Cascade Reaction of <i>In Situ</i> Generated <i>N</i> -Heteroiminophosphoranes with α,β-Unsaturated Compounds: A Direct Entry to Diverse Pyrazoles Bearing Oxindole Units

An efficient method for the <i>in situ</i> generation of <i>N</i>-heteroiminophosphoranes through the electrophilic addition of the terminal <i>N</i>-atom of oxindole diazocompounds to triphenylphosphine has been developed. The subsequent Wittig/6π 1,5-electrocyclization/1,3-H shift cascade reaction of <i>N</i>-heteroiminophosphoranes with α,β-unsaturated aldehydes, ketoesters, and ketones proceeded smoothly in one pot. A series of functionalized pyrazoles bearing an oxindole unit and atropisomeric biarylpyrazoles have been afforded by this tool with good yields and high functional group tolerance under mild conditions.