Mechanistic and chemoselective insights on sp3- and sp2-C–H bond aminations: Fe- vs. Ir-based catalysis

Abstract

The mechanisms of sp³- and sp²-C–H bond aminations of Fe- and Ir-based catalyses for a styryl substrate have been studied using the BPW91 method, with an emphasis on the origin of sp³-to-sp²-C–H amination chemoselectivity. Our calculations show that Fe-catalyzed sp³-C–H bond amination proceeds as a doublet H-atom abstraction/radical rebound mechanism accompanied with a single-electron transfer from the Fe center to the nitrene-N atom, which is made easier by electron-donating octamethylporphyrin. A more electron-rich porphyrin and a smaller steric hindrance between the catalyst and three adjacent rings of the styryl substrate lead to a remarkable preference for sp³- over sp²-C–H bond aminations for Fe-based catalysis. Conversely, Ir-catalyzed sp²-C–H bond amination undergoes a singlet C–N formation/1′,2′-H shift mechanism in which the electronic properties of the aminated bond become the controlling factor of the chemoselectivity. In comparison to the σ_C–H bond, the π bond of CC breaks more easily to provide two electrons to the electron-deficient nitrene-N atom, and hence Ir-based catalysis exhibits only the sp²-C–H amination product.