Computation Study on Copper-Catalyzed Aerobic Intramolecular Aminooxygenative C═C Bond Cleavage to Imides: Different Roles of Mononuclear and Dinuclear Copper Complexes

Abstract

Cu-catalyzed aerobic reactions are a powerful protocol for the synthesis of value-added chemicals based on the ideal oxidant O2. Despite the long research history, the mechanistic studies clarifying the details of the whole catalytic cycle, where Cu-O2 complexes and their derivatives directly participate in the conversion of substrates, are limited, leaving the mechanisms of emerging aerobic reactions far from understanding. Herein, a computational study on the mechanism of Cu-catalyzed aerobic aminooxygenation of alkene-tethered amides to imides is reported. It is found that the Cu(I) precursor is not the active species but can generate two types of Cu(II) complexes LCu(OAc)OH and LCu(OAc)OOR to start the aminooxygenation through the successive formation of dinuclear Cu(III) oxo complex, dinuclear Cu(II) hydroxide complex, and hetero-dinuclear Cu(II)-Cu(I) complex, followed by alkylperoxo radical capture with Cu(I) species. LCu(OAc)OH catalyzes the aminooxygenation via a mononuclear mechanism, while LCu(OAc)OOR is an active intermediate therein. In the initial catalytic stage, LCu(OAc)OH transforms alkene-tethered amides to α-amidated aldehydes through N–H activation, amide isomerization, cyclization, alkyl radical release, alkyl radical capture by O2, alkylperoxo radical capture by in situ-generated Cu(I) species to LCu(OAc)OOR, acetate-assisted proton-coupled electron transfer (PCET), and concerted PCET/O–O bond cleavage. In the second catalytic stage for the generation of imides from α-amidated aldehydes, the previously proposed aldehyde Cα–H pathway is possible, but it is more likely to generate CO2 and H2 as the byproducts. Instead, a more feasible pathway involving C(O)–H activation to acyl radical, decarbonylation, and radical capture to LCu(OAc)OOR′ was discovered. The C(O)–H activation pathway generates CO and H2O as the byproducts and is consistent with the experimental observations. The concerted PCET/O–O bond cleavage steps generating α-amidated aldehydes and imides have close energy barriers and both can be the rate-determining steps. The presented outcome revised and expanded the knowledge of Cu-catalyzed aerobic conversion of C═C bonds and amide N–H bonds, highlighting the different roles of mononuclear and dinuclear copper complexes in the aerobic reactions and the in situ generation of Cu(II) catalysts, respectively.