An efficient two-step synthesis of a wide range of 3-methylene-1,2-diazetidines has been developed through application of a Cu(I)-catalyzed 4-exo ring closure. The double bond of this new class of strained heterocycle can be functionalized in a stereocontrolled manner by using palladium-catalyzed Heck reactions. Moreover, chemoselective reduction of 3-alkylidene-1,2-diazetidines gives access to saturated 1,2-diazetidines and vicinal diamines.