Divergent Bimetallic Mechanisms in Copper(II)-Mediated C–C, N–N, and O–O Oxidative Coupling Reactions

Copper-catalyzed aerobic oxidative coupling of diaryl
imines provides
a route for conversion of ammonia to hydrazine. The present study
uses experimental and density functional theory computational methods
to investigate the mechanism of N–N bond formation, and the
data support a mechanism involving bimolecular coupling of Cu-coordinated
iminyl radicals. Computational analysis is extended to CuII-mediated C–C, N–N, and O–O coupling reactions
involved in the formation of cyanogen (NC–CN) from HCN, 1,3-butadiyne
from ethyne (i.e., Glaser coupling), hydrazine from ammonia, and hydrogen
peroxide from water. The results reveal two different mechanistic
pathways. Heteroatom ligands with an uncoordinated lone pair (iminyl,
NH2, OH) undergo charge transfer to CuII, generating
ligand-centered radicals that undergo facile bimolecular radical–radical
coupling. Ligands lacking a lone pair (CN and CCH) form bridged binuclear
diamond-core structures that undergo C–C coupling. This mechanistic
bifurcation is rationalized by analysis of spin densities in key intermediates
and transition states, as well as multiconfigurational calculations.
Radical–radical coupling is especially favorable for N–N
coupling owing to energetically favorable charge transfer in the intermediate
and thermodynamically favorable product formation.