Reactivity and Regioselectivity of Palladium-Catalyzed Direct Arylation

Jun 27, 2012 - ... of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, ... nation (CMD) pathway was evaluated for both noncooperative...
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Reactivity and Regioselectivity of Palladium-Catalyzed Direct Arylation in Noncooperative and Cooperative Processes Serge I. Gorelsky* Center for Catalysis Research and Innovation and Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, K1N 6N5 Canada S Supporting Information *

ABSTRACT: Recent discovery (J. Am. Chem. Soc 2012, 134, 3683) of the involvement of the cyclometalated [Pd (tBu2PCMe2CH2)(OAc)]2 complex in direct arylation of pyridine N-oxide suggested that the mechanism of this reaction may involve a process in which C−H activation occurs at one Pd center and the aryl group undergoes coupling with another aryl group at a second Pd center (a cooperative catalysis). In this work, cleavage of arene C−H bonds of different (hetero)arenes via a concerted metalation−deprotonation (CMD) pathway was evaluated for both noncooperative and cooperative processes so that the two processes could be compared in terms of reactivity and regioselectivity. The distortion−interaction analysis was performed to quantify the various contributions to the CMD transition states. Calculated barriers of the C−H bond cleavage in the two processes indicate that the cooperative and noncooperative processes lead to the same regioselectivity of arylation. Differences in contributions to the activation barriers between the two processes are fairly minor. This allows us to use the existing data about (hetero)arene C−H reactivity and regioselectivity in the noncooperative arylation and apply it to predict reactivity and regioselectivity of arylation in the cooperative process.

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barriers. 24 On the basis of the distortion−interaction contributions, arenes can be divided into three different categories to allow an easier understanding of their reactivity. Class I includes (hetero)arenes for which the regioselectivity of C−H bond metalation is controlled by the arene distortion energies, Edist(ArH). Class II includes (hetero)arenes for which catalyst−arene electronic interaction energies define the most reactive C−H bonds. Class III includes (hetero)arenes for which both the arene distortion and catalyst−arene interaction energies contribute to make a particular C−H bond most reactive. Metal coordination to heteroarenes also can be used to enhance and tune the CMD reactivity.29 Recently, Hartwig and co-workers conducted a mechanistic investigation of the direct arylation of pyridine N-oxide using palladium−acetate catalysts in toluene.21 They proposed that the cyclometalated species [Pd(tBu2PCMe2CH2)(OAc)], which

ransition-metal-catalyzed functionalization of (hetero)arene C−H bonds has emerged over the past few years as a rapidly growing and increasingly reliable alternative to traditional cross-coupling reactions.1−11 In direct arylation, the reaction of aryl halides with (hetero)arenes to form (hetero)biaryls, the functionalization of C−H bonds depends on the directing groups or relies on the intrinsic C−H bond reactivity parameters such as C−H bond acidities and nucleophilicities of carbon sites. Mechanistic studies12−22 on palladium-catalyzed direct arylation and related arene C−H bond cleavage reactions supported the concerted metalation−deprotonation (CMD) mechanism (Scheme 1).11,23,24 In a CMD transition state, proton abstraction from a C−H bond by a carboxylate ligand occurs while a metal−carbon bond is being formed. Instead of CMD, alternative names such as ambiphilic metal ligand activation25 and internal electrophilic substitution26 have also been used in the literature. The CMD pathway is at play in C− H bond cleavage by PdII−acetate complexes for a very wide range of (hetero)arenes, including electron-rich (hetero)arenes.23,24 The calculated activation barriers predicted accurately the regioselectivity observed experimentally for the palladium-catalyzed direct arylation. They are also in agreement with the relative reactivity of the various (hetero)arenes.27 The recent report that C3-selective arylation of pyridines28 can be achieved using a PdII−acetate catalyst and pyridine is more reactive than benzene is also consistent with the CMD mechanism.29 The distortion−interaction analysis of C−H bond cleavage using the [Pd(C6H5)(PR3)(OAc)] catalyst allowed quantification of various contributions to the activation © 2012 American Chemical Society

Received: March 20, 2012 Published: June 27, 2012 4631

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contains the phosphine as a ligand and participates in oxidative addition of aryl halide and in reductive elimination of biaryl. Such a cooperative mechanism opens up new ways to tune C− H bond functionalization. Since the current research is aimed at finding direct arylation conditions under which it is possible to enhance the reactivity of nonactivated (hetero)arenes and to switch reactivity from one C−H bond to another, it is important to investigate if the regioselectivity of (hetero)arene C−H bond cleavage in the cooperative process catalyzed by the cyclometalated species is different from the regioselectivity of (hetero)arene C−H bond cleavage in the noncooperative CMD process catalyzed by [Pd(C6H5)(PR3)(OAc)]. The CMD barriers for cleavage of C−H bonds of a range of arenes using the palladium−acetate catalyst [Pd(C6H5)(PMe3)(OAc)] have been evaluated by density functional theory (DFT) with the B3LYP31,32 exchange-correlation functional (Figures 1 and 2 and Table 1) and have been reported

Scheme 1. Noncooperative and Cooperative Mechanisms for Direct Arylation of Arenes using Palladium-Acetate Catalysts (R = tBu)

Figure 1. Lowest energy CMD TS structures for C−H bond cleavage at the C2 site of pyridine N-oxide using the [Pd(C6H5)(PMe3)(OAc)] catalyst (A) and the [Pd(Me2PCMe2CH2)(OAc)] catalyst with the Pd−C(arene) bond in positions trans (B) and cis (C) to the Pd−P bond. Relevant bond distances (Å) are shown. H atoms not involved in the reaction step are not shown for clarity. Red arrows indicate atomic movements that correspond to a normal mode with an imaginary frequency in the direction toward products.

is generated from the palladium−acetate dimer (eq 1; R = t Bu),30 acts as a catalyst in the C−H bond cleavage (Scheme 1). Our calculations indicate that the formation of the monomer [Pd(Me 2 PCMe 2 CH 2 )(OAc)] from the dimer [Pd(Me2PCMe2CH2)(OAc)]2 requires 3.3 kcal mol−1 at 298 K (ΔGr,298 K) but the dimer−monomer equilibrium (eq 1, ΔSr = 75.5 cal K−1 mol−1) is expected to shift to the right at temperatures greater than 75 °C. The cyclometalated palladium−acetate dimer forms as a result of a reaction between palladium acetate and PtBu3 and from decomposition of the PtBu3-ligated arylpalladium acetate complex (eq 2).

previously.23,24,29 The CMD barriers for cleavage of C−H bonds of thirteen of these arenes using the [Pd(Me2PCMe2CH2)(OAc)] catalyst21 were evaluated at the same level of theory as before (Figures 1 and 2 and Table 1). The CMD barriers for cleavage of C−H bonds of six additional substrates (pentachlorobenzene, pyrrole, 1-methylpyrrole, pyridazine N-oxide, pyrimidine N-oxide, and 1-methyl1,2,3-triazole) using the [Pd(C6H5)(PMe3)(OAc)] and the [Pd(Me2PCMe2CH2)(OAc)] catalysts21 were evaluated at the same level of theory (Figures 1 and 2). Due to the less symmetrical structure of the [Pd(R2PCMe2CH2)(OAc)] catalyst (four atoms of the metallacycle (Pd, P, and two carbons) do not lie in the same plane and the Pd−C(arene) bond being formed in the CMD transition states can be in a position cis or trans to the phosphine ligand), several transition state structures have to be calculated for each (hetero)arene substrate in order to find the lowest energy CMD barrier. The CMD barriers for cleavage of C−H bonds of the new heteroarenes are in agreement with the experimentally observed regioselectivity of Pd-catalyzed arylation: C2 for pyrroles,40−44 C5 for 1-methyl-1,2,3-triazole,44 and C6 for pyridazine Noxide45 and pyrimidine N-oxide45 (Figure 2). The analysis of the distortions and interactions in the CMD transition state structures for each C−H bond (distortion− interaction analysis)33−35 was performed (Table S1 in the Supporting Information). This analysis quantifies different contributions to the CMD barrier (Figure S2 in the Supporting

This reaction proceeds by an intermolecular CMD step in which the acetate ligand abstracts the proton from the C−H bond of the phosphine ligand (Figure S1 in the Supporting Information). The calculated activation free energy of this reaction step, 26.8 kcal mol−1, is comparable to the activation energies of C−H bond cleavage of many (hetero)arenes (see below). Thus, if the reaction temperature is high enough to overcome this reaction barrier, the phosphine−acetate complex can be converted to the cyclometalated species. Harwig and coworkers proposed21 that C−H activation in the cooperative process occurs at one Pd center (the cyclometalated site, Scheme 1) and the aryl group undergoes coupling with another aryl group at another Pd center. This second palladium center 4632

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Figure 2. Gibbs free energies of activation (ΔG⧧298 K, kcal mol−1) for C−H bond functionalization of arene substrates via the CMD pathway using the [Pd(C6H5)(PMe3)(OAc)] (values shown in black) and [Pd(Me2PCMe2CH2)(OAc)] (values shown in blue) catalysts. Data for the most reactive C−H bonds are shown in boldface.

[Pd(Me2PCMe2CH2)(OAc)] catalyst (Figure 2). Thus, the most reactive C−H bond for a given (hetero)arene remains the same despite a change in the palladium−acetate catalyst. In agreement with Hartwig’s findings for pyridine N-oxide,21 the Gibbs activation barriers (ΔG⧧298 K) for the cleavage of the most reactive C−H bonds of the (hetero)arenes using the [Pd(Me2PCMe2CH2)(OAc)] catalyst are 0.2−2.3 kcal mol−1 lower than the corresponding barriers for C−H bond cleavage using the [Pd(C6H5)(PMe3)(OAc)] catalyst. For pyridine N-oxide, the difference in Gibbs free energies of activation (ΔΔG⧧298 K) for the two pathways is only 0.7 kcal mol−1 (B3LYP31,32 calculations) and 1.1 kcal mol−1 (CAMB3LYP37 calculations) (Table 1) and both pathways result in arylation at the C2 site. In this comparison, the energy of the dimer−monomer equilibrium (eq 1) is not included in the calculation of the CMD barrier for the cooperative pathway. Calculations using the CAM-B3LYP functional37 (in which the treatment of long-range interactions is improved) instead of the B3LYP functional31,32 result in ∼2 kcal mol−1 lower activation energies for the both pathways (Table 1). A higher ΔG⧧298 K value (33 kcal mol−1) was obtained for the C−H bond cleavage at the C2 site in pyridine N-oxide using the [Pd(C6H5)(PtBu3)(OAc)] complex.21 In comparison to a reaction with [Pd(C6H5)(PMe3)(OAc)], a higher CMD activation energy with the [Pd(C6H5)(PtBu3)(OAc)] catalyst partially originates from the deficiencies of the B3LYP exchange-correlation functional to properly account for long-range dispersion interactions.38,39 These interactions are more important to consider in larger, sterically crowded structures. The CAMB3LYP calculations for C−H bond cleavage at the C2 site in pyridine N-oxide using the [Pd(C6H5)(PtBu3)(OAc)] complex results in a ΔG⧧298 K value of 31.4 kcal mol−1, while the ΔG⧧298 K value for cleavage with [Pd(tBu2PCMe2CH2)(OAc)] is the same as with the [Pd(Me2PCMe2CH2)(OAc)] catalyst (24.4 kcal mol−1). On comparison of the CMD reaction steps with [Pd(Me2PCMe2CH2)(OAc)] and [Pd(C6H5)(PMe3)(OAc)], the electronic energy barrier (ΔE⧧) is 0.6−1.1 kcal mol−1 lower for the C−H bond cleavage with the [Pd(C6H5)(PMe3)(OAc)] catalyst (Table 1). The C−H bond cleavage with [Pd(Me2PCMe2CH2)(OAc)] benefits from a lower arene

Table 1. Energies and Distortion−Interaction Analysis (kcal mol−1) for CMD Transition State Structures of Pyridine NOxide using the [Pd(C6H5)(PMe3)(OAc)] (1) and [Pd(Me2PCMe2CH2)(OAc)] (2) Catalysts cat.

position

1

C2 C3 C4 C2 C3 C4

2f

ΔG⧧298 K 27.2e 31.1 30.5 26.5g 30.0 29.8

ΔE⧧ a

Edist (PdL)b

Edist (ArH)

Eintc

B(Pd− C)d

16.5e 21.6 20.8 17.1g 20.7 19.9

15.5 15.7 16.5 17.5 17.9 19.0

36.8 40.1 42.8 34.1 37.3 39.4

-35.7 −34.2 −38.5 -34.5 −34.5 −38.6

0.434 0.473 0.511 0.380 0.456 0.505

Electronic energy (kcal mol−1) of the transition state. bDistortion energy (kcal mol−1) for the palladium−acetate catalyst. cElectronic interaction energy (kcal mol−1) between the palladium−acetate catalyst and the arene fragment. dMayer bond order36 between the PdII atom and the C site of the arene substrate. eIn the CAM-B3LYP calculations, ΔG⧧298 K and ΔE⧧ are 25.5 and 14.1 kcal mol−1, respectively. fThe trans configuration (Figure 1B) is lowest in energy. However, the Gibbs free energy difference of the cis and trans configurations (Figure 1) is only 0.1 kcal mol−1. gIn the CAM-B3LYP calculations, ΔG⧧298 K and ΔE⧧ are 24.4 and 15.2 kcal mol−1, respectively. a

Information): (a) the energetic cost (distortion energy, ΔEdist) associated with the distortion of the palladium complex and the arene from their ground state structures to their geometries in the TS structure and (b) the energy gain (interaction energy, ΔEint) resulting from the electronic interaction of distorted catalyst and arene fragments to form the TS structure. Pyridazine and pyrimidine N-oxides belong to class I (hetero)arenes, for which the distortion energy of the arene controls the regioselectivity of arylation. Pyrroles and 1-methyl1,2,3-triazole belong to class III (hetero)arenes. The calculated CMD barriers for (hetero)arene C−H bonds using the [Pd(C6H5)(PMe3)(OAc)] and [Pd(Me2PCMe2CH2)(OAc)] catalysts indicate that, for this set of substrates, the regioselectivities of arylation are identical between the noncooperative process with the [Pd(C6H5)(PMe3)(OAc)] catalyst and the cooperative process with the 4633

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distortion energy (for the C2 site: 36.8 kcal mol−1 for [Pd(Me2PCMe2CH2)(OAc)] and 34.1 kcal mol−1 for [Pd(C6H5)(PMe3)(OAc)]) and suffers from a higher catalyst distortion energy (for the C2 site: 17.5 kcal mol−1 for [Pd(Me2PCMe2CH2)(OAc)] and 15.5 kcal mol−1 for [Pd(C6H5)(PMe3)(OAc)]). The regioselectivity of arylation is determined by the low distortion energy of pyridine N-oxide at the C2 site (Table 1). If nucleophilicity were a determining factor for the regioselectivity of pyridine N-oxide arylation, functionalization at the C4 site would be expected (the most negative value for the catalyst−arene interaction energy and the highest Pd−C(arene) bond order in the corresponding CMD TS structure; Table 1). In summary, the calculated CMD barriers of the C−H bond cleavage for 19 (hetero)arenes in the cooperative and noncooperative processes indicate that the two processes have very similar activation energies for C−H bond cleavage and lead to the same regioselectivity of direct arylation. Differences in contributions to the activation barriers between the two processes are fairly minor. We expect that these findings will apply to direct arylation reactions of other (hetero)arenes. This allows one to take the existing data about (hetero)arene C−H reactivity and regioselectivity in the noncooperative arylation24,29 and apply it to predict reactivity and regioselectivity of arylation in the cooperative process.

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ASSOCIATED CONTENT

S Supporting Information *

Computational details, figures, and a table giving atomic coordinates and absolute energies of CMD transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Center for Catalysis Research and Innovation (CCRI) and the University of Ottawa for supporting this work.

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REFERENCES

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