Synthesis of Cyclobutenes by Highly Selective Transition‐Metal‐Catalyzed Ring Expansion of Cyclopropanes

A highly chemoselective, regioselective, and stereospecific synthesis of polysubstituted cyclobutenes, by catalyst-controlled ring expansion of cyclopropanes via metal carbene intermediates, is reported. Transition-metal catalysts showed profound effects on the reactivity and selectivity of metal carbene intermediates in this ring-expansion reaction.