Metal-Mediated CH Bond Activation in a Carbon-Substituted Hemiporphyrazine

摘要

Unlike many CH bond activations,[1] nucleophilic substitution reactions on aromatic rings with hydride as the leaving group can be difficult to achieve. Nucleophilic substitution of arenes by halide loss has been known for many decades. However, CH aromatic substitutions have been known only since the late 1970s and proceed only under certain reaction conditions.[2] Electron-withdrawing substituents are required on the aromatic ring, and frequently only strong nucleophiles such as NH2- can readily attack. Alternative methods can be used to assist replacement of an aromatic hydride. Vicarious nucleophilic aromatic substitution can be used in the activation of arene CH bonds, which is promoted by loss of HX.[3] In addition, oxidants can also be employed to effect loss of hydride in a process known as oxidative nucleophilic substitution of hydrogen (ONSH).[4] Herein, we present an unusual nucleophilic aromatic substitution with the relatively weak nucleophile pyridine in the dicarbahemiporphyrazine macrocycle, which may occur through an ONSH mechanism. Upon metalation with copper, nucleophilic attack of the internal carbon atom with pyridine is promoted by the reduction of the metal from CuII to CuI. This reaction proceeds under very mild conditions in air and represents a potential new means to activate CH bonds with transition metals.