Rhodium-catalyzed (5+1) annulations between 2-alkenylphenols and allenes: A practical entry to 2,2-disubstituted 2H-chromenes

Readily available alkenylphenols react with allenes under rhodium catalysis to provide valuable 2,2-disubstituted 2H-chromenes. The whole process, which involves the cleavage of one C-H bond of the alkenyl moiety and the participation of the allene as a one-carbon cycloaddition partner, can be considered a simple, versatile, and atom-economical (5+1) heteroannulation. The reaction tolerates a broad range of substituents both in the alkenylphenol and in the allene, and most probably proceeds through a mechanism involving a rhodium-catalyzed C-C coupling followed by two sequential pericyclic processes.