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

An efficient method for the in situ generation of N-heteroiminophosphoranes through the electrophilic addition of the terminal N-atom of oxindole diazocompounds to triphenylphosphine has been developed. The subsequent Wittig/6π 1,5-electrocyclization/1,3-H shift cascade reaction of N-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.