Ru(ii)-catalyzed regioselective [4 + 1] redox-neutral spirocyclization of aryl amidines with diazopyrazolones: direct access to spiro[indole-3,4′-pyrazol]-5′-ones

Abstract

An efficient protocol to construct spiro[indole-3,4′-pyrazol]-5′-ones via ruthenium(II)-catalyzed selective C–H bond activation/[4 + 1] spirocyclization starting from easily available N-aryl amidines and diazopyrazolones has been developed. A series of spiropyrazolones could be easily obtained under mild reaction conditions with high step and atom economy. Mechanistically, the formation of spirocycles involves Ru(II)-catalyzed C(sp²)–H bond cleavage, metal carbene formation, migratory insertion, intramolecular nucleophilic addition, and elimination. This protocol features obviating the use of external oxidants, excellent regioselectivity and broad generality of substrates.