Formal [2+1] Cycloadditions of a Metallacyclopentadiene Unit with Alkynes: Constructing Tetracyclic Conjugated Frameworks with Bridgehead Metals

Comprehensive Summary Metallacyclopentadienes play a vital role in transition‐metal mediated and catalyzed cycloaddition reactions of alkynes. Though versatile reactions of metallacyclopentadienes with alkynes have been disclosed, [2+1] cycloadditions of metallacyclopentadienes and alkynes have never been discovered. In present work, we report the formal [2+1] cycloadditions of a metallacyclopentadiene unit with a broad scope of commercial alkynes, providing a facile strategy to construct tetracyclic conjugated compounds. The deuterated experiment indicates a metal vinylidene intermediate has been involved in [2+1] cycloaddition. Moreover, the electrophilic substitution reaction of the tetracyclic conjugated compound with the aid of density functional theory (DFT) calculated Fukui functions is investigated. These tetracyclic conjugated compounds exhibit broad absorption spectra in the whole visible region which could be employed as potential photovoltaic materials. A facile approach to tetracyclic conjugated metallacycles by the formal [2+1] cycloadditions of a metallacyclopentadiene unit with alkynes was investigated.