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 (ΔG⧧ ≈ 30.0 kcal/mol) and of the high yields (≈90%) observed
in the case of [Pd]-Cl-2–4 complexes (ΔG⧧ ≈ 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.