Homogeneous oxidative coupling catalysts. Mechanism of catalysts formation by oxidation of [(Pip)nCuX]4 (n = 1 or 2, Pip = piperidine, X = Cl, Br or I) by dioxygen in aprotic media

This paper reports the mechanism of formation of oxidative coupling catalysts [(Pip)nCuX]4O2, n = 1 or 2 and X = Cl, Br or I, which represent half of the catalytical cycle, Scheme 1. The mechanism has been described as a pre-equilibrium between [(Pip)nCuX]4 and O2. K values are very sensitive to how strong the hydrogen-bonding between copper (I) tetranuclear and incoming dioxygen is, such association is also sensitive to the variation of X. The pronounced pre-equilibrium is the reason behind the oxidation of [(Pip)nCuI]4, which is not the case for pyridine type of ligands. The pre-equilibrium followed by rate determining step k2, which is responsible to the formation of the oxidative coupling catalysts [(Pip)nCuX]4O2. The overall reaction is a second-order process, first order in each [[(Pip)nCuX]4] and [O2], with rate constant kon (kon = Kk2) and exothermic ΔH≠ varying from −3 to −12 kcal mol−1 and ΔS≠ varying from −87 to −65 cal deg−1 mol−1. kon were found to be very sensitive to n value 1 or 2 and to the type of X (Cl, Br or I).