A borocyclopropanation of (E)- and (Z)-allylic ethers and styrene derivatives via the Simmons-Smith reaction using a novel boromethylzinc carbenoid is described. The carbenoid precursor is prepared via a 3-step sequence from inexpensive and commercially available starting materials. This methodology allows for the preparation of 1,2,3-substituted borocyclopropanes in high yields and diastereoselectivities. Several postfunctionalization reactions were also performed to illustrate the versatility of these building blocks.