Mechanistic study of copper-catalyzed intramolecular ortho-C-H activation/carbon-nitrogen and carbon-oxygen cyclizations

Intramolecular ortho-C-H activation and C-N/C-O cyclizations of phenyl amidines and amides have recently been achieved under Cu catalysis. These reactions provide important examples of Cu-catalyzed functionalization of inert C-H bonds, but their mechanisms remain poorly understood. In the present study the several possible mechanisms including electrophilic aromatic substitution, concerted metalation-deprotonation (CMD), Friedel-Crafts mechanism, radical mechanism, and proton-coupled electron transfer have been theoretically examined. Cu(II)-assisted CMD mechanism is found to be the most feasible for both C-O and C-N cyclizations. This mechanism includes three steps, i.e. CMD with Cu(II), oxidation of the Cu(II) intermediate, and reductive elimination from Cu(III). Our calculations show that Cu(II) mediates the C-H activation through an six-membered ring CMD transition state similar to that proposed for many Pd-catalyzed C-H activation reactions. It is also interesting to find that the rate-limiting steps are different for C-N and C-O cyclizations: for the former it is concerted metalation-deprotonation with Cu(II), whereas for the latter it is reductive elimination from Cu(III). The above conclusions are consistent with the experimental kinetic isotope effects (1.0 and 2.1 for C-O and C-N cyclizations, respectively), substituent effects, and the reactions under O2-free conditions.