Transition-Metal-Free Cyclopropanation of Alkenes with Dichloromethane and Chloroform via Halogen Atom Transfer Using Amine Carboxyborane

Cyclopropanes are privileged motifs in medicinal chemistry due to their role as bioisosteres of arenes, alkenes, and small alkyl groups. Herein, we report a transition-metal-free, photoredox-catalyzed cyclopropanation of diverse alkenes by halogen atom transfer between dichloromethane (CH₂Cl₂) as a C1 synthon and amine-ligated boryl radicals generated from amine carboxyborane. This method proceeds under mild conditions, exhibits broad substrate scope, and is scalable. The synthetic utility is further highlighted by deuterium incorporation using CD₂Cl₂ and the formation of chlorocyclopropane products using chloroform (CHCl₃) and CDCl₃, enabling access to valuable chlorinated and isotopically labeled cyclopropanes.