Article pubs.acs.org/JPCA
Theoretical Investigations on Rh(III)-Catalyzed CrossDehydrogenative Aryl−Aryl Coupling via C−H Bond Activation Dan Zhao,*,†,‡ Xiaoxi Li,‡,§ Keli Han,†,‡ Xingwei Li,⊥ and Yong Wang*,‡ †
School of Chemistry, Dalian University of Technology, Dalian 116024, People’s Republic of China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ⊥ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China § State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: The reaction mechanism of Rh(III)-catalyzed cross-dehydrogenative aryl−aryl coupling between benzamides and haloarenes was investigated through detailed density functional theoretical (DFT) studies in terms of regioselectivity and deuterium kinetic isotope effects (KIEs). Three possible routes including one PivO−-assisted reaction route and two non-PivO−-assisted reaction routes have been studied. The calculated results refute the proposed mechanism (without PivO−-assisted process) in the experimental paper and demonstrate that the PivO−-assisted reaction mechanism is the most favored. Meanwhile, the calculation revealed that the PivO− anion plays a crucial role as a proton acceptor in the C−H bond activation, especially when the second C−H activation of haloarenearene proceeds via a SE3 mechanism. The SE3 mechanism is presented for the Rh(III)-catalyzed aryl−aryl reaction for the first time. Our mechanism is evaluated by the calculations of the para-/meta-regioselectivity and KIEs. And it is found that the second activation process is the rate-determining step of the whole catalytic cycle. All these calculated properties agree well with the experiment and Glorius’s proposal that the Rh(III)-catalyzed cross-dehydrogenative C−C coupling reaction proceeds by dual C−H activations. Our theoretical studies suggest that the Rh(III) complex catalyst strongly affects the mechanisms of the second C−H activation step and thus this work might provide insight into the design of new catalytic systems.
1. INTRODUCTION The formation of carbon−carbon bonds has become one of the most essential and fundamental research topics because this allows the construction of many important biaryl frameworks in pharmaceuticals and in organic electronic materials. Driven by this need, an increasing number of researchers have focused on the methodologies of C−C bond formation.1,2 Using C−H bonds as starting materials under metal catalysis for this purpose has been recognized as an advantageous strategy owing to the abundance of arenes and the high atom- and stepeconomy associated.3−12 Various transition metal compounds (Pd, Ru, Rh, Ir) are known to effectively activate the aryl C−H bonds for aryl−aryl coupling or aryl−alkynyl coupling.13−18 Recently, C−H activation catalyzed by stable Rh(III) complexes has been extensively studied.19−22 Fagnou, Miura and Satoh, Glorius, Li and Rovis have reported Rh(III)catalyzed oxidative coupling of arenes with unsaturated susbtrates.23−30 Notably, Glorius and co-workers developed Rh(III)-catalyzed oxidative aryl−aryl (Ar−Ar) bond formation between benzamides and haloarenes via a 2-fold direct C−H bond activation pathway (Scheme 1).2 Importantly, catalytic cross-dehydrogenative coupling reactions represent a wastereducing and step-economic process. However, the construc© 2015 American Chemical Society
Scheme 1. Dehydrogenative Cross-Coupling Reaction
tion of biaryl building blocks catalyzed by Rh(III) complexes lagged behind. The rapid growth of catalytic systems in rhodium catalysis inspired us to elucidate the mechanism of this important dual C−H bond activation process. The computational studies in this regard might serve to broaden the scope and patterns of rhodium-catalyzed C−H activation systems. In Glorius’s mechanistic proposal (Scheme 2), there is a dual C−H bond activation for the Rh(III)-catalyzed aryl−aryl (Ar− Ar) system. The first C−H activation may include a σ-bond metathesis or concerted metalation−deprotonation (CMD)31−35 mechanism, with the help of a PiVO− as proton Received: November 18, 2014 Revised: February 3, 2015 Published: February 18, 2015 2989
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The Journal of Physical Chemistry A Scheme 2. Possible Mechanisms Proposed by Glorius et al.2
the weak interaction of dimer, we added dispersion interaction calculations using Grimme’s D3(BJ) to test. The TS method is employed for locating transition states. Transition states were ascertained by vibrational frequency analysis to possess a single mode among the reaction path with only one imaginary frequency. Ggas and vibrational entropy corrections were computed at 298.15 K. As Ho et al.39 pointed out, when dielectric continuum solvent models (such as PCM, IEFPCM, and CPCM, etc.) were employed, it is more suitable to evaluate solution-phase free energy (Gsolv) with gas-phase free energy (Ggas) plus solvation energy correction (ΔGsolvation). Thus, solution-phase free energy (Gsolv) was calculated according to the following equation, Gsolv = Ggas + ΔGsolvation
(1) 40
Truhlar and co-workers’ SMD solvation model was employed in the self-consistent reaction field (SCRF) calculation to obtain solvation energy correction ΔGsolvation and bromobenzene was taken (ε = 5.40) as the solvent. The kinetic isotope effects (KIEs) were determined using the frequency data and semiclassical Eyring equation, ⎡ (G ‡ − G R ) − (G ‡ − G R ) ⎤ ⎛ kH ⎞ H D D ⎥ ⎜ ⎟ = exp⎢ − H ⎥⎦ ⎢⎣ RT ⎝ kD ⎠S
(2)
‡
where G denotes the free energy of the transition state, GR denotes the intermediate species, and s denotes semiclassical. GH and GD were calculated at 413.15 K to allow a comparison between experiment and theory. The frequency was scaled by 0.9704. Detailed formulation and corresponding Fortran codes had been issued in our previous paper.41
acceptor. The second C−H activation is proposed in two possible pathways, one is via the formation of Rh(III), the other one is via Rh(V) species. In the plausible pathway of the second C−H activation process, PiVO− is not involved. In Glorius’s work, a key strategy was to introduce a catalytic amount of CsOPiv and an equimolar amount of PivOH. However, the mechanism of this remarkable process has yet to be explored. The role of CsOPiv salt in catalysis remains elusive. The first C−H activation process is found to be a CMD process as proposed by the Glorius et al. However, the second C−H activation process is more challenging. In the present work, non-PivO−assisted reaction mechanism proposed by Glorius et al. involving Rh(III) and Rh(V) species is discussed. The calculated properties of this mechanism are not supported by the experimental data. After that, we advance a new mechanism for the second C−H activation process, namely a SE3 mechanism. Meanwhile, our computational studies revealed that CsOPiv salt is involved in the C−H activation process to minimize the activation energy. It is worth mentioning that our PivO−-assisted Rh(III) complex catalyzed aryl−aryl reaction vis the SE3 mechanism is the first presented. This mechanism is theoretically rationalized by comparison of the calculated regiochemistry and KIEs with the experimental ones.
3. RESULTS AND DISCUSSION The Rh(III)-catalyzed aryl−aryl (Ar−Ar) coupling contained two main steps, according to the C−H bond activation. In the first step, the C−H bond of benzamide is activated (Scheme 3, RC → IM1). This is called the first C−H bond activation mechanism. The second step is the C−H bond of bromobenzene activation. This process is referred as the nonPiVO−-assisted process as in Glorius’s proposal (Scheme 3, IM1′ → IM2a and IM2b) and the PiVO−-assisted process as in our mechanism (Scheme 4, IM1′ → IM2). After two steps of C−H bond activation, a C−C coupling is followed. To explore the experimenter proposed mechanisms shown in Scheme 2, we performed DFT studies on the case without an additional PivO− anion assistant. Benzamide and bromobenzene are used as the substrates and the cationic Cp*Rh species acts as the active catalyst in the reaction. Scheme 3 shows the mechanism of non-PivO−-assisted reaction revealed by DFT calculations, and the corresponding free energy profiles are depicted in Figure 1. From our calculation study, the Rhcatalyzed aryl−aryl cross-coupling reaction is realized by means of double C−H bond activation. The first C−H bond activation is started with the reagent complex (RC), in which the Rh(III) coordinates with Cp* ligation, a κ2 PivO− ligand, and the oxygen atom of the amide ligand. This ligation geometry orientates the ortho-hydrogen of the phenyl group close to the oxygen of the PivO− anion. In this case, the first C−H bond activation occurs via TS1, in which the acetate moiety acts as the base to deprotonate the ortho aromatic proton, with concomitant formation of an Rh−aryl bond. By the first C−H activation step, the Rh(III)−aryl intermediate (IM1) and
2. THEORETICAL CALCULATIONS DFT calculations were carried out with Gaussian 09 suite of quantum chemical packages.36 {RhIII(Cp*)[PhCON(iPr)2](PivO −)} + was taken as the catalyst; benzamide and bromobenzene, as the substrate. Two basis sets were used: sdd(Rh,Br)/6-31G*(H,C,N,O) involving Stuttgart/Dresden ECPs for optimization and sdd(Rh,Br)/6-311+G**(H,C,N,O) for single point energy and solvation corrections at the PBE0 density functional level.37,38 To consider dispersion affection on 2990
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The Journal of Physical Chemistry A Scheme 3. Mechanisms of Non-PivO−-Assisted Reactions Revealed by DFT Calculations
Scheme 4. New PivO−-Assisted Reactions Mechanisms Based on Our DFT Calculations
process by Glorius et al.2 As shown in Scheme 2, after the ligand exchange, the subsequent C−H activation may proceed by two alternative reaction routes, the Rh(III) and Rh(V)− hydride routes, respectively. Herein, imido−hydroxyl intermediate (IM2a) is formed via proton transfer to the acyl group
PivOH are formed. The activation energy is about 20 kcal/mol for the first C−H activation process, which can be really easily reached in the experimental condition. Non-PivO−-Assisted Reaction Mechanism. The second C−H activation is proposed to be a non-PiVO−-assisted 2991
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Figure 1. Free energy profiles (kcal/mol) of the non-PivO−-assisted reaction process.
Figure 2. Geometric information on two important structures before and after the ligand exchange.
energy of TS3b with the maximum transition state energy during route b is about 53.6 kcal/mol relative to RC. It is clear that the activation energies of route b is lower than that of route a. However, both reaction routes hold very high energy barriers (>50 kcal/mol), which are unlikely to occur under Glorius’s experimental conditions. Furthermore, the ratelimiting step of route a is the reductive elimination, which indicates that the reaction should have a very small kinetic isotopic effect. This is opposite to the experimental observations, where a strong KIE is found. PivO − -Assisted Reaction Mechanism. To find a reasonable reaction pathway of the Rh(III) complex catalyzed aryl−aryl reaction, we have proposed a new mechanism route referred to as PivO− anion assisted. Scheme 4 shows the detailed mechanism of the PivO−-assisted reaction. As mentioned in experimental work, when CsOPiv was used alone with Rh catalyst and Ag salt (but without PivOH), no desired product was formed.2 Keeping a certain PivOH
of the amide ligand in route a, whereas the Rh(V)−hydride intermediate (IM2b) is generated via proton transfer to Rh(V) in route b. The corresponding free energy profiles are depicted in Figure 1. The formation of Rh(III) species IM2a is endergonic by 29 kcal/mol with an activation energy of 32.1 kcal/mol. In the following step, the energy of TS3a is 86.9 kcal/ mol and biaryl product PCa is formed with a high activation energy of 35.5 kcal/mol. This step is known as reductive elimination, whereas, for the Rh(V)−hydride species, intermediate IM2b is at 46.8 kcal/mol above the starting reactant RC, and its activation barrier is lower than that of IM2a. The proton transfer from the Rh(V)−hydride group to the Cp* ligand leads to the formation of cyclopenta-1,3-diene with Rh(III) species, IM3b, which is endergonic by 3.5 kcal/mol with an activation energy of 6.8 kcal/mol, in comparison with the case for IM2b. In the last step, aryl−aryl coupling via the reductive elimination mechanism generates biaryl product PCb, which is exergonic by 40.3 kcal/mol, relative to IM3b. The 2992
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of PivOH, the ligand exchange of PivOH with bromobenzne occurred simultaneously, producing IM1′, which is higher in energy by 11.8 kcal/mol. In IM1′, bromobenze forms η1 coordination with Rh(III) and H-bond interaction with an additional PivO− anion under the optimized conditions. Corresponding distances of Rh−C2 and H−O2 are 2.494 and 2.477 Å, respectively, which facilitates the transfer of proton from bromobenzene to PivO− anion and the formation of Rh−aryl bond via the SE3 mechanism. The Rh−O and Rh− C1 distances in IM1′ are much shorter than those in IM1. IM2 obtained via TS2 has a relative energy of 1.2 kcal/mol below the starting point. This step is exergonic by 23.6 kcal/mol relative to IM1′. Comparing with the non-PivO−-additionassisted reaction mechanism, this mechanism not only has the lower energy barrier but also can prevent forming the higher energy intermediates (IM2a and IM2b). Proton activated transfers to the PivO− anion should be feasible. For TS2, the distances of Rh−C2, C2−H, and H−O2 are 2.240, 1.305, and 1.334 Å, respectively, and the angle of C2−H−O2 is 157.5°. The Rh interacts strongly with the C at the para-position of bromobenzene. As a result, the Rh−C2 distance is shortened from 2.494 to 2.026 Å. The final step is C−C bond coupling via reductive elimination, which holds an activation barrier of 28.0 kcal/mol. For TS3, the Rh−C1 and Rh−C2 distances are 2.023 and 2.188 Å, respectively, and C1−Rh−C2 angle is 48.6°. In PC, the Rh−O1 distance is elongated from 2.089 to 2.132 Å. Calculated results suggest that the reaction mainly includes CMD and electrophilic metalation. Additionally, computational studies of this reaction have suggested that electrophilic metalation is in line with the speculation of Glorius et al.2 Furthermore, in the case of our investigation, the PivO− anion plays a pivotal role in electrophilic metalation. The presence of PivO− as a hydrogen acceptor during C−H bond activation provides a low energy pathway for aryl−aryl bond formation with Rh(III). The key step of the second C−H activation in the proposed route proceeds via the SE3 mechanism.7 The support for this SE3 mechanism was put forth in the previous work of Flegeau et al. on the mechanism of Ru(II) complex in catalyzing the C− H bond activation of 2-phenylpyridine.15 It is noteworthy that the SE3 mechanism in Rh(III)-catalyzed aryl−aryl crosscoupling reaction has never been reported before. By comparison of the mechanistic details of reaction routes, we can reasonably conclude that the second C−H activation (activation of bromobenzene) is the key step in the whole catalytic reaction. Casper’s investigation reported the close cycle of Phcatalyzed allylic C−H activation.43 In Glorius’s work, they proposed a plausible catalytic cycle. But they did not mention how to regenerate the catalyst in experiment. In such a case, we have not been able to finish the regeneration in the plausible catalytic cycle. Although the calculated mechanism is not a cycle, our calculation is compatible with the experiment. The exact mechanism of the Rh(III) catalyst regeneration step is not clear at the present time; we hope this problem will be solved in the further research. Regioselectivity Investigation. In Glorius’s experiment,2 only the para- and the meta-position cross-coupling products were observed. Furthermore, the corresponding reaction rates of both positions are nearly identical. To elucidate this point, we have investigated the regioselectivity of the aryl−aryl crosscoupling reaction. The calculation is started from the species IM1′ of the bromobenzene ligated complex. The optimization
concentration avoids the newly formed PivOH in IM1 from being deprotonated or replaced by PivO−. As it is known that PivO− has a much stronger coordination interaction with Rh than PivOH, it is more difficult to be replaced by bromobenzene. With the help of external PivO− to pull out the PivOH through the hydrogen bond and bromobenzene close to the Rh, the IM1′ is obtained. In fact, the formation of the following intermediate (IM1′) is very complicated and crucial for the second C−H activation. The structures of corresponding complexes before (IM1) and after (IM1′) the ligand exchange are shown in Figure 2. In the complex before the ligand exchange, the O1−H1, O2−H1m and O3−H2 distances are 1.029, 1.511, and 2.504 Å, respectively, whereas in the complex after the ligand exchange, those relative distances are 1.103, 1.324, and 2.031 Å, respectively. The O3−Rh distance elongated from 2.085 to 4.363 Å before and after the ligand exchange. In IM1′, bromobenzene weakly coordinates to the Rh(III) at the para- and meta-position, in an η1 fashion. Then, the second C−H bond activation occurs via SE3 mechanism to generate the Rh(III)−bromobenzene intermediate complex IM2 along with a second molecule of PivOH. It is the key mechanism feature which PivO− anion is as a proton acceptor and bromobenzene interacts with the Rh center in this step. The presence of additional PivO− anion holds strong Hbond interaction with ligated bromobenzene to prevent the formation of a Rh(V)−hydride intermediate previous discussed. In the final step, aryl−aryl coupling occurs via reductive elimination to generate the biaryl product and the Rh(I) complex, which can be oxidized by Cu(OAc)2 and complete the catalytic cycle. The Gibbs free energy profile of the PivO−-assisted reaction process for para-regioselectivity is shown in Figure 3, and the
Figure 3. Gibbs free energy profile (kcal/mol) of the reaction process for para-regioselectivity.
geometric features of the key reaction intermediates are depicted in Figure 4. According to Figure 3, we see that the CMD step is endergonic by 10.6 kcal/mol, and the formation of IM1 holds an activation energy barrier of 20.0 kcal/mol. The distance of Rh−O1 shortens from 2.125 Å in TS1 to 2.098 Å in IM1, and the distance of Rh−O3 lengthens from 2.101 Å in TS1 to 2.194 Å in IM1, respectively. It means the PivOH will break away. The corresponding transition state (TS1) (Figure 3) shows that the Rh−C, C−H, and H−O distances are 2.225, 1.321, and 1.307 Å, respectively, and the angle of C−H−O is 166.0°, which agrees well with the traditional geometric features of the CMD mechanism.42 Compared with the bond in RC, the Rh···O coordination in IM1 is weaker with a slightly longer Rh−O3 distance (2.190 Å vs 2.150 Å). The dissociation and ligand exchange is geometrically favorable. With the release 2993
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Figure 4. Geometric information on the key reaction intermediates.
Figure 5. Comparison of free energy profiles (kcal/mol) of the para- and meta-reaction routes.
on the ortho-, para-, and meta-ligation retains the initial ligation state; that is, the ortho-, para-, or meta-carbons of bromobenzene form an η1 coordination bond with Rh(III). Here, we can give an explanation about why the ortho C−H bond activation was not observed by comparison of related energies. Not only is the energy of TS2ortho higher than that of the other position transition states by more than 139 kcal/mol, but also the energy of IMortho is rather high, about 147 kcal/mol relative to RC. The pathway of ortho-ligation is not lower than the 209 kcal/mol free energy barrier. So it suggests that the ortho-position ligation is impossible. Therefore, only the free energy profile of the para- and meta-position is presented in Figure 5. The optimized geometries of all the species involving the meta-position reaction route are given in Figure 6. IM2meta is generated from IM1′ via TS2meta with an activation energy
barrier of 6.8 kcal/mol and is exergonic by 24.5 kcal/mol relative to IM1′. The relative energy gap between TS2meta and IM1′ (6.8 kcal/mol) is close to that between TS2para and IM1′ (6.9 kcal/mol). For meta- and para-position reaction intermediates, IM2meta and IM2para are lower in energy by −2.1 and −1.2 kcal/mol, respectively, compared to RC. It is observed that IM2 meta is more stable than IM2 para . Furthermore, the key step is C−H activation via a SE3 mechanism with high activation energies of 29.2 and 29.3 kcal/ mol for the meta- and para-reaction routes, respectively. This result demonstrates that the meta-reaction route undergoes the same rate-determining step as the para-reaction route. Geometric parameters of both transition states (TS2s shown in Figure 4 and Figure 6) are similar. For the para-route, the distance of Rh−C2 is 2.240 Å, C2−H is 1.305 Å, and H−O2 is 2994
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Figure 6. Geometric information on the key reaction intermediates in meta-reaction route.
1.334 Å, and the angle of C2−H−O2 is 157.5°; for the metaroute, the distance of Rh−C2 is 2.253 Å, C2−H is 1.300 Å, and H−O2 is 1.341 Å, and the angle of C2−H−O2 is 157.1°. The formation of PCmeta is endergonic by 4.2 kcal/mol with activation energy of 29.4 kcal/mol, whereas PCpara is endergonic by 4.7 kcal/mol with activation energy of 28.0 kcal/mol. The PCmeta with the lower energy indicates it is more stable than PCpara. Moreover, we have added a dispersion correction calculated using Grimme’s D3(BJ) for intermediates and translation states (from RC to IM2meta/IM2para). This process involved two important C−H activation steps. The first C−H activation step is exergonic by 10.4 kcal/mol, and the activation energy barrier is higher than that of the neglect of the dispersion interaction by 1.0 kcal/mol. In the second C−H activation step TS2para and TS2meta with the energy barriers of 6.5 and 5.6 kcal/mol are obtained, respectively. Although the energies of TS2para and TS2meta increase to compare with those with the ignored dispersion correction, the energy barrier added dispersion corrections are lower. The IM2para and IM2meta are +0.5 and −1.3 kcal/mol, respectively. It can be also concluded that the meta-ligand is more favored than the para-ligand. In addition, the result confirms that the dispersion correction cannot affect the regioselectivities. Our theoretical results are in perfect agreement with Glorius’s experimental observations,2 in which the coupling at the meta-position is higher than that of para-position in two regioisomers mixture (m/p = 2.8:1). Kinetic Isotope Effect (KIE) Investigation. To further probe the mechanism we proposed, the kinetic isotope effect (KIE) is also investigated (Table 1). Our calculated KIE for the first C−H activation step is 3.4 and that for the second C−H activation step is 3.6 (meta-route) and 3.7 (para-route) at 413.15 K (140 °C), respectively. The experimental KIE value of
Table 1. Experimental and Calculated Kinetic Isotope Effects KIEexp KIEcal
1st C−H activation
2nd C−H activation
1.8−2.0 3.4
3.4−3.5 3.7 (para), 3.6 (meta)
the first C−H activation observed by Glorius et al.2 is only 1.8 to 2.0, whereas that of the second step is 3.4 to 3.5, which means that the second activation process is the ratedetermining step of the whole catalytic reaction. The KIE is calculated on the base of the C−H bond activation as the ratedetermining step. The rate-determining step is the only one in the whole mechanism. So it is impossible for the two calculated KIEs to agree with the experimental values at the same time. The deviation between the calculated results and experimental observations (3.4 vs 1.8−2.0) for the first C−H activation reveals that this process is not the rate-determining step, whereas, the good consistency between theoretical and experimental results for the second C−H activation (3.6, 3.7 vs 3.4−3.5) indicates that this step is rate-determining and the SE3 mechanism is more favorable. As found in the potential energy curve in Figure 3, TS2 suffers the highest energy barrier in the whole reaction. Thus, both the calculated and experimental KIE evidence support our new mechanism. The noticeable H/D scrambling on coupling partners results clearly indicate that real C−H bond activation occurs in the reaction. From our calculations and Glorius’s mechanistic proposal,2 three alternative mechanisms for this Rh-catalyzed transformation (second C−H activation) are found. However, two of them are intramolecular processes, route a and route b, which can be safely excluded by the fact that the relatively higher reaction barriers are found for both routes as discussed above. The only reasonable one involves an intermolecular SE3 2995
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The Journal of Physical Chemistry A mechanism with the assistance of an additional PivO− anion. This preferred pathway has not only the low activation barrier but also the consistency of the calculated regioselectivity and kinetic isotope effect. A rather high activation energy is found for the coupling at the ortho-position ligation so that no coupling at the ortho-position of bromobenzene is confirmed by the theoretical study. The most favorable products are found to be meta- and para-position ligations, which are observed in the regioselective experiment. The calculated KIE also shows a good agreement with experiment results.
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4. CONCLUSIONS In present article, we gained novel mechanistic insights into the Rh(III)-catalyzed aryl−aryl bond formation via a dual C−H activation pathway. The para-/meta-regioselectivity and deuterium kinetic isotope effects were investigated by means of density functional theory (DFT) calculation. The calculated results revealed that the presence of an additional PivO− anion is vital to the catalytic activity. With such an external proton acceptor, the second C−H activation is activated by a SE3 mechanism. The second activation process is the ratedetermining step of the whole catalytic cycle. Our new mechanism involving the assistance of additional PivO− anion is evaluated by the calculations of the para-/meta-regioselectivity and KIEs. All these calculated properties are consistent with the experimental observations. Furthermore, this mechanism supports Glorius’s proposal that Rh(III)-catalyzed CMD reaction proceeds by 2-fold direct C−H activations on both coupling partners.
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates of the reaction species. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Y. Wang. E-mail: [email protected]. *D. Zhao. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge Dr. Jinchong Xiao, University of Hebei, for language polishing. The authors gratefully acknowledge research support of this work by the NFSC (grants 21173211 and 21233008 to Y.W.).
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REFERENCES
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Theoretical Investigations on Rh(III)-Catalyzed CrossDehydrogenative Aryl−Aryl Coupling via C−H Bond Activation Dan Zhao,*,†,‡ Xiaoxi Li,‡,§ Keli Han,†,‡ Xingwei Li,⊥ and Yong Wang*,‡ †
School of Chemistry, Dalian University of Technology, Dalian 116024, People’s Republic of China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ⊥ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China § State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: The reaction mechanism of Rh(III)-catalyzed cross-dehydrogenative aryl−aryl coupling between benzamides and haloarenes was investigated through detailed density functional theoretical (DFT) studies in terms of regioselectivity and deuterium kinetic isotope effects (KIEs). Three possible routes including one PivO−-assisted reaction route and two non-PivO−-assisted reaction routes have been studied. The calculated results refute the proposed mechanism (without PivO−-assisted process) in the experimental paper and demonstrate that the PivO−-assisted reaction mechanism is the most favored. Meanwhile, the calculation revealed that the PivO− anion plays a crucial role as a proton acceptor in the C−H bond activation, especially when the second C−H activation of haloarenearene proceeds via a SE3 mechanism. The SE3 mechanism is presented for the Rh(III)-catalyzed aryl−aryl reaction for the first time. Our mechanism is evaluated by the calculations of the para-/meta-regioselectivity and KIEs. And it is found that the second activation process is the rate-determining step of the whole catalytic cycle. All these calculated properties agree well with the experiment and Glorius’s proposal that the Rh(III)-catalyzed cross-dehydrogenative C−C coupling reaction proceeds by dual C−H activations. Our theoretical studies suggest that the Rh(III) complex catalyst strongly affects the mechanisms of the second C−H activation step and thus this work might provide insight into the design of new catalytic systems.
1. INTRODUCTION The formation of carbon−carbon bonds has become one of the most essential and fundamental research topics because this allows the construction of many important biaryl frameworks in pharmaceuticals and in organic electronic materials. Driven by this need, an increasing number of researchers have focused on the methodologies of C−C bond formation.1,2 Using C−H bonds as starting materials under metal catalysis for this purpose has been recognized as an advantageous strategy owing to the abundance of arenes and the high atom- and stepeconomy associated.3−12 Various transition metal compounds (Pd, Ru, Rh, Ir) are known to effectively activate the aryl C−H bonds for aryl−aryl coupling or aryl−alkynyl coupling.13−18 Recently, C−H activation catalyzed by stable Rh(III) complexes has been extensively studied.19−22 Fagnou, Miura and Satoh, Glorius, Li and Rovis have reported Rh(III)catalyzed oxidative coupling of arenes with unsaturated susbtrates.23−30 Notably, Glorius and co-workers developed Rh(III)-catalyzed oxidative aryl−aryl (Ar−Ar) bond formation between benzamides and haloarenes via a 2-fold direct C−H bond activation pathway (Scheme 1).2 Importantly, catalytic cross-dehydrogenative coupling reactions represent a wastereducing and step-economic process. However, the construc© 2015 American Chemical Society
Scheme 1. Dehydrogenative Cross-Coupling Reaction
tion of biaryl building blocks catalyzed by Rh(III) complexes lagged behind. The rapid growth of catalytic systems in rhodium catalysis inspired us to elucidate the mechanism of this important dual C−H bond activation process. The computational studies in this regard might serve to broaden the scope and patterns of rhodium-catalyzed C−H activation systems. In Glorius’s mechanistic proposal (Scheme 2), there is a dual C−H bond activation for the Rh(III)-catalyzed aryl−aryl (Ar− Ar) system. The first C−H activation may include a σ-bond metathesis or concerted metalation−deprotonation (CMD)31−35 mechanism, with the help of a PiVO− as proton Received: November 18, 2014 Revised: February 3, 2015 Published: February 18, 2015 2989
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The Journal of Physical Chemistry A Scheme 2. Possible Mechanisms Proposed by Glorius et al.2
the weak interaction of dimer, we added dispersion interaction calculations using Grimme’s D3(BJ) to test. The TS method is employed for locating transition states. Transition states were ascertained by vibrational frequency analysis to possess a single mode among the reaction path with only one imaginary frequency. Ggas and vibrational entropy corrections were computed at 298.15 K. As Ho et al.39 pointed out, when dielectric continuum solvent models (such as PCM, IEFPCM, and CPCM, etc.) were employed, it is more suitable to evaluate solution-phase free energy (Gsolv) with gas-phase free energy (Ggas) plus solvation energy correction (ΔGsolvation). Thus, solution-phase free energy (Gsolv) was calculated according to the following equation, Gsolv = Ggas + ΔGsolvation
(1) 40
Truhlar and co-workers’ SMD solvation model was employed in the self-consistent reaction field (SCRF) calculation to obtain solvation energy correction ΔGsolvation and bromobenzene was taken (ε = 5.40) as the solvent. The kinetic isotope effects (KIEs) were determined using the frequency data and semiclassical Eyring equation, ⎡ (G ‡ − G R ) − (G ‡ − G R ) ⎤ ⎛ kH ⎞ H D D ⎥ ⎜ ⎟ = exp⎢ − H ⎥⎦ ⎢⎣ RT ⎝ kD ⎠S
(2)
‡
where G denotes the free energy of the transition state, GR denotes the intermediate species, and s denotes semiclassical. GH and GD were calculated at 413.15 K to allow a comparison between experiment and theory. The frequency was scaled by 0.9704. Detailed formulation and corresponding Fortran codes had been issued in our previous paper.41
acceptor. The second C−H activation is proposed in two possible pathways, one is via the formation of Rh(III), the other one is via Rh(V) species. In the plausible pathway of the second C−H activation process, PiVO− is not involved. In Glorius’s work, a key strategy was to introduce a catalytic amount of CsOPiv and an equimolar amount of PivOH. However, the mechanism of this remarkable process has yet to be explored. The role of CsOPiv salt in catalysis remains elusive. The first C−H activation process is found to be a CMD process as proposed by the Glorius et al. However, the second C−H activation process is more challenging. In the present work, non-PivO−assisted reaction mechanism proposed by Glorius et al. involving Rh(III) and Rh(V) species is discussed. The calculated properties of this mechanism are not supported by the experimental data. After that, we advance a new mechanism for the second C−H activation process, namely a SE3 mechanism. Meanwhile, our computational studies revealed that CsOPiv salt is involved in the C−H activation process to minimize the activation energy. It is worth mentioning that our PivO−-assisted Rh(III) complex catalyzed aryl−aryl reaction vis the SE3 mechanism is the first presented. This mechanism is theoretically rationalized by comparison of the calculated regiochemistry and KIEs with the experimental ones.
3. RESULTS AND DISCUSSION The Rh(III)-catalyzed aryl−aryl (Ar−Ar) coupling contained two main steps, according to the C−H bond activation. In the first step, the C−H bond of benzamide is activated (Scheme 3, RC → IM1). This is called the first C−H bond activation mechanism. The second step is the C−H bond of bromobenzene activation. This process is referred as the nonPiVO−-assisted process as in Glorius’s proposal (Scheme 3, IM1′ → IM2a and IM2b) and the PiVO−-assisted process as in our mechanism (Scheme 4, IM1′ → IM2). After two steps of C−H bond activation, a C−C coupling is followed. To explore the experimenter proposed mechanisms shown in Scheme 2, we performed DFT studies on the case without an additional PivO− anion assistant. Benzamide and bromobenzene are used as the substrates and the cationic Cp*Rh species acts as the active catalyst in the reaction. Scheme 3 shows the mechanism of non-PivO−-assisted reaction revealed by DFT calculations, and the corresponding free energy profiles are depicted in Figure 1. From our calculation study, the Rhcatalyzed aryl−aryl cross-coupling reaction is realized by means of double C−H bond activation. The first C−H bond activation is started with the reagent complex (RC), in which the Rh(III) coordinates with Cp* ligation, a κ2 PivO− ligand, and the oxygen atom of the amide ligand. This ligation geometry orientates the ortho-hydrogen of the phenyl group close to the oxygen of the PivO− anion. In this case, the first C−H bond activation occurs via TS1, in which the acetate moiety acts as the base to deprotonate the ortho aromatic proton, with concomitant formation of an Rh−aryl bond. By the first C−H activation step, the Rh(III)−aryl intermediate (IM1) and
2. THEORETICAL CALCULATIONS DFT calculations were carried out with Gaussian 09 suite of quantum chemical packages.36 {RhIII(Cp*)[PhCON(iPr)2](PivO −)} + was taken as the catalyst; benzamide and bromobenzene, as the substrate. Two basis sets were used: sdd(Rh,Br)/6-31G*(H,C,N,O) involving Stuttgart/Dresden ECPs for optimization and sdd(Rh,Br)/6-311+G**(H,C,N,O) for single point energy and solvation corrections at the PBE0 density functional level.37,38 To consider dispersion affection on 2990
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The Journal of Physical Chemistry A Scheme 3. Mechanisms of Non-PivO−-Assisted Reactions Revealed by DFT Calculations
Scheme 4. New PivO−-Assisted Reactions Mechanisms Based on Our DFT Calculations
process by Glorius et al.2 As shown in Scheme 2, after the ligand exchange, the subsequent C−H activation may proceed by two alternative reaction routes, the Rh(III) and Rh(V)− hydride routes, respectively. Herein, imido−hydroxyl intermediate (IM2a) is formed via proton transfer to the acyl group
PivOH are formed. The activation energy is about 20 kcal/mol for the first C−H activation process, which can be really easily reached in the experimental condition. Non-PivO−-Assisted Reaction Mechanism. The second C−H activation is proposed to be a non-PiVO−-assisted 2991
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The Journal of Physical Chemistry A
Figure 1. Free energy profiles (kcal/mol) of the non-PivO−-assisted reaction process.
Figure 2. Geometric information on two important structures before and after the ligand exchange.
energy of TS3b with the maximum transition state energy during route b is about 53.6 kcal/mol relative to RC. It is clear that the activation energies of route b is lower than that of route a. However, both reaction routes hold very high energy barriers (>50 kcal/mol), which are unlikely to occur under Glorius’s experimental conditions. Furthermore, the ratelimiting step of route a is the reductive elimination, which indicates that the reaction should have a very small kinetic isotopic effect. This is opposite to the experimental observations, where a strong KIE is found. PivO − -Assisted Reaction Mechanism. To find a reasonable reaction pathway of the Rh(III) complex catalyzed aryl−aryl reaction, we have proposed a new mechanism route referred to as PivO− anion assisted. Scheme 4 shows the detailed mechanism of the PivO−-assisted reaction. As mentioned in experimental work, when CsOPiv was used alone with Rh catalyst and Ag salt (but without PivOH), no desired product was formed.2 Keeping a certain PivOH
of the amide ligand in route a, whereas the Rh(V)−hydride intermediate (IM2b) is generated via proton transfer to Rh(V) in route b. The corresponding free energy profiles are depicted in Figure 1. The formation of Rh(III) species IM2a is endergonic by 29 kcal/mol with an activation energy of 32.1 kcal/mol. In the following step, the energy of TS3a is 86.9 kcal/ mol and biaryl product PCa is formed with a high activation energy of 35.5 kcal/mol. This step is known as reductive elimination, whereas, for the Rh(V)−hydride species, intermediate IM2b is at 46.8 kcal/mol above the starting reactant RC, and its activation barrier is lower than that of IM2a. The proton transfer from the Rh(V)−hydride group to the Cp* ligand leads to the formation of cyclopenta-1,3-diene with Rh(III) species, IM3b, which is endergonic by 3.5 kcal/mol with an activation energy of 6.8 kcal/mol, in comparison with the case for IM2b. In the last step, aryl−aryl coupling via the reductive elimination mechanism generates biaryl product PCb, which is exergonic by 40.3 kcal/mol, relative to IM3b. The 2992
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of PivOH, the ligand exchange of PivOH with bromobenzne occurred simultaneously, producing IM1′, which is higher in energy by 11.8 kcal/mol. In IM1′, bromobenze forms η1 coordination with Rh(III) and H-bond interaction with an additional PivO− anion under the optimized conditions. Corresponding distances of Rh−C2 and H−O2 are 2.494 and 2.477 Å, respectively, which facilitates the transfer of proton from bromobenzene to PivO− anion and the formation of Rh−aryl bond via the SE3 mechanism. The Rh−O and Rh− C1 distances in IM1′ are much shorter than those in IM1. IM2 obtained via TS2 has a relative energy of 1.2 kcal/mol below the starting point. This step is exergonic by 23.6 kcal/mol relative to IM1′. Comparing with the non-PivO−-additionassisted reaction mechanism, this mechanism not only has the lower energy barrier but also can prevent forming the higher energy intermediates (IM2a and IM2b). Proton activated transfers to the PivO− anion should be feasible. For TS2, the distances of Rh−C2, C2−H, and H−O2 are 2.240, 1.305, and 1.334 Å, respectively, and the angle of C2−H−O2 is 157.5°. The Rh interacts strongly with the C at the para-position of bromobenzene. As a result, the Rh−C2 distance is shortened from 2.494 to 2.026 Å. The final step is C−C bond coupling via reductive elimination, which holds an activation barrier of 28.0 kcal/mol. For TS3, the Rh−C1 and Rh−C2 distances are 2.023 and 2.188 Å, respectively, and C1−Rh−C2 angle is 48.6°. In PC, the Rh−O1 distance is elongated from 2.089 to 2.132 Å. Calculated results suggest that the reaction mainly includes CMD and electrophilic metalation. Additionally, computational studies of this reaction have suggested that electrophilic metalation is in line with the speculation of Glorius et al.2 Furthermore, in the case of our investigation, the PivO− anion plays a pivotal role in electrophilic metalation. The presence of PivO− as a hydrogen acceptor during C−H bond activation provides a low energy pathway for aryl−aryl bond formation with Rh(III). The key step of the second C−H activation in the proposed route proceeds via the SE3 mechanism.7 The support for this SE3 mechanism was put forth in the previous work of Flegeau et al. on the mechanism of Ru(II) complex in catalyzing the C− H bond activation of 2-phenylpyridine.15 It is noteworthy that the SE3 mechanism in Rh(III)-catalyzed aryl−aryl crosscoupling reaction has never been reported before. By comparison of the mechanistic details of reaction routes, we can reasonably conclude that the second C−H activation (activation of bromobenzene) is the key step in the whole catalytic reaction. Casper’s investigation reported the close cycle of Phcatalyzed allylic C−H activation.43 In Glorius’s work, they proposed a plausible catalytic cycle. But they did not mention how to regenerate the catalyst in experiment. In such a case, we have not been able to finish the regeneration in the plausible catalytic cycle. Although the calculated mechanism is not a cycle, our calculation is compatible with the experiment. The exact mechanism of the Rh(III) catalyst regeneration step is not clear at the present time; we hope this problem will be solved in the further research. Regioselectivity Investigation. In Glorius’s experiment,2 only the para- and the meta-position cross-coupling products were observed. Furthermore, the corresponding reaction rates of both positions are nearly identical. To elucidate this point, we have investigated the regioselectivity of the aryl−aryl crosscoupling reaction. The calculation is started from the species IM1′ of the bromobenzene ligated complex. The optimization
concentration avoids the newly formed PivOH in IM1 from being deprotonated or replaced by PivO−. As it is known that PivO− has a much stronger coordination interaction with Rh than PivOH, it is more difficult to be replaced by bromobenzene. With the help of external PivO− to pull out the PivOH through the hydrogen bond and bromobenzene close to the Rh, the IM1′ is obtained. In fact, the formation of the following intermediate (IM1′) is very complicated and crucial for the second C−H activation. The structures of corresponding complexes before (IM1) and after (IM1′) the ligand exchange are shown in Figure 2. In the complex before the ligand exchange, the O1−H1, O2−H1m and O3−H2 distances are 1.029, 1.511, and 2.504 Å, respectively, whereas in the complex after the ligand exchange, those relative distances are 1.103, 1.324, and 2.031 Å, respectively. The O3−Rh distance elongated from 2.085 to 4.363 Å before and after the ligand exchange. In IM1′, bromobenzene weakly coordinates to the Rh(III) at the para- and meta-position, in an η1 fashion. Then, the second C−H bond activation occurs via SE3 mechanism to generate the Rh(III)−bromobenzene intermediate complex IM2 along with a second molecule of PivOH. It is the key mechanism feature which PivO− anion is as a proton acceptor and bromobenzene interacts with the Rh center in this step. The presence of additional PivO− anion holds strong Hbond interaction with ligated bromobenzene to prevent the formation of a Rh(V)−hydride intermediate previous discussed. In the final step, aryl−aryl coupling occurs via reductive elimination to generate the biaryl product and the Rh(I) complex, which can be oxidized by Cu(OAc)2 and complete the catalytic cycle. The Gibbs free energy profile of the PivO−-assisted reaction process for para-regioselectivity is shown in Figure 3, and the
Figure 3. Gibbs free energy profile (kcal/mol) of the reaction process for para-regioselectivity.
geometric features of the key reaction intermediates are depicted in Figure 4. According to Figure 3, we see that the CMD step is endergonic by 10.6 kcal/mol, and the formation of IM1 holds an activation energy barrier of 20.0 kcal/mol. The distance of Rh−O1 shortens from 2.125 Å in TS1 to 2.098 Å in IM1, and the distance of Rh−O3 lengthens from 2.101 Å in TS1 to 2.194 Å in IM1, respectively. It means the PivOH will break away. The corresponding transition state (TS1) (Figure 3) shows that the Rh−C, C−H, and H−O distances are 2.225, 1.321, and 1.307 Å, respectively, and the angle of C−H−O is 166.0°, which agrees well with the traditional geometric features of the CMD mechanism.42 Compared with the bond in RC, the Rh···O coordination in IM1 is weaker with a slightly longer Rh−O3 distance (2.190 Å vs 2.150 Å). The dissociation and ligand exchange is geometrically favorable. With the release 2993
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Figure 4. Geometric information on the key reaction intermediates.
Figure 5. Comparison of free energy profiles (kcal/mol) of the para- and meta-reaction routes.
on the ortho-, para-, and meta-ligation retains the initial ligation state; that is, the ortho-, para-, or meta-carbons of bromobenzene form an η1 coordination bond with Rh(III). Here, we can give an explanation about why the ortho C−H bond activation was not observed by comparison of related energies. Not only is the energy of TS2ortho higher than that of the other position transition states by more than 139 kcal/mol, but also the energy of IMortho is rather high, about 147 kcal/mol relative to RC. The pathway of ortho-ligation is not lower than the 209 kcal/mol free energy barrier. So it suggests that the ortho-position ligation is impossible. Therefore, only the free energy profile of the para- and meta-position is presented in Figure 5. The optimized geometries of all the species involving the meta-position reaction route are given in Figure 6. IM2meta is generated from IM1′ via TS2meta with an activation energy
barrier of 6.8 kcal/mol and is exergonic by 24.5 kcal/mol relative to IM1′. The relative energy gap between TS2meta and IM1′ (6.8 kcal/mol) is close to that between TS2para and IM1′ (6.9 kcal/mol). For meta- and para-position reaction intermediates, IM2meta and IM2para are lower in energy by −2.1 and −1.2 kcal/mol, respectively, compared to RC. It is observed that IM2 meta is more stable than IM2 para . Furthermore, the key step is C−H activation via a SE3 mechanism with high activation energies of 29.2 and 29.3 kcal/ mol for the meta- and para-reaction routes, respectively. This result demonstrates that the meta-reaction route undergoes the same rate-determining step as the para-reaction route. Geometric parameters of both transition states (TS2s shown in Figure 4 and Figure 6) are similar. For the para-route, the distance of Rh−C2 is 2.240 Å, C2−H is 1.305 Å, and H−O2 is 2994
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Figure 6. Geometric information on the key reaction intermediates in meta-reaction route.
1.334 Å, and the angle of C2−H−O2 is 157.5°; for the metaroute, the distance of Rh−C2 is 2.253 Å, C2−H is 1.300 Å, and H−O2 is 1.341 Å, and the angle of C2−H−O2 is 157.1°. The formation of PCmeta is endergonic by 4.2 kcal/mol with activation energy of 29.4 kcal/mol, whereas PCpara is endergonic by 4.7 kcal/mol with activation energy of 28.0 kcal/mol. The PCmeta with the lower energy indicates it is more stable than PCpara. Moreover, we have added a dispersion correction calculated using Grimme’s D3(BJ) for intermediates and translation states (from RC to IM2meta/IM2para). This process involved two important C−H activation steps. The first C−H activation step is exergonic by 10.4 kcal/mol, and the activation energy barrier is higher than that of the neglect of the dispersion interaction by 1.0 kcal/mol. In the second C−H activation step TS2para and TS2meta with the energy barriers of 6.5 and 5.6 kcal/mol are obtained, respectively. Although the energies of TS2para and TS2meta increase to compare with those with the ignored dispersion correction, the energy barrier added dispersion corrections are lower. The IM2para and IM2meta are +0.5 and −1.3 kcal/mol, respectively. It can be also concluded that the meta-ligand is more favored than the para-ligand. In addition, the result confirms that the dispersion correction cannot affect the regioselectivities. Our theoretical results are in perfect agreement with Glorius’s experimental observations,2 in which the coupling at the meta-position is higher than that of para-position in two regioisomers mixture (m/p = 2.8:1). Kinetic Isotope Effect (KIE) Investigation. To further probe the mechanism we proposed, the kinetic isotope effect (KIE) is also investigated (Table 1). Our calculated KIE for the first C−H activation step is 3.4 and that for the second C−H activation step is 3.6 (meta-route) and 3.7 (para-route) at 413.15 K (140 °C), respectively. The experimental KIE value of
Table 1. Experimental and Calculated Kinetic Isotope Effects KIEexp KIEcal
1st C−H activation
2nd C−H activation
1.8−2.0 3.4
3.4−3.5 3.7 (para), 3.6 (meta)
the first C−H activation observed by Glorius et al.2 is only 1.8 to 2.0, whereas that of the second step is 3.4 to 3.5, which means that the second activation process is the ratedetermining step of the whole catalytic reaction. The KIE is calculated on the base of the C−H bond activation as the ratedetermining step. The rate-determining step is the only one in the whole mechanism. So it is impossible for the two calculated KIEs to agree with the experimental values at the same time. The deviation between the calculated results and experimental observations (3.4 vs 1.8−2.0) for the first C−H activation reveals that this process is not the rate-determining step, whereas, the good consistency between theoretical and experimental results for the second C−H activation (3.6, 3.7 vs 3.4−3.5) indicates that this step is rate-determining and the SE3 mechanism is more favorable. As found in the potential energy curve in Figure 3, TS2 suffers the highest energy barrier in the whole reaction. Thus, both the calculated and experimental KIE evidence support our new mechanism. The noticeable H/D scrambling on coupling partners results clearly indicate that real C−H bond activation occurs in the reaction. From our calculations and Glorius’s mechanistic proposal,2 three alternative mechanisms for this Rh-catalyzed transformation (second C−H activation) are found. However, two of them are intramolecular processes, route a and route b, which can be safely excluded by the fact that the relatively higher reaction barriers are found for both routes as discussed above. The only reasonable one involves an intermolecular SE3 2995
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The Journal of Physical Chemistry A mechanism with the assistance of an additional PivO− anion. This preferred pathway has not only the low activation barrier but also the consistency of the calculated regioselectivity and kinetic isotope effect. A rather high activation energy is found for the coupling at the ortho-position ligation so that no coupling at the ortho-position of bromobenzene is confirmed by the theoretical study. The most favorable products are found to be meta- and para-position ligations, which are observed in the regioselective experiment. The calculated KIE also shows a good agreement with experiment results.
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4. CONCLUSIONS In present article, we gained novel mechanistic insights into the Rh(III)-catalyzed aryl−aryl bond formation via a dual C−H activation pathway. The para-/meta-regioselectivity and deuterium kinetic isotope effects were investigated by means of density functional theory (DFT) calculation. The calculated results revealed that the presence of an additional PivO− anion is vital to the catalytic activity. With such an external proton acceptor, the second C−H activation is activated by a SE3 mechanism. The second activation process is the ratedetermining step of the whole catalytic cycle. Our new mechanism involving the assistance of additional PivO− anion is evaluated by the calculations of the para-/meta-regioselectivity and KIEs. All these calculated properties are consistent with the experimental observations. Furthermore, this mechanism supports Glorius’s proposal that Rh(III)-catalyzed CMD reaction proceeds by 2-fold direct C−H activations on both coupling partners.
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates of the reaction species. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Y. Wang. E-mail: [email protected]. *D. Zhao. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge Dr. Jinchong Xiao, University of Hebei, for language polishing. The authors gratefully acknowledge research support of this work by the NFSC (grants 21173211 and 21233008 to Y.W.).
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REFERENCES
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