Vitamin B 12 ‐Photocatalyzed Cyclopropanation of Electron‐Deficient Alkenes Using Dichloromethane as the Methylene Source**

Abstract The cyclopropyl group is of great importance in medicinal chemistry, as it can be leveraged to influence a range of pharmaceutical properties in drug molecules. This report describes a Vitamin B 12 ‐photocatalyzed approach for the cyclopropanation of electron‐deficient alkenes using dichloromethane (CH 2 Cl 2 ) as the methylene source. The reaction proceeds in good to excellent yields under mild conditions, has excellent functional group compatibility, and is highly chemoselective. The scope could also be extended to the preparation of D 2 ‐cyclopropyl and methyl‐substituted cyclopropyl adducts starting from CD 2 Cl 2 and 1,1‐dichloroethane, respectively.