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Computational Study on Cs2CO3-Assisted Palladium-Catalyzed X—H(X=C,O,N, B) Functionalization Reactions

Bai Wenji , Shi Yubing , Mu Weihua , Li Jiangping , Yu Jiawei

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (10) : 2283 -2301.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (10) : 2283-2301 DOI: 10.7536/PC220132
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Computational Study on Cs2CO3-Assisted Palladium-Catalyzed X—H(X=C,O,N, B) Functionalization Reactions
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Abstract

Palladium-catalyzed X—H (X=C, O, N, B) functionalization reaction is an important organic synthesis strategy, which can build C—C and C—X (X=O, N, B) bonds in an atomically economical way by employing small molecules such as aryl halides, alkenes or alkynes as substrates. Due to its high yield, good reactivity and wide substrate compatibility, Cs2CO3-assisted palladium-catalyzed X—H (X=C, O, N, B) functionalization reaction has become one of the hot spots in the field of organic synthesis in recent years, and has played a crucial role in constructing C—C and C—X bonds in polycyclic natural product skeletons. Based on previous experimental results, density functional theory (DFT) has been employed to study the Cs2CO3-assisted palladium-catalyzed X—H (X=C,O,N,B) functionalization reaction in detail, so as to help people understand the essence of this type of reaction at microscopic level, and provide inspiration for designing new experimental synthetic routes. Herein, the latest density functional theory research results on Cs2CO3-assisted palladium-catalyzed X—H (X=C,O,N,B) functionalization reactions have been summarized, with corresponding computational results about microcosmic reaction mechanism and role of Cs2CO3 additive emphasized. The present issues and prospects of future development in this field are also summarized and forecast in the end.

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palladium catalysis / X—H functionalization / Cs2CO3 / reaction mechanism / selectivity / DFT

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Bai Wenji, Shi Yubing, Mu Weihua, Li Jiangping, Yu Jiawei. Computational Study on Cs2CO3-Assisted Palladium-Catalyzed X—H(X=C,O,N, B) Functionalization Reactions. Progress in Chemistry, 2022, 34(10): 2283-2301 DOI:10.7536/PC220132

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Contents

1 Introduction
2 Cs2CO3-assisted palladium-catalyzed functionalization of C—H
2.1 Cs2CO3-assisted palladium-catalyzed functionalization of C(sp2)—H
2.2 Cs2CO3-assisted palladium-catalyzed functionalization of C(sp3)—H
2.3 Cs2CO3-assisted palladium-catalyzed functionalization of C(sp)—H
3 Cs2CO3-assisted palladium-catalyzed functionalization of O—H
4 Cs2CO3-assisted palladium-catalyzed functionalization of N—H
5 Cs2CO3-assisted palladium-catalyzed functionalization of B—H
6 Conclusion and outlook

1 Introduction

Transition metal catalysis plays an important role in organic synthesis, materials and medicine[1⇓~3]. According to their different mechanisms, transition metal catalysts can be divided into two types: the first type is the reaction of transition metal as Lewis acid, and the second type is the reaction catalyzed by transition metal through oxidation/reduction mechanism.Among them, transition metal palladium has attracted great attention because of its stable oxidation States such as Pd (0), Pd (Ⅱ) and Pd (Ⅳ), and has played an important role in catalyzing the functionalization of X — H (X = C, O, N, B) and the construction of functionalized organic compounds containing C — C, C — X bonds, polycyclic natural products with complex structures and drugs[4][5⇓⇓⇓⇓~10][11][12⇓⇓⇓⇓⇓⇓⇓⇓~21].
With the development of research, it has been found that the addition of basic additives such as Cs2CO3 in the palladium-catalyzed X — H (X = C, O, N, B) functionalization reaction can accelerate the reaction rate or improve the yield, and the mechanism of Cs2CO3 accelerating the reaction is speculated to be due to its good solubility in organic solvents such as alcohols, N, N-dimethylformamide (DMF), ether and so on[22⇓⇓⇓~26][27,28]. However, due to the existence of d orbital of transition metal and its extensive participation in the catalytic reaction, the microscopic mechanism of Cs2CO3 assisted palladium-catalyzed X — H (X = C, O, N, B) functionalization reaction is extremely complex[29,30]. In order to clarify the Kinetic mechanism of the reaction, some experimental researchers used Kinetic Isotope Effect (KIE) experiments to predict the rate-determining step of the reaction, and found that most of them were the activation step of X-H (X = C, O, N, B) bonds[31⇓~33]. However, it is difficult to observe or detect the microscopic mechanism between the reaction substrate, palladium catalyst and Cs2CO3 in traditional experiments. In order to effectively solve this problem, in recent years, many theoretical workers have used quantum chemical methods to calculate the Density Functional Theory assisted palladium-catalyzed X — H (X = C, O, N, B) functionalization reactions, among which Density Functional Theory (DFT) is the most widely used method[34⇓⇓⇓⇓~39]. Through the computational simulation of the reaction system, they obtained the structural and energy information of each stationary point on the reaction path, and based on this, they revealed the microscopic mechanism and selective control mechanism of the reaction, which provided important help for further understanding the nature of Cs2CO3 assisted palladium-catalyzed X — H (X = C, O, N, B) bond activation. At present, the theoretical research results of palladium-catalyzed X — H (X = C, O, N, B) functionalization have been reviewed and widely cited by excellent journals at home and abroad, but there is no comprehensive summary and review of the theoretical research results of palladium-catalyzed X — H (X = C, O, N, B) functionalization assisted by Cs2CO3[40,41][42⇓⇓⇓⇓⇓⇓⇓⇓~51].
In fact, palladium-catalyzed X — H (X = C, O, N, B) functionalization has made great progress in the past decade, especially in the field of palladium-catalyzed X — H functionalization assisted by Cs2CO3.Excellent experimental results have been reported, and many theoretical researchers have used density functional theory to study the microscopic mechanism of this kind of reaction, the origin of selectivity and the mechanism of Cs2CO3[52][53⇓⇓⇓~57][58⇓~60]. Studies have shown that the complexing ability of Cs2CO3 is very strong, and it can form polymers or complexes with Cs2CO3 itself, ligands or additives to enhance the reactivity and reduce the reaction energy barrier[61,62]. In 2019, Liu Jingyao et al. Theoretically calculated the monomer, dimer and tetramer of Cs2CO3, and found that the tetramer was the most stable, and it was more reasonable to use 1/4(Cs2CO3)4 as the energy reference zero[63]. However, when discussing the reaction mechanism and reaction selectivity of Cs2CO3 assisted palladium-catalyzed X — H (X = C, O, N, B) functionalization, most authors still use unimolecular Cs2CO3 to calculate because the difficulty of reaction and the result of reaction selectivity are less affected by the selection of zero energy, and usually only one Cs2CO3 molecule can directly intervene in the activation process[58,64,65].
In view of the important application value of palladium-catalyzed X — H (X = C, O, N, B) functionalization reaction in the construction of polycyclic natural products, pharmaceuticals and pesticides, in order to let more researchers understand the mechanism of Cs2CO3 in palladium-catalyzed X — H (X = C, O, N, B) functionalization reaction from the microscopic level.In this paper, based on the theoretical research experience of our group in transition metal-catalyzed reactions, the DFT calculation research results of Cs2CO3 assisted palladium-catalyzed C — H, O — H, N — H and B — H functionalization reactions in the past decade at home and abroad are summarized and reviewed[66⇓⇓~69].

2 Cs2CO3 Assisted Palladium-Catalyzed C — H Functionalization

C — H bond is the most common chemical bond in organic compounds, which can be divided into C(sp3)—H, C(sp2)—H, C (sp) — H and so on according to the hybridization type of the attached carbon atom. The C(sp3)—H are mainly alkyl C—H,C(sp2)—H, including aryl, alkenyl and aldehyde C — H, and C (sp) — H is mainly alkynyl C — H. At present, the research on Cs2CO3 assisted palladium-catalyzed C(sp2)—H functionalization is the most thorough, while the theoretical studies on Cs2CO3 assisted palladium-catalyzed alkyl C(sp3)—H functionalization and alkynyl C (sp) — H functionalization are relatively few. In this section, the theoretical research results of Cs2CO3 assisted palladium-catalyzed C(sp2)—H, C(sp3)—H and C (sp) — H functionalization are summarized and discussed in turn.

2.1 Cs2CO3 Assisted Palladium-Catalyzed C(sp2)—H Functionalization

According to the different ways of Cs2CO3 assisted palladium-catalyzed C(sp2)—H bond activation, it can be divided into two types: Cs2CO3 directly assisted and Cs2CO3 indirectly assisted. The Cs2CO3 directly assisting palladium-catalyzed C(sp2)—H activation means that when only the Cs2CO3 is used as a reaction additive, the Cs2CO3 directly assists the C(sp2)—H activation of the substrate. The indirect assistance of Cs2CO3 means that when other additives or ligands and Cs2CO3 assist the palladium-catalyzed C(sp2)—H activation reaction, the Cs2CO3 finally activates the C(sp2)—H by assisting the O — H activation in other additives or ligands.

2.1.1 Palladium-catalyzed C(sp2)—H activation directly assisted by Cs2CO3

In the past decade, theoretical studies on the direct palladium-catalyzed activation of aryl C(sp2)—H by Cs2CO3 have been reported from time to time, mainly involving reactions to obtain complex functional molecules through coupling, spirocyclization, or fused cyclization processes[58,63,70⇓~72][73][74][75]. The common aryl C(sp2)—H activation mechanisms can be divided into oxidative addition type (A), halogen synergistic type (B), external base assisted type (C) and internal base assisted type (D), etc. (Scheme 1)[9,76]. Among them, the external base assisted type generally corresponds to the coordination of base and halogen, and the removal of CsX·CsHCO3 clusters or CsX and CsHCO3 after C — H activation. In the internal base-assisted type, the halogen/ligand ion is usually replaced by carbonate, which coordinates with palladium while removing CsX (X is the halogen/ligand ion), and then the CsC O 3 - coordinates with palladium to assist C — H activation and remove the CsHCO3. This sub-section mainly deals with two types, namely, external alkali-assisted type (C) and internal alkali-assisted type (D), which are directly assisted by Cs2CO3.
In 2017, at the PCM (DME)/M06/6-311 + + G (d, p) -SDD (Pd, P, I, Cs)//B3LYP/6-31 G (d) -LANL2DZ (Pd, P, I, Cs) level, Bi Siwei et al carried out DFT calculations on the palladium-catalyzed cross-coupling reaction of o-methyl iodobenzene, benzoic anhydride and ethyl acrylate reported by Xu Pengfei et al[77][70]. The results show that the reaction mainly involves oxidative addition, norbornene insertion, C — H activation, carbon-carbon bonding, norbornene removal and reductive elimination (Scheme 2). In the activation process of aryl C(sp2)—H (INT2 → INT3), the Gibbs free energy barrier for the reaction to cross the transition state TS1 is only 21.1 kcal·mol-1 due to the assistance of Cs2CO3 in the form of external base assistance.The energy barrier (25.1 kcal·mol-1) is 4.0 kcal·mol-1 lower than that of the rate-step transition state TS2, indicating that the assistance of Cs2CO3 is of great significance for the smooth progress of the reaction.
In order to further explore the external base and internal base assisted mechanism in the palladium-catalyzed C — H bond activation reaction, Lautens et al. Conducted a DFT study on the reaction mechanism of palladium-catalyzed acrylamide spirocyclization to construct nitrogen-containing spiroheterocycles at the IEFPCM (Tol)/M06L/def2-TZVPP//B3LYP/6-31G (d, p) -LANL2DZ (Pd, Cs, I) level in 2018[58]. The calculation results show that the catalyst PdL2 first undergoes oxidative addition with the substrate R4 to form the intermediate INT7 (Scheme 3A), and then the intermediate INT8 undergoes C — C bonding to form the five-membered ring intermediate INT9. The C(sp2)—H in INT9 is activated under the assistance of the Cs2CO3, and is converted into the spirocyclic intermediate INT12 through two possible paths of Path A and Path B (Scheme 3B), wherein Path A corresponds to the process that the Cs2CO3 exchanges with the iodine ligand to remove CsI and assists the activation of the aryl C(sp2)—H in the form of an internal assistant, and the Gibbs free energy barrier of the 25.8 kcal·mol-1(TS3) needs to be overcome; Path B corresponds to the process of aryl C(sp2)—H activation assisted by Cs2CO3 in the form of external assistance to remove CsHCO3 and CsI, which only needs to cross the Gibbs free energy barrier of 14.2 kcal·mol-1(TS4), indicating that the C — H activation reaction is more inclined to proceed through the mechanism of external base assistance. Finally, substrate R5 reacts with the intermediate INT12 to generate the P2 product via ligand exchange, alkyne insertion, and reductive elimination (Scheme 3A). The results of this theoretical study have important foundational significance for the further study of the external base and internal base assisted mechanism of palladium-catalyzed C(sp2)—H activation assisted by Cs2CO3.
In 2019, Koley et al. Performed a DFT study on the coupling mechanism of palladium-catalyzed 4-methoxybromobenzene and polyfluorocarbon carbonyl at the SMD (Tol)/B3LYP-D3 (BJ)/6-311 + + G * * -LANL2TZ (f) (Pd, Cs)/B3LYP-D3/6-31 + G * * -LANL2DZ (Pd, Cs) level[71]. The calculated results show that the reaction undergoes CO ligand exchange, oxidative addition, CO migration and insertion, C(sp2)—H activation, C — C bonding and reductive elimination (Fig. 4A). The activation of aryl C(sp2)—H (INT19 → TS5 → INT20) assisted by Cs2CO3 in the form of internal assistance is the rate-determining step of the whole reaction (Scheme 4B), and the Gibbs free energy barrier to be crossed is 20.2 kcal·mol-1. They also made a comparative study on the reaction assisted by K2CO3 and Na2CO3, and found that the reaction mechanism was similar to that assisted by Cs2CO3, but the Gibbs free energy barrier of the rate-determining step was higher than that assisted by Cs2CO3 (Fig. 4A), indicating that Cs2CO3 had a better effect on the reaction. In the same year, Liu Jingyao et al. Studied the reaction mechanism of palladium-catalyzed iodobenzene, norbornene and di-tert-butyldiazirine at the PCM (Tol)/M06L/6-311 + G * * -LANL2DZ (Pd, P, I, Cs)//M06L/6-31G (d, p) -LANL2DZ (Pd, P, I, Cs) level[63]. The calculated results show that the reaction undergoes C — I oxidative addition, CsI·CsHCO3 cluster formation, norbornene insertion, aryl C(sp2)—H activation, N — N oxidative addition, C(sp2)-N formation, C(sp3)-N formation and reductive elimination (Fig. 5A). The activation of aryl C(sp2)—H (INT25 → TS7 → INT26) is assisted by Cs2CO3 in the reaction (Fig. 5B), and the corresponding Gibbs free energy barrier is 26.6 kcal·mol-1, which is ①(① compared with the energy barrier of the transition state TS8 of the rate-determining step. In this paper, the difference between the intermediate INT27 and the transition state TS8 is used to calculate the energy barrier of the rate-determining step. ) (29.4 kcal·mol-1) low 2.8 kcal·mol-1. It can be seen that the assistance of Cs2CO3 makes the activation of C(sp2)—H easier. They also compared the response of K2CO3 and Cs2CO3 assistance.It is found that the Gibbs free energy barrier of the rate-determining step assisted by K2CO3 is 2.1 kcal·mol-1 higher than that of the rate-determining step assisted by Cs2CO3 (29.4 kcal·mol-1), which indicates that the auxiliary effect of Cs2CO3 on the Heck reaction is better.
Inspired by the above research results of Koley et al., in 2020, Itami et al. Studied the reaction mechanism of palladium-catalyzed 2-chlorobiphenyl to fused ring compounds at the IEFPCM (DCM)/B3LYP-D3 (BJ)/6-311 + G (d, p) -SDD (Pd, Cs)//B3LYP-D3 (BJ)/6-31G (d, p) -LANL2DZ (Pd, Cs) level[71][72]. The calculated results show that the reaction undergoes the processes of Cs2CO3 exchange with phosphine ligand, C — Cl cleavage, aryl C(sp2)—H activation, C — Cl cleavage, C — C bonding, aryl C(sp2)—H activation, C — C bonding and reductive elimination (Fig. 6). Cs2CO3 participates in the whole reaction process many times and assists the aryl C(sp2)—H activation twice in the form of internal assistance, and the corresponding Gibbs free energy barriers are 31.4 and 22.7 kcal·mol-1, respectively, which are lower than that of the C — Cl cleavage rate step (INT34→INT35,35.8 kcal·mol-1). In addition, the Gibbs free energy barrier of the first assisted aryl C(sp2)—H activation by Cs2CO3 is 5.3 kcal·mol-1 lower than that of the assisted by Na2CO3 (36.7 kcal·mol-1),It indicates that Cs2CO3 is more beneficial to enhance the reaction rate of C(sp2)—H activation.
The above theoretical research results of Liu Jingyao et al., Koley et al. And Itami et al. Show that there are great differences in the energy barriers of palladium-catalyzed C — H functionalization reactions assisted by alkali metal carbonate Na2CO3, K2CO3 and Cs2CO3.Moreover, the energy barrier of palladium-catalyzed C — H functionalization assisted by Cs2CO3 is generally lower than that of palladium-catalyzed C — H functionalization assisted by Na2CO3 and K2CO3, which provides a very valuable theoretical reference for the experimental screening of base additives[63][71][72].
In the same year, Poater et al. Performed a DFT study on the mechanism of the palladium-catalyzed reaction of phenyl triisopropylsilyl ethynyl ether with cinnamyl pivalate at the PCM (Tol)/M06/cc-pVTZ-SDD (Pd)//BP86-D3/SVP-SDD (Pd) level[78]. The calculated results show that the reaction mainly involves alkenyl coordination, Cs2CO3 coordination, aryl C(sp2)—H activation, C — C bonding, H migration, C — C bonding, alkyl C(sp3)—H activation and reductive elimination (Fig. 7A). In the conversion of intermediate INT39 to intermediate INT44 (scheme 7b),Cs2CO3 also assists the aryl C(sp2)—H activation in the form of an inner assistant, which is the rate-determining step of the whole reaction, and the corresponding Gibbs free energy barrier is 35.9 kcal·mol-1(TS10), which is slightly higher at 140 ° C.
Based on the above theoretical results, in the palladium-catalyzed aryl C(sp2)—H activation process, the Cs2CO3 is mostly involved in the form of internal assistance, but in a small number of reactions, the pathway in which the Cs2CO3 is involved in the form of external assistance is more energetically favorable. Therefore, in the theoretical study of different reaction systems, the possibility of the participation of Cs2CO3 in the form of external assistance and internal assistance should be fully considered.
Compared with Cs2CO3 assisted palladium-catalyzed aryl C(sp2)—H functionalization, there are relatively few reports on Cs2CO3 assisted palladium-catalyzed alkenyl C(sp2)—H functionalization, and the reason is that the carbon-carbon double bond cleavage in the alkenyl group is easier than the C(sp2)—H activation in the alkenyl group[79⇓~81]. However, the construction of complex molecules by functionalization of alkenyl C(sp2)—H can keep the carbon-carbon double bonds intact, which is essential for further functionalization of products[82,83]. At present, the functionalization of alkenyl C(sp2)—H has been successfully achieved by using Cs2CO3 assisted palladium catalysis, and the microscopic mechanism of the reaction has been revealed by theoretical calculation[84,85][86⇓~88].
For example, in 2015, Patil et al. Prepared N-aryl-1,2,3-triazole derivatives by using N-aryl-1,2,3-triazole and iodobenzene as substrates, Pd(OAc)2 as catalyst, and Cs2CO3 as base at 100 ° C, and conducted DFT study on the reaction mechanism at M06L/6-311G * * -LANL2DZ (Pd, Cs) level[86]. The results show that the oxidative addition of Pd (0) to iodobenzene to intermediate INT49 only needs to overcome the Gibbs free energy barrier of 0.3 kcal·mol-1 (Fig. 8), and then the Cs2CO3 reacts with intermediate INT49 to convert to intermediate INT50 through ligand exchange. After the substrate R15 enters, the activation of alkenyl C(sp2)—H is achieved by Cooperative Metallization-Deprotonation (CMD). Among them, the Gibbs free energy barriers for the activation of C (5) — H or C (4) — H assisted by Cs2CO3 in the form of internal assistance are 25.8 (TS12) and 30.2 kcal·mol-1(TS13), respectively, indicating that the reaction prefers to form the product P7 activated by C (5) — H, and the regioselectivity of the reaction arises from this. This work not only opens up a new method for the preparation of N-aryl-1,2,3-triazole derivatives, but also provides a new insight into the mechanism of Cs2CO3 assisted palladium-catalyzed activation of alkenyl C(sp2)—H. The origin of regioselectivity revealed by theoretical calculation has positive guiding significance for the experimental design of schemes to obtain products with specific regioselectivity.
In the context of the remarkable achievements of Cs2CO3 assisted palladium-catalyzed functionalization of aryl C(sp2)—H and alkenyl C(sp2)—H, Cs2CO3 assisted palladium-catalyzed functionalization of aldehyde C(sp2)—H has also attracted much attention. In 2018, Xueliang Huang et al. Reported the palladium-catalyzed intermolecular acylation of aryl diazoesters with o-bromobenzaldehyde, which is the first reaction example about the Cs2CO3 assisted palladium-catalyzed C(sp2)—H activation of aldehyde group[65]. They performed DFT calculations for this reaction at the SMD (Tol)/M06/def2-TZVP//B3LYP/6-31G (d) -LANL2DZ (Pd, Br, Cs) level. The results showed that the oxidative addition of o-bromobenzaldehyde to palladium catalyst needs to overcome the Gibbs free energy barrier of 12.6 kcal·mol-1 (TS14, Fig. 9), and then the substrate R17 reacts with the intermediate INT58 to form the carbene intermediate INT59 through the transition state TS15, which is the rate-determining step of the whole reaction and needs to overcome the Gibbs free energy barrier of 34.3 kcal·mol-1. After that, Cs2CO3 acts as an endo-auxiliary to assist the aldehyde group C(sp2)—H activation in the intermediate INT60, and the corresponding Gibbs free energy barrier is 29.0 kcal·mol-1(TS17).The energy barrier is 5.3 kcal·mol-1 lower than that of the rate-determining step, indicating that the activation of aldehyde C(sp2)—H becomes easier with the assistance of Cs2CO3. This work is of fundamental significance for further exploring the mechanism of C(sp2)—H activation of aldehyde group.

2.1.2 Palladium-catalyzed C(sp2)—H activation indirectly assisted by Cs2CO3

In contrast to the direct Cs2CO3 assisted palladium-catalyzed C(sp2)—H activation described above, the Cs2CO3 assisted palladium-catalyzed C(sp2)—H activation can also be performed in an indirect manner. For example, in 2018, Alb AlbÉniz et al. Used DFT theory on the level of SMD (pyridine)/M06/6-311 + + G (d, p) -LANL2TZ (f) (Pd, Cs, Br, I)//M06/6-31 + G (d) -LANL2TZ (f) (Pd, Cs, Br, I) to study the Cs2CO3 assisted palladium-catalyzed C(sp2)—H activation process involved in the palladium-catalyzed arylation coupling reaction of pyridine and p-trifluor[87]. The results showed that the reaction underwent oxidative addition, ligand exchange, C(sp2)—H activation and reductive elimination (Fig. 10a), and there were two possible pathways for C(sp2)—H activation (Fig. 10B). Path A corresponds to the process in which the ligand 2,2 '-bipyridine-4-oxygen (bipy-4-O) first assists the activation of the C(sp2)—H, which requires crossing the total Gibbs free energy barrier (TS20) of the 27.5 kcal·mol-1. After the formation of intermediate INT66, the Cs2CO3 acts as an exo-auxiliary to assist the O — H activation of the ligand, and through the transition state TS21, it overcomes the Gibbs free energy barrier of the 14.9 kcal·mol-1 and transforms into intermediate INT68. Path B corresponds to a process in which Cs2CO3 directly assists the C(sp2)—H activation of the pyridine ring in an exo-auxiliary form, and is converted into an intermediate INT68 through a transition state TS22 and across the total Gibbs free energy barrier of the 30.9 kcal·mol-1. The above results indicate that the C(sp2)—H activation reaction is more inclined to proceed in the way of Path A, which has a good inspiration for people to further understand the microscopic mechanism of palladium-catalyzed C(sp2)—H activation indirectly assisted by Cs2CO3.
In 2019, Xu et al. Reported the palladium-catalyzed carbonylation of tosyl-protected diarylmethyl amines to 4-phenyl-substituted isoquinolinones with a yield of 87%[89]. They conducted a DFT study on the activation mechanism of aryl C(sp2)—H at the M06L/6-31G (d, p) -LANL2DZ (Pd, Cs) level, and the results showed that :Cs2CO3 first assisted tert-butoxycarbonyl-L-valine (Boc-L-Val-OH) to undergo O — H activation to form Boc-L-Val-OCs, which was combined with Pd (II) catalyst to convert to the intermediate INT69 (Fig. 11).
Then CO and substrate R20 successively enter the reaction system and are converted into intermediate INT72 through C-N bonding. After that, Boc-l-Val-OCs assist the aryl C(sp2)—H activation in the form of endo-assistance, which is the rate-determining step of the reaction, and the corresponding Gibbs free energy barrier is 14.2 kcal·mol-1(TS23). Finally, the intermediate INT74 undergoes reductive elimination to form the product P11. The theoretical results also have important reference value for people to deeply understand the microscopic mechanism of palladium-catalyzed C(sp2)—H activation indirectly assisted by Cs2CO3.

2.2 Cs2CO3 Assisted Palladium-Catalyzed C(sp3)—H Functionalization

Compared with the functionalization of C(sp2)—H, the functionalization of alkyl C(sp3)—H is more difficult and usually requires harsher reaction conditions, such as photocatalysis, which largely hinders the application of alkyl C(sp3)—H functionalization in the field of organic synthesis[90]. With the development of research, it has been found that palladium-catalyzed reaction assisted by Cs2CO3 can effectively promote the activation of alkyl C(sp3)—H, which opens up a new world for the development and utilization of alkyl C(sp3)—H functionalization reaction[91]. In this section, we will summarize the results of DFT theoretical studies on Cs2CO3 direct and indirect assisted palladium-catalyzed alkyl C(sp3)—H activation reactions.

2.2.1 Palladium-catalyzed C(sp3)—H activation directly assisted by Cs2CO3

In 2018, Chen Gong et al. Reported the Cs2CO3 assisted palladium-catalyzed reaction of 4-methoxyiodobenzene and 3-arylpropionamide via alkyl C(sp3)—H activation to prepare the coupling product P12 (Fig. 12), and the reaction was studied by DFT at the SMD (m-xylene)/ωB97XD/def2TZVPP//ωB97XD/6-31 G (d) -LANL2DZ (Pd, Cs) level[92]. The calculated results show that the reaction proceeds through oxidative addition, ligand exchange, alkyl C(sp3)—H activation and reductive elimination. The activation of alkyl C(sp3)—H assisted by Cs2CO3 in the form of internal assistance is the rate-determining step of the whole reaction, and only the Gibbs free energy barrier (TS24) of 29.0 kcal·mol-1 needs to be overcome, which is in good agreement with the 95% P12 product obtained at 100 ℃ for 48 H.
One year later, Xie et al. Carried out a DFT study on the palladium-catalyzed reaction of (Z) -2-bromovinylbenzene with endo-N- (p-phenylmethyl) -norbornene succinimide to prepare cyclopropane skeleton at the CPCM (Tol)/M06/6-31 G (d, p) -LANL2DZ (Pd, P, Br, Cs) level[93]. The calculated results (Fig. 13) show that the reaction undergoes oxidative addition, migration and insertion of substrate R24, exchange of Cs2CO3 and CsC O 3 - with bromine ligand, activation of alkyl C(sp3)—H, intramolecular olefin insertion, β-H elimination and reductive elimination. Among them, the Gibbs free energy barrier for the activation of alkyl C(sp3)—H assisted by Cs2CO3 in the form of internal assistance is 8.9 kcal·mol-1(TS25), which is 19.6 kcal·mol-1 lower than the energy barrier (28.5 kcal·mol-1) of the rate-determining step transition state TS26; However, without Cs2CO3 assistance, the conversion of intermediate INT79 to intermediate INT87 needs to absorb the Gibbs free energy of 86.7 kcal·mol-1, which indicates that Cs2CO3 assistance is an important condition for the palladium-catalyzed alkyl C(sp3)—H activation.
Based on the previous research results of palladium-catalyzed alkyl C(sp3)—H activation assisted by Cs2CO3, Chen Gong et al. Prepared β-lactam product (P14) with 3-phenylpropionamide as substrate and aryl iodide as oxidant at 100 ℃ in 2020, with a yield of 92%[92,93][94]. They carried out a mechanistic study on the chemoselectivity of the reaction at the SMD (m-xylene)/M06/6-311 + G (d, p) -SDD (Pd, I)//B3LYP-D3/6-31 G (d) -SDD (Pd, I) level, and found that the reaction underwent alkyl C(sp3)—H activation, oxidative addition, ligand exchange, C — C/C — N bonding, and reductive elimination processes (Fig. 14a). The role of Cs2CO3 in the reaction is to assist the activation of alkyl C(sp3)—H, and then to assist the reductive elimination of C — C/C — N in the form of HC O 3 -. In addition, they respectively used o-methoxyiodobenzene (R26) and 2-methoxy-5-chloroiodobenzene (R27) as oxidants to study the chemoselectivity (Fig. 14 B), and the results showed that the energy barrier of the C-C/C-N reductive elimination reaction with the intermediate INT89 as the zero point (the path without the assistance of HC O 3 -) was higher than that with the intermediate INT90 as the zero point (the path with the assistance of HC O 3 -), indicating that the HC O 3 - assistance reaction can effectively reduce the energy.
The above theoretical research results on the palladium-catalyzed alkyl C(sp3)—H activation directly assisted by Cs2CO3 show that the participation of the auxiliary form in the reaction within :Cs2CO3 can effectively reduce the relative Gibbs free energy of the key intermediate and the energy barrier of the alkyl C(sp3)—H activation reaction, so that the reaction can be carried out under relatively mild experimental conditions. These theoretical research results have certain theoretical guiding significance for designing and developing other simple and feasible alkyl C(sp3)—H activation reaction strategies in the future.

2.2.2 Palladium-catalyzed C(sp3)—H activation indirectly assisted by Cs2CO3

In 2013, Fu Yao et al performed DFT calculations on the mechanism of palladium-catalyzed amidation of o-methylnaphthalene aminomethylformyl chloride at the SMD (mesitylene)/M06/6-311 + G (d, p) -LANL2DZ (Pd, Cs)//B3LYP/6-31 G (d) -LANL2DZ (Pd, Cs) level[95]. The results showed that the reaction underwent oxidative addition, alkyl C(sp3)—H activation and reductive elimination (Fig. 15A). There are two possible reaction paths for alkyl C(sp3)—H activation: Path A and Path B (Scheme 15B). Path A corresponds to the process in which the Cs2CO3 assists the activation of alkyl C(sp3)—H in the form of internal assistance, and the Gibbs free energy barrier to be overcome is that 27.9 kcal·mol-1(TS28);Path B corresponds to the process in which the Cs2CO3 assists pivaloyl hydroxylamine first.The Gibbs free energy barrier to be crossed in the process of O — H activation in Piv = pivaloyl) to generate PivNHOCs and then assist the activation of alkyl C(sp3)—H is 23.7 kcal·mol-1(TS27), which is 4.2 kcal·mol-1 lower than that of TS28. The above results indicate that when PivNHOH and Cs2CO3 co-assist the palladium-catalyzed alkyl C(sp3)—H activation reaction, it is more favorable for Cs2CO3 to assist the reaction in an indirect manner.
One year later, K Kündig et al. Reported the palladium-catalyzed preparation of 2,3-substituted indoles from N-isopropylcarbamates, and performed DFT calculations on the mechanism of this reaction at the M06L/6-31 G (d, p) -LANL2DZ (Pd, Br) level[96]. The results showed that the reaction underwent oxidative addition, ligand exchange, alkyl C(sp3)—H activation and reductive elimination (Fig. 16). In which that activation of the alkyl C(sp3)—H is carry out :Cs2CO3 by first indirectly assisting the O — H activation of pivalic acid (tBuCOOH) to produce tBuCOOCs,tBuCOOCs then assists in the activation of alkyl C(sp3)—H in the form of internal assistance, which is converted to the intermediate INT101 through the transition state TS29 or to the intermediate INT102 through the transition state TS30.
Since the energy barrier of TS30 is 3.4 kcal·mol-1 higher than that of TS29, the reaction prefers to generate the P17 product. The theoretical calculation results are consistent with the experimental phenomena, which reveals the origin of the chemoselectivity of the reaction, and provides a valuable theoretical reference for the experimental development of new palladium-catalyzed selective functionalization of C(sp3)—H.
Based on the research results of the mechanism of palladium-catalyzed C(sp3)—H activation indirectly assisted by Cs2CO3, Liang Yongmin et al. Recently prepared the spirocyclization product by palladium-catalyzed Narasaka-Heck/C(sp3)—H activation of γ, δ-unsaturated oxime esters, and the yield could reach 75%[95,96][97]. They further performed DFT calculations at the PCM (Tol)/B3LYP/def2-SVP level for this reaction. The results showed that the reaction underwent oxidative addition, intramolecular Heck cyclization, C(sp3)—H activation and reductive elimination (Fig. 17A). Among them, C(sp3)—H activation is the rate-determining step of the whole reaction, and the Gibbs free energy barrier to be crossed with the assistance of ligand C6F5C O 2 - is 31.9 kcal·mol-1(TS31, Scheme 17 B), which is more reasonable at the experimental temperature of 150 ° C. The subsequent conversion of the intermediate INT105 to the five-membered spiropalladacycle intermediate INT106 is a barrierless process, in which Cs2CO3 assists the activation of O — H in C6F5CO2H, promotes the removal of C6F5C O 2 - from the system, and releases the Gibbs free energy of 27.3 kcal·mol-1.

2.3 Cs2CO3 Assisted Palladium-Catalyzed C (sp) — H Functionalization

Compared with the Cs2CO3 assisted palladium-catalyzed C(sp2)—H and C(sp3)—H functionalization reactions, the DFT theoretical studies on the Cs2CO3 assisted palladium-catalyzed alkynyl C (sp) — H functionalization reactions are obviously less reported. In 2017, the mechanism of Sonogashira cross-coupling reaction between phenylacetylene and iodobenzene was studied by DFT at SMD (DMF)/M06/6-311 + G (d, p) -SDD (Pd, Cu, I, Cs)//B3LYP/6-31 + G * -LANL2DZ (Pd, Cu, I, Cs) level[98]. The results show that copper catalysis and palladium catalysis are two relatively independent and closely linked processes. In the palladium-catalyzed cycle, the reaction undergoes oxidative addition, alkyne insertion, and reductive elimination processes (Scheme 18 A). At the same time, the CsC O 3 - generated in the palladium-catalyzed cycle participates in the copper-catalyzed cycle by coordinating with copper (Scheme 18 B), so that CuI is converted into the intermediate INT109, which is 17.3 kcal·mol-1 lower in energy than the intermediate INT110 formed by coordination of CuI with alkyne. After that, CsC O 3 - assists the alkynyl C (sp) — H activation in an endo-auxiliary form, and the corresponding Gibbs free energy barrier is as low as 8.7 kcal·mol-1(TS32), so that the intermediate INT109 is rapidly converted into the intermediate INT112 via INT111 and the transition state TS32 and participates in the palladium catalytic cycle. The theoretical calculation results show that the Cs2CO3 plays an indispensable role in the activation of alkynyl C (sp) -H and the realization of the two catalytic cycles.

3 Palladium-Catalyzed O-H Functionalization Assisted by Cs2CO3;

In addition to the Cs2CO3 assisted palladium-catalyzed C — H functionalization, the Cs2CO3 assisted palladium-catalyzed O — H functionalization reaction has received great attention from both experimental and theoretical chemistry researchers[99⇓⇓~102]. For example, in 2011, Fern Fernández et al. Performed a DFT study on the mechanism of palladium-catalyzed intramolecular coupling reaction of γ- (2-iodoanilino) ketone at the PCM (THF)/B3LYP/def2-SVP level[103]. The results showed that the substrate R33 was oxidatively added to the palladium catalyst (Fig. 19), crossing the Gibbs free energy barrier of the 9.2 kcal·mol-1 to form the intermediate INT113, and then the intermediate INT114 was transformed into the four-membered palladacycle intermediate INT115 by Pd-N bonding, which needed to cross the Gibbs free energy barrier of the 16.1 kcal·mol-1 (TS33). Then, the intermediate INT116 is converted to the intermediate INT117 by O — H activation assisted by Cs2CO3, and the Gibbs free energy of 46.2 kcal·mol-1 is released, which plays a vital role in driving the formation of P21 product.
In order to further clarify the reaction mechanism of palladium-catalyzed O — H activation assisted by Cs2CO3, Zhang Songlin et al. Studied the reaction mechanism of palladium-catalyzed cross-coupling of bromobenzene and 4-methyl-4-hydroxy-2-pentanone to α-aryl ketones at SMD (Tol)/M06/6-311 + G (d, p) -SDD (Pd) -LANL2DZ (Cs)//B3LYP/6-31G (d) -LANL2DZ (Pd, p, Cs) level in[104]. The calculated results show that the reaction undergoes oxidative addition, O — H activation, C — C cleavage and reductive elimination (Fig. 20A). Among them, the O — H activation process of intermediate INT120 to INT121 only needs to absorb the Gibbs free energy of 11.9 kcal·mol-1 (Fig. 20 B), indicating that the O — H activation process is easier due to the assistance of Cs2CO3.
The above theoretical results of Fern Fernández et al. And Zhang Songlin et al. Show that the assistance of Cs2CO3 can make the palladium-catalyzed O — H activation reaction easier, but the detailed reaction mechanism of Cs2CO3 participation is not given[103,104]. In 2019, Yonggui Zhou et al. Reported the palladium-catalyzed reaction of 1-bromo-2- (1-phenylethenyl) benzene with 1-phenylcyclobutanol to prepare the cross-coupling product P23 (92% yield), and the microscopic mechanism of this reaction was investigated by DFT at the B3LYP/6-311 + + G (2df, 2p)/SMD (Tol)/M05-2X/6-31 G (d) -SDD/B3LYP/6-31 + G (d) -SDD level[88]. The calculation results show that after the substrate R36 is converted into the intermediate INT123 by oxidative addition with the palladium catalyst (Figure 21), the auxiliary form other than the CsC O 3 - assists the O — H activation through the transition state TS36, and this process only needs to overcome the Gibbs free energy barrier of the 5.4 kcal·mol-1.
Next, the intermediate INT124 undergoes a C — C cleavage ring opening, crosses the Gibbs free energy barrier of 15.5 kcal·mol-1(TS37) to convert to the intermediate INT125, and releases a large amount of heat. Finally, the intermediate INT125 undergoes reductive elimination to form the product P23. It can be seen from the figure that the assistance of Cs2CO3 in this reaction plays a key role in reducing the palladium-catalyzed O — H activation energy barrier and ensuring the smooth progress of the reaction.
In the above palladium-catalyzed O — H activation reactions, the relative energy of the key intermediate or transition state involved in the O — H activation process is effectively reduced due to the assistance of Cs2CO3, and the corresponding reaction energy barrier is also significantly reduced, thus ensuring the smooth occurrence of the reaction.

4 Cs2CO3 Assisted Palladium-Catalyzed N-H Functionalization

Compared with the Cs2CO3 assisted palladium-catalyzed C — H and O — H functionalizations, there are few reports on both experimental and theoretical studies on the Cs2CO3 assisted palladium-catalyzed N — H functionalization, due to the high bond energy of the N — H bond, which is difficult to activate and cleave. However, with the development of research, palladium-catalyzed N — H bond activation assisted by Cs2CO3 has been realized, and it has been widely used in novel reactions to construct C — N bonds in an atom-economical manner[59,105⇓⇓⇓~109].
For example, in 2019, Liang Yongmin et al. Reported the palladium-catalyzed reaction of o-iodoaniline, N-benzoyloxyamine and norbornadiene to prepare 4-aminoindole by cross-coupling, and the reaction mechanism was studied by DFT at the PCM (Tol)/B3LYP-D3/6-31G (d, p) -LANL2DZ (Pd, P, Cs, I) level[59]. The results showed that when R in substrate R38 was tert-butoxycarbonyl, the Gibbs free energy barriers of Cs2CO3 to assist N — H and C — H activation in the form of internal assistance were 16.9 (TS38) and 24.5 kcal·mol-1(TS39, respectively, Fig. 22 a, B), indicating that the reaction preferred N — H activation and P24 was the main product. When R in substrate R38 is tert-butyl, the Gibbs free energy barriers for Cs2CO3 to assist N — H and C — H activation in the form of internal assistance are 26.2 (TS41) and 25.3 kcal·mol-1(TS42, respectively, Fig. 22 a, C), indicating that the reaction prefers C — H activation and P25 is the main product. The above theoretical calculation results are consistent with the experimental conclusions, and also show that the energy barrier of the Cs2CO3 assisted N — H activation process will be affected by the substituent of the substrate, which has important reference value for screening and expanding the types of substrates for C-N coupling reactions.

5 Cs2CO3 Assisted Palladium-Catalyzed B-H Functionalization

Palladium-catalyzed B — H functionalization assisted by Cs2CO3 is of great significance for the construction of C — B bonds, but the experimental research in this field is still in its infancy, and there are only a handful of theoretical reports available[110,111][60,112]. In 2018, Xie Zuowei et al. Reported the palladium-catalyzed asymmetric B — H activation for the synthesis of chiral caged o-carboranes[60]. This reaction with the assistance of chiral ligand L * and Cs2CO3 can prepare (S) -P26 main product in up to 98% yield (90% ee) (Scheme 23). They speculated that the reaction underwent oxidative addition, B — H activation and reductive elimination, and DFT calculations were performed at the B3LYP/6-31G (d, p) -LANL2DZ (Pd) level for the B — H activation process.
The results showed that the :Cs2CO3 assisted B — H bond activation in an endo-assisted manner, and the relative Gibbs free energy of the transition state TS43-S, which was required for the formation of (S) -P26 product, was 2.0 kcal·mol-1 lower than that of the transition state TS43-R, which was required for the formation of (R) -P26 product, indicating that the reaction preferred to form (S) -P 26 product. Their theoretical calculations not only reproduced the experimental results well, but also confirmed the feasibility of palladium-catalyzed B — H activation assisted by Cs2CO3. In 2019, our research group carried out DFT study on Cs2CO3 assisted nickel-catalyzed B — H activation reaction at the PCM (Tol)/LC-ωPBE/DGDZVP-SDD (Cs) level, and found that the Gibbs free energy barrier for B — H activation assisted by auxiliary forms other than Cs2CO3 (using K2CO3 instead of calculation) is only 23.1 kcal·mol-1, which indicates that Cs2CO3 can not only assist palladium-catalyzed B — H-activation, but also has a broad application in assisting other transition metal-catalyzed B — H-activation reactions[112].

6 Conclusion and prospect

In the last ten years, a great deal of theoretical research on palladium-catalyzed X — H (X = C, O, N, B) functionalization assisted by Cs2CO3 has been carried out by chemists at home and abroad, which provides great help for people to deeply understand the role of Cs2CO3 in palladium-catalyzed X — H (X = C, O, N, B) bond activation reaction and its microscopic reaction mechanism. Based on the calculation results using B3LYP, M06L and B3LYP-D3 methods, Cs2CO3 usually assists palladium-catalyzed X — H (X = C, O, N, B) bond activation reactions in both direct and indirect ways, and the direct assistance can be divided into two types: internal base assisted and external base assisted. In the palladium-catalyzed X — H (X = C, O, N, B) bond activation reactions, the Cs2CO3 are mostly assisted by direct means, in which the internal base assisted type is in the majority. In most of the reactions, the relative Gibbs free energies of the corresponding intermediates and transition States are reduced by the direct participation of Cs2CO3, which effectively reduces the activation energy barrier of the reaction and makes the reaction easy to proceed. It is removed from the reaction in the form of CsX (X = Cl, Br, I..), CsHCO3, or CsX·CsHCO3 clusters. The clarification of the above reaction mechanism provides important theoretical guidance for experimental researchers to change the experimental conditions pertinently to reduce the energy barrier of the rate-determining step, thereby promoting the rapid and efficient occurrence of the reaction. In addition, the effect of different alkali additives on the reaction energy barrier and the calculation and verification results of the reaction selectivity will be helpful to screen out the optimal alkali additive in the experiment, which will lay a foundation for the subsequent experiments to take reasonable measures to obtain the expected product on the basis of reasonably evaluating the ratio of the main by-products of the X — H (X = C, N, O, B) functionalization reaction.
With the unremitting efforts of researchers, many remarkable achievements have been made in the Cs2CO3 assisted palladium-catalyzed X — H (X = C, O, N, B) functionalization reactions, and the understanding of the auxiliary activation mode and mechanism of Cs2CO3 has become more and more mature.However, there are still some problems to be solved: (1) In terms of experimental research, the types of palladium-catalyzed N — H and B-H functionalization reactions assisted by Cs2CO3 need to be expanded, and other X — H functionalization reactions assisted by palladium-catalyzed Cs2CO3, such as Si-H functionalization reaction, need to be further developed and tried. (2) In the aspect of theoretical research, the mechanism of Cs2CO3 assisted palladium-catalyzed O — H and N — H functionalization reaction needs to be further studied, and the microscopic mechanism, regioselectivity, chemoselectivity and other issues of Cs2CO3 assisted palladium-catalyzed B-H functionalization reaction are still lack of systematic theoretical study and discussion.
Therefore, it is a potential development direction in this field to continue to expand the reaction types of Cs2CO3 assisted palladium-catalyzed X — H (X = O, N, B, Si, …) functionalization and carry out DFT theoretical studies, and to design and develop more economical and efficient Cs2CO3 assisted palladium-catalyzed X — H functionalization reaction systems and apply them to the construction of C — X bonds on the basis of in-depth study of the microscopic mechanism and reaction selectivity theory.

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Funding

National Natural Science Foundation of China(21763033)

Top young talents of Yunnan ten thousand people plan

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