Role of Brønsted Acids in Promoting Pd(OAc)₂-Catalyzed Chlorination of Phenol Carbamates Using N-Chlorosuccinimide

Abstract

Numerous studies have demonstrated that Brønsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc) 2catalyzed functionalization of C−H bonds. However, the rationale for using these acids as a promoter is not yet completely obvious. The purpose of this work is to provide a detailed explanation for this observation with the aid of density functional theory calculations. This is accomplished by investigating the chlorination mechanism of phenol carbamates (DG∼C−H) with N-chlorosuccinimide (NCS) using HOTf as a promoter and Pd(OAc) 2 as a catalyst. Typically, in order for Pd(OAc) 2 to activate the C−H bond, it is believed that the trinuclear precatalyst Pd 3 (OAc) 6 reacts with the substrate DG∼C−H to generate the chelated complex [Pd(OAc) 2 (DG∼C−H)], from which C−H activation occurs via a concerted metalation−deprotonation mechanism. Because the substrate DG∼C−H binds relatively weak to palladium, the corresponding chelated complex lies much higher in energy than the reference structure Pd 3 (OAc) 6 , resulting in a very high energy barrier for C−H activation. The Brønsted acid HA is capable of undergoing ligand-exchange reactions with both Pd 3 (OAc) 6 and [Pd(OAc) 2 (DG∼C−H)] to form Pd 3 (OAc) 6−x (A) x and [Pd(OAc)(A)(DG∼C−H)], respectively. Our calculations demonstrate that while the formation of [Pd(OAc)(A)(DG∼C−H)] from [Pd(OAc) 2 (DG∼C−H)] is highly exergonic, that of Pd 3 (OAc) 6−x (A) x from Pd 3 (OAc) 6 is either nearly thermoneutral or endergonic. This feature significantly reduces the energy difference between the reference structure and the chelated complex, resulting in a significant decreased energy barrier for C−H activation. We also found that the acidity of the employed HA influences the energy difference between the trinuclear reference structure and the chelated complex [Pd(OAc)(A)(DG∼C−H)]; the more acidic the HA, the smaller the energy difference, and the lower the activation energy of C−H activation. In addition, our calculations show that the presence of HA not only lowers the overall energy barrier for C−H activation but also accelerates the chlorination step by protonating one of the oxygen atoms in NCS rather than the N atom.

Stereochemistry of the Reactions between Palladacycle Complexes and Primary Alkyl Iodides

“…It was found that the major donor–acceptor interaction involved in TS3a is the electron donation from the d z 2 -type orbital of the Pd center to the σ*­(C–I)-type orbital of iodide (Figure a). A similar donor–acceptor interaction was proposed by Ariafard and co-workers in an S N 2-type oxidative addition TS of a phenol carbamate-derived palladacycle complex with N -chlorosuccinimide . In contrast, in TS3c , both the donation from the Pd d z 2 -type orbital to σ*­(C–I)-type orbital and the back donation from the nonbonding electron pair of iodine to the empty d x 2 – y 2 -type orbital of Pd are significant, with the latter being more profound (Figure b)…”

“…A similar donor−acceptor interaction was proposed by Ariafard and coworkers in an S N 2-type oxidative addition TS of a phenol carbamate-derived palladacycle complex with N-chlorosuccinimide. 33 -type orbital of Pd are significant, with the latter being more profound (Figure 4b). 34 Based on the above analysis, we conclude that the σdonation ability of the ligands is a key factor affecting the alkylation mechanism, which is supported by the computed energy levels of the palladacycles of interest (Figure 5).…”

The Stereochemistry of the Reactions between Palladacycle Complexes and Primary Alkyl Iodides

“…Similar donor-acceptor interaction was proposed by Ariafard and co-workers in an S N 2-type oxidative addition TS of a phenol carbamate-derived palladacycle complex with N-chlorosuccinimide. 30 In contrary, in TS3c both the donation from the Pd d z 2 -type orbital to σ*(C-I)-type orbital and the back donation form the nonbonding electron pair of iodine to the empty d x 2y 2 -type orbital of Pd are significant, with the latter being more profound (Figure 3b). Based on the above analysis, we purpose that the σ-donation ability of the ligands is a key factor affecting the alkylation mechanism, which is supported by the computed energy levels of the palladacycles of interest (Figure 4).…”

Mechanism of CO2 Electroreduction to Multicarbon Products over Iron Phthalocyanine Single-Atom Catalysts

“…All DFT calculations were performed using Gaussian 16 at the M06-L level of theory . The accuracy of M06-L in calculating the energy of organometallic reactions has been shown in numerous studies. − For all the calculations, solvent effects were considered using the SMD solvation model with water as the solvent. The SDD basis set , with effective core potential was chosen to describe iron.…”