• palladium
• mechanistic study
• imidazole
• direct arylation
• C-H functionalization
• catalysis
• CMD

We have studied some mechanistic aspects of the palladium-catalyzed direct arylation of imidazole derivatives at the C5 position with aryl halides. Differently from previously reported studies on similar transformations, our focus was not only on the C-H functionalization step, but on the whole catalytic cycle.
We have shown that PPh3-ligated organopalladium species are unlikely intermediates of the catalytic cycle and the interactions within 1,2-dimethylimidazole (dmim) and PPh3-ligated organopalladium complexes have been studied in detail. PPh3, when used as a supporting ligand in conjunction with Pd(OAc)2, is oxidized to PPh3O under the reaction conditions, thus the reaction proceeds through imidazole-ligated organopalladium intermediates. The kinetics of the oxidative addition of aryl halides to dmim-ligated Pd(0) species has been characterized.

Deuterium kinetic isotope effect (KIE) demonstrated that C-H bond cleavage is involved in the turnover limiting step of the catalytic cycle. Complexes of formula ArPd(dmim)3I have been isolated and characterized for the first time. They behave as a mixture of trans-[ArPd(dmim)2I] and dmim in apolar solvent, while in polar solvents they partially dissociate to give the [ArPd(dmim)3]+. The latter cationic species is active towards the direct arylation of dmim at room temperature in the presence of AcO- as a base. Preliminary kinetic data point out that C-H bond cleavage occurs by a concertated metalation-deprotonation mechanism with AcO- acting as an outer-sphere base. Reactivity studies on 1-aryl-2-methylimidazole showed that the reaction under study is faster for electron-poor imidazole substrates.

We propose a revised catalytic cycle, consistent with all the experimental data. We are confident that our results will be useful for the development of enhanced catalytic systems for C-H activation.