CoCl to keep the free alkyl radical at low\nconcentration, as elucidated by the relationship between catalytic\nactivity and reduction potentials of four (<sup><i>t</i>Bu</sup>PNN)Co complexes with various electronic properties.