Copper(ii) chloride mediated (aza)oxindole synthesis by oxidative coupling of Csp2–H and Csp3–H centers: substrate scope and DFT study

Abstract

A CuCl₂ mediated direct intramolecular oxidative coupling of C_sp²–H and C_sp³–H centers gives access to 3,3-disubstituted oxindoles containing aromatic, heteroaromatic and alkyl substituents as well as a heteroatom at the quaternary center in good to excellent yields. The reaction is carried out in the presence of NaOtBu and CuCl₂ in DMF at 110 °C. The key step of this reaction is the formation of an amidyl radical by one electron oxidation of amide enolate followed by an intramolecular radical cyclization reaction (homolytic aromatic substitution reaction). A detailed DFT study shows that the cyclization of the amidyl radical is the rate-limiting step in the oxindole synthesis, whereas the second single electron transfer (SET) becomes the rate-determining step in the aza-oxindole formation. Computational data are in agreement with the experimentally observed relative reactivity and regioselectivity.