Mechanistic study of the oxidative N-dealkylation reactions of bis(μ-oxo)dicopper complexes

The results of experiments designed to probe the mechanism by which the bis(μ-oxo)dicopper complexes [(LCu)₂(μ-O)₂](ClO₄)₂ (L = 1,4,7-tribenzyl-,1,4,7-triisopropyl-, and 1,4-diisopropyl-7-benzyl-1,4,7-triazacyclononane ligands; L(Bn₃), L(iPr₃), and L(iPr₂Bn), respectively) decompose to products arising from macrocyclic ligand N-dealkylation are described. After removal of copper from the decomposed solutions, analysis of the organic products revealed N-dealkylated ligands and aldehyde or ketone, the oxygen atoms in the latter being derived from the bis(μ-oxo)dicopper core as shown by ¹⁸O-isotope labeling experiments. Thus, the overall N-dealkylation is akin to monooxygenase reactions carried out by various metalloenzymes such as cytochrome P450, dopamine β-monooxygenase, and peptidyl glycine α-amidating monooxygenase. Direct, intramolecular attack of the bis(μ-oxo)dicopper core at a ligand substituent C-H bond during the rate-determining step was indicated by the observed first-order kinetics, the results of H/D- and ¹⁶O/¹⁸O-isotope and double labeling experiments, large primary kinetic deuterium isotope effects (KIEs), and Eyring activation parameters. Tunneling was implicated in the C-H bond cleavage step by the temperature dependence of the KIEs. A Hammett study of the decay of suitably functionalized derivatives of L(Bn₃) revealed a ρ value of -0.8, consistent with the diamagnetic bis(μ-oxo)dicopper core behaving as an electrophile during C-H bond scission like the active oxidant in cytochrome P450. Subsequent hydroxyl 'rebound' or a concerted mechanism is then proposed to generate a carbinolamine intermediate that decomposes to secondary amine and ketone or aldehyde final products.