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)(n)CuX](4)O-2, 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)(n-)CuX](4) and O-2. K values are very sensitive to how strong the hydrogen-bonding between copper (1) 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)(n)CuI](4), which is not the case for pyridine type of ligands. The pre-equilibrium followed by rate determining step k(2), which is responsible to the formation of the oxidative coupling catalysts [(PiP)(n)CuX](4)O-2. The overall reaction is a second-order process, first order in each [[(PiP)(n)CuX](4)] and [O-2], with rate constant k(on) (k(on) = Kk(2)) and exothermic DeltaH(not equal) varying from -3 to -12 kcal mol(-1) and DeltaS(not equal) varying from -87 to -65 cal deg(-1) mol(-1). k(on) were found to be very sensitive to n value 1 or 2 and to the type of X (Cl, Br or I). (C) 2004 Elsevier B.V. All rights reserved.