Selective Synthesis of Pyrazolonyl Spirodihydroquinolines or Pyrazolonyl Spiroindolines under Aerobic or Anaerobic Conditions

Abstract

Presented herein is a condition-controlled selective synthesis of pyrazolonyl spirodihydroquinolines or pyrazolonyl spiroindolines through formal [5 + 1] or [4 + 1] spiroannulation of 2-alkenylanilines with diazopyrazolones. Mechanistically, the formation of the title products involves initial generation of a pyrazolonyl spiro-fused seven-membered ruthenacycle species serving as a key intermediate through Ru(II)-catalyzed C-H/N-H bonds metalation, carbene formation, and its migratory insertion. When the reaction is carried out under air, the key intermediate undergoes reductive elimination to afford spirodihydroquinoline. When the reaction is run under argon, the key intermediate undergoes protonation and intramolecular nucleophilic addition to furnish spiroindoline. This work provides an atom-economical protocol for the effective functionalization of alkenyl C(sp2)-H bond, allowing rapid and selective assembly of valuable spiroscaffolds with a broad range of substrates.