A novel one-step [2 + 1] cycloaddition synthesis of cyclopropanes has been developed. Reaction of (E)-1-(phenylseleno)-2-silylethenes 1a,b with vinyl ketones 2a-d and acrolein (2e) in the presence of SnCl4 gave cyclopropane products by a formal [2 + 1] cycloaddition accompanied by 1,2-silicon migration rather than by [2 + 2] cycloaddition. This facile 1,2-silicon shift is rationalized by a remarkable selenium effect. A generated beta-silicon-stabilized zwitterion A is transformed by a 1,2-silicon shift to the more stable selenium-bridged intermediate C. Ab initio MO calculations for model compounds clearly demonstrate that the intermediate C is more stable than A. The selenium-bridged geometry of C shows that preference is for formation of a cyclopropane ring instead of a cyclobutane ring.