Metal‐Catalyzed Cascade Reactions: Useful Synthetic Tools for the Preparation of Polycyclic Arenes

This account summarizes our recent efforts to synthesize numerous important and interesting polycyclic arenes under mild conditions using metal‐catalyzed protocols. The palladium‐catalyzed annulations of 2‐iodobiphenyls or 2,2′‐diiodobiphenyls with alkynes efficiently generated phenanthrene derivatives. This synthetic method was utilized as the key step when preparing phenanthrene‐based alkaloids, tetrabenzopyracylenes and persubstituted [8]circulenes. Depending on whether a palladium or nickel catalytic system was used, 1‐ethynyl‐8‐iodonaphthalenes underwent either a cyclodimerization or a nitrile‐incorporated cascade reaction to produce zethrenes or pyrroloarenes, respectively. Methylene‐bridged polyarenes are generated easily from 2‐halo‐2′‐methylbiaryls through benzylic C – H bond activation and subsequent carbon–carbon bond formation, and palladium complexes promote the arylation of methylene carbons. The palladium‐catalyzed annulations of 1,8‐bis(arylethynyl)naphthalene derivatives with o ‐diiodoarenes yielded benzo[ k ]fluoranthene‐based linear acenes, which can be applied to synthesize highly curved fragments of fullerenes. The self‐reactions of diarylethynes formed either dihydrocyclopenta[ a ]indenes or octaaryl‐1,3,5,7‐octatetraenes through palladium‐catalyzed cycloisomerization or nickel‐catalyzed tetramerization, respectively. In the presence of palladium catalysts, the hydroalkynylation of terminal arylalkynes directly generated angular trimerization adduct dienynes.