Iridium-Catalyzed Formal [4+1] Cycloaddition of Biphenylenes with Alkenes Initiated by C-C Bond Cleavage for the Synthesis of 9,9-Disubstituted Fluorenes

An Ir-catalyzed intermolecular reaction of biphenylenes as a C4 unit with various alkenes as a Cl unit gave 9,9-disubstituted fluorenes in moderate to high yields. Preliminary mechanistic studies revealed that this formal [4 + I] cycloaddition probably proceeds via C-C bond cleavage, alkene insertion, beta-hydrogen elimination, intramolecular alkene insertion, and then reductive elimination. An example of enantioselective reaction was also disclosed.