Rh(III)- or Ru(II)-Catalyzed C–H Annulation with Vinylene Carbonate and an Unexpected Aerobic Oxidation/Deprotection Cascade to Yield Cinnolin-4(1H)-ones

Abstract

Transition metal-catalyzed C–H annulation reactions have been extensively utilized for the synthesis of cinnolines, especially the N-protected ones; however, none of them can yield cinnolin-4(1H)-ones, a significant class of bioactive skeletons. Herein, we disclose one-pot access to cinnolin-4(1H)-ones through Rh(III)- or Ru(II)-catalyzed C–H activation/annulation of N-aryl cyclic hydrazides with vinylene carbonate, followed by an O₂/K₂CO₃-promoted aerobic oxidation/deprotection cascade. The π-conjugation of the directing groups plays a crucial role in facilitating this transformation. Notably, seven-membered enolic Rh species IMC is characterized via electrospray ionization mass spectroscopy for the first time, which, along with systematic control experiments, provides compelling evidence for the mechanistic pathway encompassing alkenyl insertion, β-oxygen elimination, protonation, and dehydration.