Phenanthrene Synthesis by Iron-Catalyzed [4+2] Benzannulation between Alkyne and Biaryl or 2-Alkenylphenyl Grignard Reagent

The [4 + 2] benzannulation reaction of internal or terminal alkynes with 2-biaryl, 2-heteroarylphenyl, or 2-alkenylphenyl Grignard reagents in the presence of Fe(acac)3, 4,4'-di-tert-butyl-2,2'-bipyridyl, and 1 ,2-dichloro-2-methylpropane takes place at room temperature in I h to give 9-substituted or 9,10-disubstituted phenanthrenes and congeners in moderate to excellent yields. The reaction tolerates sensitive functional groups such as bromide and olefin. When applied to a 1,3-diyne, the annulation reaction takes place on both acetylenic moieties to give a bisphenanthrene derivative.