Dissociative Electron Transfer to Alkyl Halides of Relevance to Atom Transfer Radical Polymerization. Mechanism of Activation by Copper(I) Complexes

Atom transfer radical polymerization (ATRP) is one of the most studied techniques of controlled living radical polymerization. It can be applied to a wide range of monomers for the preparation of many polymers with controlled molecular weights and well-defined architectures. ATRP is catalyzed by transition metal complexes, especially Cu(I) complexes with nitrogen ligands (L), and its success mainly relies on the establishment of an equilibrium in which a dormant alkyl halide (RX) reversibly reacts with an activator (CuIL) to produce the propagating radical (R•) and a deactivator (XCuIIL). However, the precise mechanism of this activation reaction is still under investigation. Three possible mechanisms, namely halogen atom transfer, outer-sphere electron transfer and concerted dissociative electron transfer (DET), are currently considered. The aim of this work was to discriminate between the possible activation mechanisms, particularly concerted DET and halogen atom transfer. To this end, homogeneous reduction of haloacetonitriles, methyl 2-halopropionates and ethyl haloacetates by electrogenerated aromatic radical anions (Ar•-) or Cu(I) complexes was investigated in CH3CN at 25 °C. Kinetic analysis of the reaction between RX and Ar•- has shown that these compounds undergo a concerted DET with the formation of weakly interacting fragments. A comparison of the kinetics of the activation of RX by Cu(I) with those obtained for Ar•- has shown that Cu(I) complexes are highly more reactive than expected according to a dissociative electron transfer mechanism. It is therefore concluded that the copper amine complexes used in ATRP react with the dormant halide species by a halogen atom transfer mechanism.