Ring Contraction of Metallacyclobutadiene to Metallacyclopropene Driven by π- and σ-Aromaticity Relay

Abstract π-Aromaticity is an important driving force in directing the synthesis of aromatic compounds; in contrast, reactions induced by σ-aromaticity are uncommon. Herein, we report a strategy based on π- and σ-aromaticity relays to realize the first structurally defined ring contraction of metallacyclobutadiene to metallacyclopropene. This reaction involves the release of the π-antiaromaticity of metallacyclobutadiene in 2a to afford a π-aromatic intermediate (4A), followed by ring reclosure to generate σ-aromatic metallacyclopropene in 3a. The ring-opening-reclosing mechanism and versatile switching of the aromaticity of the metallacyclic species are supported by experimental results and theoretical calculations. This work demonstrates the importance of the synergistic effects of π- and σ-aromaticity as driving forces in reactions and sheds light on possible applications in other challenging transformations.