ABSTRACT
Herein, we describe the regioselective functionalization of unsymmetrical ketones using imine directing groups, Cu, and H2O2. The C-H hydroxylation of the substrate-ligands derived from 2-substituted benzophenones occurred exclusively at the γ-position of the unsubstituted ring due to the formation of only one imine stereoisomer. Conversely, the imines derived from 4-substituted benzophenones produced E/Z mixtures that upon reacting with Cu and H2O2 led to two γ-C-H hydroxylation products. Contrary to our initial hypothesis, the ratio of the hydroxylation products did not depend on the ratio of the E/Z isomers but on the electrophilicity of the reactive [LCuOOH]1+. A detailed mechanistic analysis suggests a fast isomerization of the imine substrate-ligand binding the CuOOH core before the rate-determining electrophilic aromatic hydroxylation. Varying the benzophenone substituents and/or introducing electron-donating and electron-withdrawing groups on the 4-position of pyridine of the directing group allowed for fine-tuning of the electrophilicity of the mononuclear [LCuOOH]1+ to reach remarkable regioselectivities (up to 91:9 favoring the hydroxylation of the electron-rich arene ring). Lastly, we performed the C-H hydroxylation of alkyl aryl ketones, and like in the unsymmetrical benzophenones, the regioselectivity of the transformations (sp3 vs sp2) could be controlled by varying the electronics of the substrate and/or the directing group.
