Chinese Journal of Chemistry, 2008, 26, 358—362 Full Paper

* E-mail: fuyao@ustc.edu.cn; Fax: 0086-0551-3606689 Received June 17, 2007; revised September 17, 2007; accepted October 19, 2007. Project supported by the National Natural Science Foundation of China (No. 20602034) and the Natural Science Foundation of Anhui Province (No.

070416237).

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Theoretical Study on Catalyst Activation of Palladacycles in Heck Reaction

WANG, Chen(王晨) FU, Yao*(傅尧) LI, Zhe(李哲) GUO, Qing-Xiang*(郭庆祥)

Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China

A computational study with the B3PW91 density functional theory was carried out on the activation process of palladacycles as catalysts in the Heck reaction. Two possible pathways (i.e. anion reductive cleavage of the Pd—C bond, and olefin insertion into the Pd—C bond followed by β-H elimination) were taken into consideration. Com-putational results indicate that the palladacycles are activated via olefin insertion into the Pd—C bond followed by β-H elimination in the reaction conditions.

Keywords palladacycle, olefin insertion, density functional theory, reductive elimination

Introduction

Palladacycles are a class of organopalladium com-pounds whose Pd centers coordinate with an anionic four-electron (bidentate) or six-electron (tridentate) do-nor ligand (Scheme 1). They are now applied as a new family of catalyst precursors for an array of reactions.1-5 These complexes are often air- and moisture-stable, eas-ily prepared through a C—H activation process, and once catalytically active (usually above 100 ℃), they can attain high turnover numbers.

Scheme 1

Palladacycles were introduced in Heck coupling ca-talysis by Herrmann and Beller in 1995.6,7 In the early research, a catalytic mechanism including Pd(II)/Pd(IV) was proposed.8 However, it is now widely accepted that palladacycles are precatalysts which can slowly release the active Pd(0) species at a rate that is inferior to the catalytic rate and then operates via a traditional Pd(0)/ Pd(II) catalytic cycle.9-12 In 1999, Herrmann reviewed the application of palladacycles to Heck reactions,13 pointing out that acetic acid, which was formed during catalysis, was unable to cleave the Pd—C bond, nor was the NaOAc or olefins. The catalytically active species were ill-defined Pd(0) species. However, in 2005, d’Orlyé et al.14 detected a Pd(0) species generated in situ from a dimeric P,C-palladacycle in cyclic voltam-metry. They proposed the Pd(0) complex formed in an endergonic equilibrium via a reductive elimination (C—OAc bond formation), and the fast backward reaction,

which was an intramolecular oxidative addition of the monophosphine Pd(0) complex, ensured the stability of the P,C-palladacycle structure when the aryl halides were consumed in the Heck reaction. This reaction was carried out in the absence of any reducing agents, as well as olefins and aryl halides being used as reactants in Heck reaction. In the same year, Rosol15 isolated and unequivocally characterized the disassembly product of 1'-carbopalladated-4-ferrocenyl-1,3-oxazolines coupled with the olefin in a non-catalytic Heck reaction, indi-cating that it was the origin of the release of Pd(0) from the palladacycle carrier. At an earlier time of 2001, Beletskaya12 accounted that the olefin insertion into the Pd—C bond of palladacycle, which was proved im-probable for PC-system by Herrmann, may readily happen for the less strongly bonded NC-system. Alacid et al.16 also indicated that the formation of Pd(0) was via insertion of the olefin in the Pd—C bond of the cata-lyst followed by reduction when revisiting oxime pal-ladacycles.

The two main viewpoints of the activation process of palladacycles [i.e. (1) anion reductive cleavage of the Pd—C bond; (2) olefin insertion into the Pd—C bond followed by β-H reduction] could explain different ex-perimental observations (Scheme 2). For example, Herrmann found the reactivity decreased according to the series OAc＞Br＞Cl when performing the reaction of 4-bromoanisole with n-butyl acrylate without base in the presence of three different palladacycyles which differed in their anions, supporting an enhanced activity if the system was more easily reduced thermally. From the first mechanism we can interpret this as the differ-ence made by the different capacity of different anions to reductively cleave the Pd—C bond. As to the second mechanism, heating in the presence of only an olefin

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Scheme 2

resulted in practical Pd black formation if no aryl halide was present. Understanding the activation process of palladacycles can help us better recognize the full cata-lytic mechanism, optimize the reaction conditions, and design a better catalyst. Hence, in this work we present a computational study of the activation process of pal-ladacycles as catalyst in Heck reaction.

Computational details

Calculations were performed at density functional theory (DFT) level, by means of the hybrid B3PW9117-19 functional as implemented in Gaussian 03.20 Optimizations were carried out in solvent through the polarizable continuum model approach using DMF (dielectric constant ε＝38.6, radius＝3.65 Å) as solvent at a system temperature of 403.15 K, using an effective core potential (LANL2DZ) for the inner electrons, and its associated double-ζ basis set for the more external ones.21-24 In the case of Br, P atoms and Pd atom a d-polarization shell or an f-polarization shell was added (exponent 0.438, 0.387 and 1.472 respectively). The structures of the reactants, intermediates, and products were confirmed to be real minima on the potential en-ergy surface by the frequency calculations. Transition states were identified by having one imaginary fre-quency in the Hessian matrix. Solvent effects were in-troduced through PCM single-point calculations on sol-vent phase optimized geometries using LANL2DZ with an added f-polarization shell for Pd atom and 6-31+G(d) basis set for other atoms. DMF was also used as solvent at the system temperature of 403.15 K.25 Relative ener-gies were included in the respective schemes, tables and figures, including ΔGopt (change in free energies simply based on optimized results); ΔGelec (change in free ener-gies in solution without nonelectrostatic contributions) and ΔGsol (change in total free energies in solution).

Results

In the first step, considering the actual existence of the catalyst in solution, we selected three species with different ligands (X＝Br, DMF and vacancy in a1) in the trans position of the methylene, and proposed an

equilibrium existing among them. The substitution of the bromide ligand cis to the methylene in a1B by an olefin (b1) was also taken into account. The relative free energies (ΔGsol) of these species are shown in Scheme 3.

Scheme 3

The anion complex with two bromide ligands (a1B) is the most stable among these species. Dissociating the bromide trans to methylene in a1B causes a rise of en-ergy by 16.3 kJ/mol. The energy associated with the substitution of the bromide by a solvent DMF molecule is 33.9 kJ/mol. Also, the substitution of Br－ in the cis position of the methylene by olefin has an energy cost of 11.3 kJ/mol.

Path A: bromide cleavage of the Pd—C bond

In this proposed mechanism, the bromide gets close to the methylene first while the Pd—C bond and Pd—Br bond are elongated to form an approximate six-membered ring. Then an intramolecular reductive elimination occurs to cleave the Pd—C bond and the Pd—Br bond, whereas a C—Br bond is formed. The ge-ometries of the transition states are shown in Figure 1. In these transition states, the breaking Pd—C and Pd—Br distances are elongated at different degrees mean-while the forming C—Br distances are shortened. For example, in a1B and the corresponding transition state

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TSa(1-2)B, the Pd—C distances are 2.066 and 2.612 Å, the Pd—Br distances are 2.595 and 2.970 Å, and the C—Br distances are 3.297 and 2.474 Å, respectively.

Figure 1 Geometries of the transition states for path A. Dis-tances are in angstroms.

The calculated free energy profile for Path A is shown in Figure 2. Relative free energies at different levels are presented in Table 1. In the view of that the three different values presented for each entry (ΔGopt, ΔGelec and ΔGsol) have the same trend and similar rela-tive values, we will discuss ΔGsol, which is considered more exact for anionic species in the following part. For the three species, a1B, a1D, and a1V, the energy barriers for the intramolecular reductive elimination are 233.2, 238.7 and 205.1 kJ/mol, respectively. The complex with a solvent DMF molecule, a1D, has the highest energy barrier among the three species while that with a va-cancy, a1V, has the lowest energy barrier of 205.1 kJ/mol, which is also a very high energy barrier. More-over, all the three overall reactions give endothermic processes with energetic differences between reactants and products of 146.0, 154.7 and 147.2 kJ/mol, respec-tively. This energy profile indicates that the bromide cleavage of the Pd—C bond is so energy demanding that this reaction is very difficult to proceed.

Figure 2 Reaction pathway and ΔGsol (kJ/mol) for the path A.

Table 1 Relative free energies (kJ/mol) for the path A of Figure 1

ΔGopt

ΔGelec ΔGsol

a1B 0.0 0.0 0.0

TSa(1-2)B 231.8 227.5 233.2

a2B 131.1 145.3 146.0

a1V 0.0 0.0 0.0

TSa(1-2)V 183.1 209.6 205.1

a2V 144.7 150.4 147.2

a1D 0.0 0.0 0.0

TSa(1-2)D 209.7 238.9 238.7

a2D 135.0 156.2 154.7

Path B: olefin insertion into the Pd—C bond fol-lowed by β-H elimination

The process of olefin insertion into the Pd—C bond followed by β-H reduction is similar with the steps after oxidative addition in Heck reaction. The process starts with an η2 olefin complex b1, with the Pd—C bond dis-tances of 2.352 and 2.353 Å. The insertion of the olefin into the Pd—C bond takes place via a concerned four-centered transition state and then results in a seven-membered palladacycle intermediate b2. After-wards, b2 undergoes the β-H elimination and HBr elimination to form the Pd(0). The geometries of the transition states are shown in Figure 3. In the olefin in-sertion transition state TSb(1-2), the breaking Pd—Cmethylene distance is elongated from 2.071 Å in b1 to 2.337 Å, the forming C—C bond distance is 2.010 Å, and the C—C double bond in olefin is elongated from 1.376 Å in b1 to 1.454 Å close to C—C single bond. In the β-H elimination transition state TSb(2-3), the breaking Pd—C distance and C—H distance are 2.236 and 1.606 Å while the forming Pd—H distance is 1.581 Å.

The calculated free energy profile for path B is shown in Figure 4. Relative free energies at different levels are presented in Table 2. Starting with the η2 ole-fin complex b1, the first step, olefin insertion, has an energy barrier of 129.5 kJ/mol followed by formation of the seven-membered palladacycle intermediate b2 with similar energy to that of b1. The energy barrier of the β-H elimination as the second step is 89.5 kJ/mol and noticea- bly lower than that of olefin insertion. The relative energy of the corresponding intermediate b3 is 65.2 kJ/mol. In

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Figure 3 Geometries of the transition states for path B. Dis-tances are in angstroms.

Figure 4 Reaction pathway and ΔGsol (kJ/mol) for the path B.

Table 2 Relative Free energies (kJ/mol) for the path B of Figure 2

ΔGopt

ΔGelec ΔGsol

b1 0.0 0.0 0.0

TSb(1-2) 140.5 133.6 129.5

b2 16.4 1.3 －7.4

TSb(2-3) 102.3 85.8 82.1

b3 70.6 57.2 65.2

b3＋OAc－ 0 0 0

b4＋HOAc＋Br－ －9.4 －32.3 －40.5

the last step, b3 eliminates a HBr to get Pd(0) complex b4. According to our calculation, direct elimination de-mands another energy of 65.6 kJ/mol. Maybe it is be-cause that formation of HBr is not favored in solvent. In fact, experiment is usually carried out in the presence of base. So we took the experimental case in which OAc－ was added as base into our computation (the last two en-tries of Table 2). The relative energy demanding for the elimination is decreased to －40.5 kJ/mol, indicating that this step is an exothermic process. Furthermore, because the OAc－ attack and HOAc and Br－ elimination process may be concerted or alternatively stepwise, we will not discuss the process in detail but believe that it is facile.

Discussion

The activation process of model palladacycle as catalyst in Heck reaction has been analyzed by means of DFT methods. For the first step, different anionic or neutral complexes as starting species in solution were calculated. The relative energies show that the anion complex with two bromide ligands (a1B) is most stable in solution. Maybe it is because that anion complexes are favorable in polar solvent. Substitution of Br－ in the cis position to the methylene in a1B by olefin leads to a slight rise of the energy by 11.3 kJ/mol while dissocia-tion of the bromide trans to methylene in a1B or substi-tution of it by DMF demands higher energies. This is consistent with the experimental observation that the anionic, monomeric palladacycle a1B became main component in the solution as the reaction operated for a period of time.

The analysis of the bromide cleavage mechanism of Pd—C bond shows that the process with a very high energy barrier is difficult to occur whatever the starting species are. That may be the reason why Herrmann has not detected the corresponding product. Even though d'Orlyé detected the product, their experiment should be processed at a high temperature (80 ℃) with an addi-tional P(o-Tol)3 to stabilize the product.

In the olefin insertion followed by a β-H elimination mechanism, the energy barriers of the two steps are 129.5 and 89.5 kJ/mol, in which the higher one is much lower than that in bromide cleavage mechanism. In the last step of HBr elimination, we found that the added base was necessary to reduce the energy. This finding can be the explanation to Herrmann’s experiment men-tioned in the second section. When the same Heck reac-tion was performed without base in the presence of three different palladacycles which differed in their anions, the reactivity decreased according to the series OAc＞Br＞Cl in respect to that the stability and extent of formation in solution followed the series HOAc＞HBr＞HCl in the last step of Pd(0) formation. Accord-ing to our computation, the direct HBr elimination de-mands so high energy that the step would be rate-determining in this mechanism.

Computational results support the pathway in which olefin insertion followed by β-H elimination is the activa-

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tion process of palladacycles as catalysts in Heck reaction. This is not conflicting with d'Orlyé’s conclusion because their experiment was carried out without olefin. Never-theless, the activation energy of this pathway is also higher than that in the catalytic cycle in previous study, which is consistent with the viewpoint that palladacycles slowly release the active Pd(0) species at a rate that is inferior to the catalytic rate. Furthermore, we can give new explanation to those previous experimental observa-tions. In Herrmann’s study, electron-withdrawing groups of 4-substituted styrenes increased the yield of corre-sponding stilbene in the reaction with 4-brommoanisole catalyzed by a palladacycle, so they suggested that the rate-determining step was not the oxidative addition of the actual palladium catalyst, but rather the insetion of the olefin into the arylpalladium intermediate.26 We thought that maybe the 4-substituted styrene with an electron-withdrawing group is favorable in insertion into the Pd—C bond followed by β-H elimination.

Conclusion

The activation process of palladacycles as catalyst in Heck reaction has been analyzed by means of B3PW91 DFT calculations. Two main mechanisms for the activa-tion process as shown in Scheme 2 were taken into con-sideration. The results suggest that palladacycles be ac-tivated via olefin insertion into a Pd—C bond followed by β-H elimination in the reaction conditions. Based on the conclusion, we can optimize the releasing rate of active Pd(0) species from palladacycles by choosing appropriate olefins and palladacycles to achieve satis-fying catalytic results.

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(E0706172 LI, L. T.; ZHENG, G. C.)