Article pubs.acs.org/Organometallics
Mechanism of Ruthenium-Catalyzed Direct Arylation of C−H Bonds in Aromatic Amides: A Computational Study Chunhui Shan,†,§ Xiaoling Luo,†,‡,§ Xiaotian Qi,† Song Liu,† Yingzi Li,† and Yu Lan*,† †
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, People’s Republic of China School of Chemistry, Chongqing Normal University, Chongqing 400415, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Ruthenium-catalyzed arylation of ortho C−H bonds directed by a bidentate 8-aminoquinoline moiety not only is important to construct new biaryl derivates but also merges important research areas. In this study, the density functional theory (DFT) method M11-L was employed to predict the mechanism of this C−H bond arylation reaction. The computational results indicate that the initial step for this reaction is catalyst loading by electrophilic deprotonation to generate a substrate-coordinated Ru(II) intermediate, which is the key compound in the complete catalytic cycle. The catalytic cycle includes electrophilic deprotonation by carbonate, oxidative addition of bromobenzene, reductive elimination to form a new aryl−aryl bond, proton transfer to release the product, and ligand exchange to regenerate the initial Ru(II) intermediate. Theoretical calculations suggest that the oxidative addition of bromobenzene is the ratedetermining step of the whole catalytic cycle, and the apparent activation free energy is 32.7 kcal/mol. The ligand effect was considered in DFT calculations, and the calculated results agree well with experimental observations.
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INTRODUCTION Functionalization of aromatic compounds via catalytic C−H bond activation by taking advantage of an 8-aminoquinoline directing group has been extensively explored in recent years.1 In these reported works, regioselective direct conversion of C− H bonds to C−C bonds has opened new possibilities in the field of catalytic activation of C−H bonds. Transition-metalcatalyzed C−H functionalization remains a significant strategy for the construction of these synthetic targets.2 Transitionmetal catalysts initially based on palladium or rhodium complexes2b,3 were found to realize C−C bond construction via direct C−H bond activation under mild conditions. However, it is well-known that other transition-metal complexes, such as those of ruthenium, iridium, and nickel, are also highly efficient catalysts in C−H bond functionalizations.4 Among these, ruthenium catalysts have attracted considerable attention in the catalytic functionalization of C− H bonds. The introduction of aryl groups followed by the cleavage of C−H bonds has been widely used to construct biaryl derivatives.2d,5 Daugulis and co-workers reported the Pd(II)catalyzed arylation of unactivated sp3 C−H bonds,1a and the bidentate directing groups picolinamide,6 8-aminoquinoline,1b,7 N-(2-pyridylsulfonyl),8,7d sulfixamine,9 and 2-methylthioaniline10 have been subsequently investigated for C−H bond functionalization. Recently, the Chatani group reported a novel Ru(II)-catalyzed ortho arylation of aromatic amides with a bidentate directing group by the cleavage of a C−H bond (Scheme 1).11 In this reaction, 8-aminoquinoline acts as a © XXXX American Chemical Society
bidentate directing group and coordinates in an N,N fashion to the ruthenium center. Scheme 1. Ru(II)-Catalyzed C−H Bond Functionalization of Aromatic Amides with Phenyl Bromide
In our previous study on the mechanism of rhodiumcatalyzed C−H bond activation, we found that the reaction mechanism contains three steps: electrophilic deprotonation, oxidation, and reductive elimination. However, the order of these three steps is not always the same.12 For example, in Rh2(OAc)4-catalyzed oxidation of toluene, the active catalyst Rh2(OAc)4 is first oxidated by Selectfluor, followed by electrophilic deprotonation with a Rh(III) intermediate.12b Special Issue: Organometallics in Asia Received: January 25, 2016
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DOI: 10.1021/acs.organomet.6b00064 Organometallics XXXX, XXX, XXX−XXX
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Organometallics On the basis of this idea, we believe that the order of electrophilic deprotonation, oxidation, and reductive elimination is important and needs to be clarified in transition-metalcatalyzed C−H bond activation reactions. Here, we focus on the mechanism of Ru(II)-catalyzed C−H bond functionalization of aromatic amides with phenyl bromide. Generally, there are four steps in this reaction: N− H bond cleavage, C−H bond cleavage, oxidative addition of bromobenzene, and reductive elimination to form the C−C bond. Three plausible pathways are proposed in Scheme 2.
Scheme 3. Proposed Mechanisms for Ru(II)-catalyzed C−H Bond Arylation Directed by the Bidentate Group
Scheme 2. Mechanism of Ru(II)-Catalyzed C−H Functionalization
When oxidative addition of bromobenzene takes place first to form a Ru(IV) intermediate, subsequent cleavage of the N−H bond and C−H bond affords another Ru(IV) intermediate. The biaryl product and active catalyst can then be generated by reductive elimination. Alternatively, N−H bond cleavage could occur first to generate a substrate-contained Ru(II) intermediate. The corresponding C−H bond cleavage and oxidative addition could then generate the same Ru(IV) intermediate, although the order of the last two steps may be different. The mechanism of this Ru(II)-catalyzed C−H bond functionalization reaction remains unclear because of the difficulty in observing transient Ru(IV) intermediates. We have performed density functional theory (DFT) calculations to investigate the mechanism of the Ru(II)-catalyzed C−H bond functionalization reaction.
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deprotonation by carbonate occurs to generate the rutheniumcontaining [3.3.0] heterocycle D. The following release of bicarbonate and oxidative addition of bromobenzene afford Ru(IV) intermediate F. In pathway a-1-2, the oxidative addition of bromobenzene on complex D generates intermediate F. Subsequent reductive elimination forms a new C−C bond in intermediate K, which could react with amide B to generate product L. After ligand exchange, active intermediate C is regenerated to complete the catalytic cycle. In this procedure, the order of C−H bond cleavage and oxidative addition could be swapped, which is called pathway a-2. Alternatively, oxidative addition by bromobenzene could occur initially in pathway b. N−H bond cleavage by carbonate could also then afford the same intermediate G. All of these pathways were studied by DFT calculations in this work. Free energy profiles for the initiation steps of pathways a-1 and a-2 for the Ru(II)-catalyzed C−H bond arylation reaction are shown in Figure 1. All energies are with respect to ruthenium complex 1, which would be generated from the [RuCl2(p-cymene)]2 catalyst by reaction with Na2CO3 and triphenylphosphine. The coordination of quinoline 2 to the catalyst forms the intermediate 3 in an endothermic process with 9.8 kcal/mol free energy and simultaneously releases one molecule of p-cymene. Subsequently, the quinoline-directed carbonate-assisted electrophilic deprotonation proceeds through transition state 4-ts with a free energy increase of 19.8 kcal/mol. This step results in the exothermic formation of intermediate 5. The ligand exchange with carbonate then generates complex 6 irreversibly, which is the common species involved in both pathways a-1 and a-2. The free energy profile for the C−H bond cleavage and oxidative addition steps via pathway a-1 or a-2 is shown in Figure 2. For pathway a-1, from complex 6, intramolecular deprotonation by the carbonate moiety occurs through transition state 7-ts with an energy barrier of 20.6 kcal/mol, leading to the formation of inactive intermediate 8. Complex 9 is generated from intermediate 8 with the release of bicarbonate (path a-1-1). The coordination of bromobenzene then leads to
COMPUTATIONAL METHODS
All of the DFT calculations were performed using the Gaussian 09 series of programs.13 The B3-LYP14 density functional with the standard 6-31G(d) basis set (SDD basis set for Ru) was used in geometry optimizations. Harmonic vibrational frequency calculations for all stationary points were conducted to confirm the stationary points are local minima or transition structures and to derive the thermochemical corrections for the enthalpies and free energies. The M11-L functional recently proposed by Truhlar et al.15 was employed in the solvation single-point energy calculations to give more accurate energetic information.16 The solvent effects were taken into consideration using single-point calculations based on the gas-phase stationary points with the SMD continuum solvation model.17 The basis set used in the solvation single-point calculations was 6311+G(d,p) (SDD basis set for Ru). The energies reported in this paper are the M11-L calculated Gibbs free energies in toluene solvent, and the values given in parentheses are the B3-LYP calculated Gibbs free energies in the same solvent.
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RESULTS AND DISCUSSION As shown in Scheme 3, four possible pathways were considered (pathways a-1-1, a-1-2, a-2, and b). The initiation step for both pathways a-1 and a-2 is deprotonation of amide B by carbonate to form Ru(II) intermediate C. In pathway a-1-1, electrophilic B
DOI: 10.1021/acs.organomet.6b00064 Organometallics XXXX, XXX, XXX−XXX
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the formation of intermediate 10 with an energy barrier of 5.8 kcal/mol, which can be attributed to entropy loss. Oxidative addition via transition state 11-ts then generates Ru(IV) intermediate 12. The energy barrier for oxidative addition from intermediate 10 via transition state 11-ts is only 6.1 kcal/mol. However, the relative free energy of intermediate 10 is 26.6 kcal/mol higher than that of intermediate 6. Therefore, the overall energy barrier for the oxidative addition step is 34.2 kcal/mol, which suggests that the oxidative addition step is the rate-determining step and a high reaction temperature is necessary because of the high activation free energy.18 The calculated results are in accord with the experimental observations. In related experimental reports, this reaction takes place at as high as 130 °C, and the rate-determining step is not the C−H bond cleavage step.11 On the other hand, the coordination of bromobenzene to intermediate 8 would form intermediate 28 (path a-1-2). Oxidative addition via transition state 29-ts generates intermediate 30 with an energy barrier of 30.4 kcal/mol. The overall activation free energy for the oxidative addition of 8 is 39.8 kcal/mol, and the relative free energy of transition state 29-ts is 11.7 kcal/mol higher than that of 11-ts. The bond length of the breaking C−Br bond in 29-ts is 0.11 Å longer than that of 11-ts, which implies that the transition state 29-ts occurred later than 11-ts along the reaction coordinate, thereby resulting in the higher energy of 29-ts. Therefore, pathway a-1-2 is energetically unfavorable. The oxidative addition and C−H bond cleavage pathway was also considered (pathway a-2). The coordination of bromobenzene forms intermediate 18 with an energy barrier of 23.1 kcal/mol.
Figure 1. Initiation steps of the Ru(II)-catalyzed C−H bond arylation reaction. The bond lengths in the geometries are given in angstroms.
Figure 2. Free energy profile of the C−H bond cleavage and oxidative addition steps of the Ru(II)-catalyzed C−H bond arylation reaction. The bond lengths are given in angstroms. C
DOI: 10.1021/acs.organomet.6b00064 Organometallics XXXX, XXX, XXX−XXX
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molecule of product 16, the substrate is loaded in intermediate 17. Ligand exchange with carbonate then regenerates active species 6 to accomplish the complete catalytic cycle. In summary, the catalyst loading part included coordination of the substrate, N−H bond cleavage, and carbonate exchange. Following these steps, active catalytic intermediate 6 is generated. The catalytic cycle starts from intermediate 6 and contains the following steps: electrophilic deprotonation, oxidative addition by bromobenzene, reductive elimination, and proton transfer to give the product. After these steps, active species 6 is regenerated. As shown in Figure 4, pathway b was also investigated by DFT calculations. The ligand exchange of catalyst 1 with bromobenzene forms intermediate 22 with an energy increase of 2.5 kcal/mol. Oxidative addition via transition state 23-ts affords Ru(IV) intermediate 24 with an activation free energy of 31.5 kcal/mol. After loading of substrate 2, intermediate 25 is formed with an energy increase of 21.8 kcal/mol. N−H deprotonation then takes place through transition state 26-ts. Ligand exchange then generates intermediate 20 (the same intermediate as pathway a-2). The DFT calculations suggest that the overall activation free energy of the N−H bond cleavage step reaches up to 51.0 kcal/mol because of the high relative free energy of precursor 25. Therefore, pathway b is energetically unfavorable. The ligand effect of the Ru(II)-catalyzed C−H bond arylation reaction was also investigated. Figure 5 shows a comparison of the reaction involving triphenylphosphine (black) and the ligand-free reaction (blue). In this comparison, triphenylphosphine-coordinated intermediate 6 and ligand-free intermediate 6a are set to zero energy in the respective reactions. The activation free energy of the C−H bond cleavage step through 7a-ts in this ligand-free reaction is 34.5 kcal/mol, which is 13.9 kcal/mol higher than that of the triphenylphosphine-assisted reaction. The higher activation free energy of the C−H cleavage step can be attributed to the 14-electron configuration of ruthenium in 7a-ts. However, the electron configuration of ruthenium in the corresponding transition state 7-ts is 16e, owing to the coordination of triphenylphosphine. The relative free energy of 9a is also 12.6 kcal/mol higher than that of corresponding intermediate 9, which is also ascribed to the 12-electron configuration of ruthenium in intermediate 9a. Although the oxidative addition of inter-
Oxidative addition via transition state 19-ts then gives Ru(IV) intermediate 20 with the release of a bromide ion. The overall activation free energy for the oxidative addition of 6 via transition state 19-ts is 49.3 kcal/mol, and the relative free energy of 19-ts is 16.6 kcal/mol higher than that of 11-ts. Therefore, pathway a-2 is also energetically unfavorable. As depicted in Figure 3, when Ru(IV) intermediate 12 is formed, reductive elimination rapidly occurs to form a new C−
Figure 3. Reductive elimination and proton transfer steps of the Ru(II)-catalyzed C−H bond arylation. The bond lengths are given in angstroms.
C bond. The relative free energy of intermediate 14 is 43.6 kcal/mol lower than that of intermediate 12. Therefore, the reductive elimination is irreversible. Intermediate 14 reacts with reactant 2 through transition state 15-ts with an activation free energy of 17.1 kcal/mol. In this transition state, proton transfer takes place from reactant to product. After release of one
Figure 4. Free energy profile for pathway b of the Ru(II)-catalyzed C−H bond arylation. The bond lengths are given in angstroms. D
DOI: 10.1021/acs.organomet.6b00064 Organometallics XXXX, XXX, XXX−XXX
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Figure 5. Free energy profiles showing the ligand effect on the Ru(II)-catalyzed C−H bond arylation. The bond lengths are given in angstroms.
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mediate 9a is easier than that of intermediate 9, the overall activation free energy of the rate-determining step for the ligand-free reaction is still 4.1 kcal/mol higher than that of the triphenylphosphine-assisted reaction. The computational result is consistent with experimental conclusions.
Corresponding Author
*Y.L.: tel, +86-18680805840; fax, +86-023-65111067; e-mail,
[email protected].
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Author Contributions §
These authors contributed equally to this work.
CONCLUSION The DFT method M11-L has been employed to elucidate the mechanism of the ruthenium-catalyzed direct arylation of C−H bonds in aromatic amides reported by Chatani et al. Our results show a clear preference for pathway a-1-1, which consists of an initial procedure and a catalytic cycle. In the initial procedure, coordination of the substrate, N−H bond cleavage, and carbonate exchange give the active catalytic species 6, which is the starting point in the catalytic cycle. The catalytic cycle includes electrophilic deprotonation by carbonate, oxidative addition of bromobenzene, reductive elimination to form a new aryl−aryl bond, proton transfer to release the product, and ligand exchange to regenerate active species 6. The ratedetermining step of this pathway was found to be oxidative addition of bromobenzene, and the apparent activation free energy is 34.2 kcal/mol. These results are consistent with experimental observations. We also evaluated the ligand effect for this ruthenium-catalyzed arylation reaction using the triphenylphosphine ligand. The computational results suggest that the apparent activation free energy is 4.1 kcal/mol lower in the triphenylphosphine-assisted reaction (34.2 kcal/mol) than in the ligand-free reaction (36.8 kcal/mol). Reported experimental results also show that addition of triphenylphosphine can increase the yield of products.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Science Foundation of China (Grant Nos. 21372266 and 51302327). We are also thankful for the project (No.106112015CDJZR228806) supported by the Fundamental Research Funds for the Central Universities (Chongqing University).
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00064. Energies of all reported structures (PDF) Cartesian coordinates of the calculated structures (XYZ) E
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