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

Kaveh Farshadfar; Samaneh K. Tizhoush; Alireza Ariafard

doi:10.1021/acscatal.1c05512

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

Kaveh Farshadfar, Samaneh K. Tizhoush, Alireza Ariafard^*

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

8 Citations (Scopus)

Abstract

Numerous studies have demonstrated that Brønsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc)₂-catalyzed 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 Pd₃(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 Pd₃(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 Pd₃(OAc)₆ and [Pd(OAc)₂(DG∼C-H)] to form Pd₃(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 Pd₃(OAc)_6-x(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 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.

Original language	English
Pages (from-to)	2681-2693
Number of pages	13
Journal	ACS Catalysis
Volume	12
Issue number	4
Early online date	9 Feb 2022
DOIs	https://doi.org/10.1021/acscatal.1c05512
Publication status	Published - 18 Feb 2022
Externally published	Yes

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10.1021/acscatal.1c05512

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@article{359728a440634b97abf61827ee12c3e2,

title = "Role of Br{\o}nsted Acids in Promoting Pd(OAc)2-Catalyzed Chlorination of Phenol Carbamates Using N-Chlorosuccinimide",

abstract = "Numerous studies have demonstrated that Br{\o}nsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc)2-catalyzed 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 Pd3(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 Pd3(OAc)6, resulting in a very high energy barrier for C-H activation. The Br{\o}nsted acid HA is capable of undergoing ligand-exchange reactions with both Pd3(OAc)6 and [Pd(OAc)2(DG∼C-H)] to form Pd3(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 Pd3(OAc)6-x(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 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.",

keywords = "Br{\o}nsted acid, chlorination, C−H activation, DFT calculations, N-chlorosuccinimide, palladium catalysis, Pd(IV) intermediate, reaction mechanism",

author = "Kaveh Farshadfar and Tizhoush, \{Samaneh K.\} and Alireza Ariafard",

note = "Publisher Copyright: {\textcopyright} 2022 American Chemical Society",

year = "2022",

month = feb,

day = "18",

doi = "10.1021/acscatal.1c05512",

language = "English",

volume = "12",

pages = "2681--2693",

journal = "ACS Catalysis",

issn = "2155-5435",

publisher = "American Chemical Society",

number = "4",

}

TY - JOUR

T1 - Role of Brønsted Acids in Promoting Pd(OAc)2-Catalyzed Chlorination of Phenol Carbamates Using N-Chlorosuccinimide

AU - Farshadfar, Kaveh

AU - Tizhoush, Samaneh K.

AU - Ariafard, Alireza

PY - 2022/2/18

Y1 - 2022/2/18

N2 - Numerous studies have demonstrated that Brønsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc)2-catalyzed 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 Pd3(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 Pd3(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 Pd3(OAc)6 and [Pd(OAc)2(DG∼C-H)] to form Pd3(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 Pd3(OAc)6-x(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 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.

AB - Numerous studies have demonstrated that Brønsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc)2-catalyzed 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 Pd3(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 Pd3(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 Pd3(OAc)6 and [Pd(OAc)2(DG∼C-H)] to form Pd3(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 Pd3(OAc)6-x(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 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.

KW - Brønsted acid

KW - chlorination

KW - C−H activation

KW - DFT calculations

KW - N-chlorosuccinimide

KW - palladium catalysis

KW - Pd(IV) intermediate

KW - reaction mechanism

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U2 - 10.1021/acscatal.1c05512

DO - 10.1021/acscatal.1c05512

M3 - Article

AN - SCOPUS:85125053134

SN - 2155-5435

VL - 12

SP - 2681

EP - 2693

JO - ACS Catalysis

JF - ACS Catalysis

IS - 4

ER -

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

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