Directed Electrophilic Cyclizations:  Efficient Methodology for the Synthesis of Fused Polycyclic Aromatics

A versatile method for the synthesis of complex, fused polycyclic aromatic systems in high chemical yield is described. Construction is achieved using a general two-step synthetic sequence. Pd-catalyzed Suzuki and Negishi type cross-coupling chemistries allow for the preparation of nonfused skeletal ring systems in yields consistently >80%. The critical ring-forming step, which generally proceeds in very high to quantitative yield, utilizes 4-alkoxyphenylethynyl groups and is induced by strong electrophiles such as trifluoroacetic acid and iodonium tetrafluoroborate. The reaction in essence produces phenanthrene moieties which are integrated into extended polycyclic aromatic structures. Fused polycyclic benzenoids as well as benzenoid/thiophene systems may be prepared utilizing this methodology. The scope of the described cross-coupling/cyclization chemistry including mechanistic insights and problematic side reactions are described.