Palladium-Catalyzed Coupling of Allenylphosphine Oxides with <i>N</i>-Tosylhydrazones toward Phosphinyl [3]Dendralenes

Abstract

A palladium-catalyzed coupling of allenylphosphine oxides with N-tosylhydrazones, leading to phosphinyl [3]dendralenes, is established. The coupling reaction can be catalyzed by bis(triphenylphosphine)palladium chloride with sodium pivalate as a key additive, presumably via π-allyl-Pd-carbene intermediates. This protocol provides an expedient synthesis of unprecedented multisubstituted phosphinyl [3]dendralenes with a broad substrate diversity and dominant Z-stereoselectivity, depending on the substitution positions. X-ray crystallographic analyses confirm the relative stereochemistry of products and reveal multiple double bonds in a phosphinyl [3]dendralenes array with a "fanlike" dimensional orientation. Further applications of phosphinyl [3]dendralenes to intramolecular cyclization and selective oxidation demonstrate the differentiated reactivities of double bonds.