Exploring the Oxidative‐Addition Pathways of Phenyl Chloride in the Presence of Pd^IIAbnormal N‐Heterocyclic Carbene Complexes: A DFT Study

Abstract

DFT calculations were performed to elucidate the oxidative addition mechanism of the dimeric palladium(II) abnormal N-heterocyclic carbene complex 2 in the presence of phenyl chloride and NaOMe base under the framework of a Suzuki-Miyaura cross-coupling reaction. Pre-catalyst 2 undergoes facile, NaOMe-assisted dissociation, which led to monomeric palladium(II) species 5, 6, and 7, each of them independently capable of initiating oxidative addition reactions with PhCl. Thereafter, three different mechanistic routes, path a, path b, and path c, which originate from the catalytic species 5, 7, and 6, were calculated at M06-L-D3(SMD)/LANL2TZ(f)(Pd)/6-311++G**//M06-L/LANL2DZ(Pd)/6-31+G* level of theory. All studied routes suggested the rather uncommon Pd /Pd oxidative addition mechanism to be favourable under the ambient reaction conditions. Although the Pd /Pd routes are generally facile, the final reductive elimination step from the catalytic complexes were energetically formidable. The Pd /Pd activation barriers were calculated to be 11.3, 9.0, 26.7 kcal mol (ΔΔ G ) more favourable than the Pd /Pd reductive elimination routes for path a, path b, and path c, respectively. Out of all the studied pathways, path a was the most feasible as it comprised of a Pd /Pd activation barrier of 24.5 kcal mol (ΔG ). To further elucidate the origin of transition-state barriers, EDA calculations were performed for some key saddle points populating the energy profiles.

Mechanistic Exploration of the Transmetalation and Reductive Elimination Events Involving PdIV–Abnormal NHC Complexes in Suzuki–Miyaura Coupling Reactions: A DFT Study

“…To assess the role of the base and the impact of countercations on the transmetalation and reductive elimination events, we considered only the favorable pathway in the presence/absence of different countercations, as depicted in Scheme . Maintaining some similarity with our previous studies, pathways A, B, and C involve the Pd IV intermediates I A , I B , and I C , respectively, deriving from the oxidative addition reactions (Scheme ) . We previously reported that the dimethoxy Pd II complex is the active species, because a stoichiometric amount of base will convert the relatively unstable 10 B and 10 C species into 10 A (Δ G L S‐D3 ( 10 A / 10 B / 10 C )=0.0/9.3/22.0 kcal mol −1 ) .…”

“…Recently, we performed DFT calculations to investigate the mechanism of the oxidative addition of PhCl to a model variant ( 1 M ) of the cyclometalated dimeric Pd II complex 1 (see Scheme ). Our study suggested that the dimeric Pd II complex used in the experiments readily transforms into its active monomeric Pd II form through a base‐assisted decomposition mechanism, as outlined in Scheme . It was also found that the less observed and uncommon Pd II /Pd IV pathway is more favorable than the frequently observed classical Pd 0 /Pd II route.…”

“…The aNHC‐Pd II ‐catalyzed Suzuki–Miyaura cross‐coupling reaction of unreactive C−Cl bond activation in the presence of excess base was found experimentally to proceed with high yield and excellent turnover under mild reaction conditions . Our previous theoretical investigation on the oxidative addition process revealed that the reaction proceeds by unconventional Pd II /Pd IV routes . In this study we explored the detailed reaction pathways associated with the subsequent transmetalation and reductive elimination steps.…”

“… a) Suzuki–Miyaura cross‐coupling reaction of PhCl and PhB(OH) 2 in the presence of 1 , b) isolated dimeric Pd II complex 1 , and c) model dimeric Pd II complex considered in computational studies …”

A Quantum-Chemical DFT Approach to Elucidation of the Chirality Transfer Mechanism of the Enantioselective Suzuki–Miyaura Cross-Coupling Reaction

“…Since aryl-aryl bond forming cross-couplings are mechanistically quite similar, in our studies on enantioselective coupling we concentrated on the Suzuki-Miyaura reaction, which is usually applied as a method of first choice. The mechanics of the nonstereocontrolled Suzuki-Miyaura (and a few other couplings) reaction catalysed by complexes of nonchelating monophosphines has been studied in detail (Scheme 1) [20][21][22][23][24][25][26][27][28][29] and can be extrapolated to reactions catalysed by transition metal complexes of different types and to stereoselective transformations. At …”