Mechanism of the suzuki-miyaura cross-coupling reaction mediated by [Pd(NHC)(allyl)Cl] Precatalysts

Density functional theory calculations have been used to investigate the activation mechanism for the precatalyst series [Pd]-X-1-4 derived from [Pd(IPr)(R-allyl)X] species by substitutions at the terminal position of the allyl moiety ([Pd] = Pd(IPr); R = H (1), Me (2), gem-Me2 (3), Ph (4), X = Cl, Br). Next, we have investigated the Suzuki-Miyaura cross-coupling reaction for the active catalyst species IPr-Pd(0) using 4-chlorotoluene and phenylboronic acid as substrates and isopropyl alcohol as a solvent. Our theoretical findings predict an upper barrier trend, corresponding to the activation mechanism for the [Pd]-Cl-1-4 series, in good agreement with the experiments. They indeed provide a quantitative explanation of the low yield (12%) displayed by [Pd]-Cl-1 species (G 30.0 kcal/mol) and of the high yields (90%) observed in the case of [Pd]-Cl-2-4 complexes (G 20.0 kcal/mol). Additionally, the studied Suzuki-Miyaura reaction involving the IPr-Pd(0) species is calculated to be thermodynamically favorable and kinetically facile. Similar investigations for the [Pd]-Br-1-4 series, derived from [Pd(IPr)(R-allyl)Br], indicate that the oxidative addition step for IPr-Pd(0)-mediated catalysis with 4-bromotoluene is kinetically more favored than that with 4-chlorotoluene. Finally, we have explored the potential of Ni-based complexes [Ni((IPr)(R-allyl)X] (X = Cl, Br) as Suzuki-Miyaura reaction catalysts. Apart from a less endergonic reaction energy profile for both precatalyst activation and catalytic cycle, a steep increase in the predicted upper energy barriers (by 2.0-15.0 kcal/mol) is calculated in the activation mechanism for the [Ni]-X-1-4 series compared to the [Pd]-X-1-4 series. Overall, these results suggest that Ni-based precatalysts are expected to be less active than the Pd-based precatalysts for the studied Suzuki-Miyaura reaction.