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 via halogen atom transfer between dichloromethane (CH2Cl2) as a C1 synthon and amine-ligated boryl radicals generated from amine carboxyborane. This method proceeds under mild reaction conditions, exhibits a 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.