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Role of Bronsted acids in promoting Pd(OAc)2-catalyzed chlorination of phenol carbamates using N-chlorosuccinimide

Citation

Farshadfar, K and Tizhoush, SK and Ariafard, A, Role of Bronsted acids in promoting Pd(OAc)2-catalyzed chlorination of phenol carbamates using N-chlorosuccinimide, ACS Catalysis, 12, (4) pp. 2681-2693. ISSN 2155-5435 (2022) [Refereed Article]


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DOI: doi:10.1021/acscatal.1c05512

Abstract

Numerous studies have demonstrated that Brnsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc)2-catalyzed functionalization of CH 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∼CH) 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 CH bond, it is believed that the trinuclear precatalyst Pd3(OAc)6 reacts with the substrate DG∼CH to generate the chelated complex [Pd(OAc)2(DG∼CH)], from which CH activation occurs via a concerted metalationdeprotonation mechanism. Because the substrate DG∼CH binds relatively weak to palladium, the corresponding chelated complex lies much higher in energy than the reference structure Pd3(OAc)6, resulting in a very high energy barrier for CH activation. The Brnsted acid HA is capable of undergoing ligand-exchange reactions with both Pd3(OAc)6 and [Pd(OAc)2(DG∼CH)] to form Pd3(OAc)6x(A)x and [Pd(OAc)(A)(DG∼CH)], respectively. Our calculations demonstrate that while the formation of [Pd(OAc)(A)(DG∼CH)] from [Pd(OAc)2(DG∼CH)] is highly exergonic, that of Pd3(OAc)6x(A)x from Pd3(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 CH 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∼CH)]; the more acidic the HA, the smaller the energy difference, and the lower the activation energy of CH activation. In addition, our calculations show that the presence of HA not only lowers the overall energy barrier for CH activation but also accelerates the chlorination step by protonating one of the oxygen atoms in NCS rather than the N atom.

Item Details

Item Type:Refereed Article
Keywords:palladium, DFT calculation, reaction mechanism, Bronsted acid promoters
Research Division:Chemical Sciences
Research Group:Physical chemistry
Research Field:Catalysis and mechanisms of reactions
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the chemical sciences
UTAS Author:Ariafard, A (Associate Professor Alireza Ariafard)
ID Code:150056
Year Published:2022
Funding Support:Australian Research Council (DP180100904)
Deposited By:College Office - CoSE
Deposited On:2022-05-16
Last Modified:2022-05-18
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