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

A palladium-catalyzed coupling of allenylphosphine oxides with <i>N</i>-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 <i>Z</i>-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