Palladium-catalyzed trimerizations of terminal arylalkynes: Synthesis of 1,3-diaryl-2-arylethynyl-1,3-butadienes

Various 1,3-diaryl-2-arylethynyl-1,3-butadienes 7 have been prepared by the Pd-catalyzed trimerization of arylalkynes 4. Their structures and stereochemistry have been confirmed by X-ray crystal analyses. This procedure provides high regioselectivity to generate adducts Z-7 in moderate to excellent yields. The scope, limitations and regioselectivity of this reaction have been investigated. In DMSO at 180 °C, 1,3-di(1-naphthalenyl)-2-(1- naphthalenylethynyl)-1,3-butadiene (7m) underwent 6π-electrocyclization to form 2-(1-naphthalenyl)-3-(1-naphthalenyl-ethynyl)-1,2-dihydrophenanthrene (21). Under acidic conditions, (1Z)-1,3-diphenyl-2-(1-phenylethynyl)-1,3-butadiene (Z-7a) was converted to 1-methyl-3-phenyl-2-(2-phenyl-1-ethionyl)-2-indene (22) in 75% yield.