Theoretical Study on Homogeneous Hydrogen Activation Catalyzed

Article pubs.acs.org/Organometallics

Theoretical Study on Homogeneous Hydrogen Activation Catalyzed by Cationic Ag(I) Complex Yuan-Ye Jiang,† Hai-Zhu Yu,‡ and Yao Fu*,† †

Department of Chemistry, University of Science and Technology of China, Hefei 230026, China Department of Polymer Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

‡

S Supporting Information *

ABSTRACT: Recently, the Li group reported the first Ag-catalyzed hydrogenation of aldehydes in water, demonstrating the utility of Ag complexes in homogeneous catalytic transformations through hydrogen activation. In the present study, density functional theory methods have been used to study the mechanism of Ag-catalyzed hydrogen activation. Three possible pathways, including base-assisted hydrogen activation, ligand-assisted hydrogen activation, and oxidative addition were investigated. The ligand-assisted hydrogen activation is disfavored because the neutral biaryl phosphine ligand XPhos is not a competent proton acceptor and results in the destruction of the aromaticity of an aryl group. Oxidative addition of H2 on AgI complexes was also found to be unlikely. The resulting AgIII hydride complexes are highly unstable and can undergo spontaneous reduction due to the weakly electron-donating ligand and the relatively low electronegativity of hydrogen. By contrast, the base-assisted hydrogen activation mechanism is more favored. This mechanism mainly includes three steps: base-assisted heterolytic H−H bond cleavage, hydride transfer, and protonation. Hydride transfer is the rate-determining step of the whole catalytic cycle. In addition, the ligand XPhos was found to coordinate with the Ag center by both the phosphine and the isopropyl-substituted phenyl groups, and this coordination mode is able to facilitate hydrogen activation.

1. INTRODUCTION Hydrogenation is a fundamentally important reaction in finechemical, pharmaceutical, fuel, fragrance, flavoring, nutrition, and cosmetic industries.1 Transition metal complexes are powerful catalysts that can efficiently cleave the H−H bond of H2 and promote atom-economic hydrogenation. Since the appearance of the Wilkinson catalyst in the 1960s,2 transitionmetal-catalyzed hydrogenation has received wide and continuous attention. Precious transition metals such as Rh,3 Ru,4 Ir,5 Pd,6 and cheaper transition metals including Co,7 Fe,8 and Ni9 have been known to be competent to hydrogenation. Interestingly, despite the fact that silver salts have been reported to active H2 as early as 1950s,10 homogeneous Agcatalyzed H2 activation/hydrogenation is still rarely known.11 Recently, the Li group made a breakthrough and reported the first Ag-catalyzed hydrogenation of aldehydes in water (eq 1).12

It is valuable to clarify the mechanism of the emerging Agcatalyzed H2 activation/hydrogenation reaction from both academic interest and further development of related organic transformations. In accordance with the previous proposals and mechanistic studies on similar transition-metal-catalyzed hydrogen activations, three mechanisms are possible for the Agcatalyzed H2 activation.10c,d,13−18 The first mechanism is the base-assisted hydrogen activation mechanism (path A, Scheme 1). In this pathway, the heterolytic cleavage of the H−H bond is realized by the cooperation of Ag and the base and affords a silver hydride complex and a quaternary ammonium cation.10c,d,13 The hydride complex then reacts with the aldehyde to generate a Ag−O complex. Thereafter, the protonation of the resulting complex by the quaternary ammonium cation affords the alcohol and finishes the catalytic cycle. The second mechanism is the ligand-assisted hydrogen activation mechanism (path B, Scheme 1). In this pathway, the ligand acts as a proton acceptor in the H−H bond cleavage step. Similar ligand-assisted hydrogen activation has been previously proposed in some Co-,14 Fe-,15 and Ru-catalyzed16 hydrogenations with H2. In addition, the biaryl phosphine ligands are also reported to possibly participate in the chemical transformations.17 The third possible mechanism is oxidative addition18 in which the AgI inserts into the H−H bond to form a AgIII intermediate (path C, Scheme 1). Given that AgIII

Li’s reactions use AgPF6 as a catalyst precursor, XPhos as a ligand, and DIPEA (DIPEA = N,N-diisopropylethylamine) as a base and proceed in water at 100 °C under 40 bar of H2. This method can be applied to different (hetero)aromatic, alkyl, and alkenyl aldehydes to generate related alcohols in moderate to high yields. © 2014 American Chemical Society

Received: September 6, 2014 Published: November 12, 2014 6577

dx.doi.org/10.1021/om500921d | Organometallics 2014, 33, 6577−6584

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confirm that the optimized structure is either a minimum or a transition state and also to obtain the thermodynamic energy correction. Intrinsic reaction coordinate (IRC) analysis25 was conducted at the same level of theory to confirm the transition states connect correct reactants and products. To provide more accurate energies, solution-phase single-point energies were calculated by using M06 method26 and SMD solvation model with a larger basis set (i.e., SDD27 for Ag and 6-311++G(d,p) for the rest of the atoms). To avoid possible integration grid errors,28 the “ultrafine” grid was assigned for all our calculations with the M06 method. Natural bond orbital (NBO) analysis was performed by using NBO version 3.1 implemented in Gaussian 09.29 Noncovalent interaction plot was generated with the NCI plot developed by Yang’s group.30 The reported energies are the solution-phase single-point energies corrected by solution-phase Gibbs free-energy corrections, responding to 1 mol/L and 298 K.

Scheme 1. Three Possible Mechanisms for H2 Activation Catalyzed by a Cationic Ag(I) Complex

3. RESULTS AND DISCUSSION 3.1. Equilibrium between Different Cationic Ag(I) Complexes. The silver precatalyst is expected to exist as a cation because of the presence of the weakly coordinating anion PF6−.31 The XPhos-coordinated AgI cation could further be coordinated by substrates or solvent to generate other complexes, and these complexes exist in equilibrium before hydrogen activation.32 Here we considered the coordination of DIEPA, benzaldehyde, water, and H2 on [(XPhos)AgI]+ (Scheme 3). Besides, two conformations for biaryl phosphine ligands (i.e., synperiplanar33 and anticlinal)34 were examined. The DIPEA-coordinated complexes are found to be more stable than the ones coordinated by benzaldehyde, water, or H2. This is understandable because the nucleophilicity of the base DIPEA is stronger than all the other species. Moreover, the synperiplanar isomers are always more stable than the anticlinal isomers, though the former seems to be more crowded. To explore the origin of the stability of synperiplanar isomers, the molecular orbitals of 4 and 4′ were investigated first. It is found that the orbitals of Ag and the iPr-substituted phenyl group overlaps in the HOMO, HOMO−2, and HOMO−4 of 4 (Figure 1), indicating that the aryl group also coordinates with the Ag center. In contrast, there is no orbital overlap between Ag and the iPr-substituted phenyl group in 4′ because they locate far from each other. We found C−H···π interactions between the cyclohexyl group on phosphine and one aryl group in 4′ (Figure 2). However, the C−H···π interactions belong to noncovalent interaction and are slightly stronger than van der Waals interactions while weaker than the coordination interaction of the aryl group in synperiplanar isomers. Therefore, synperiplanar isomers are generally more stable than anticlinal isomers here. As a result, the synperiplanar DIPEA-coordinated complex 1 is the most stable one among all the investigated complexes. 1 was chosen as the reference point for the following mechanistic studies on hydrogen activation. 3.2. Base-Assisted Hydrogen Activation. From the cationic Ag(I) complexes, base-assisted hydrogen activation mechanism was investigated first. As shown in Figure 3, ligand exchange of complex 1 with H2 generates the complex 5, causing an energy increase of 13.9 kcal/mol. The H−H bond of hydrogen changes slightly from 0.7429 to 0.7550 Å in this step. Then the cleavage of H−H bond occurs via transition state TS1 with the aid of the base DIPEA. The free-energy barrier of this step is +19.4 kcal/mol (from 1 to TS1). In TS1, the H−H bond length increases to 0.900 Å (Figure 4). One of the hydrogen atoms approaches the Ag center and the other locates close to the nitrogen atom of DIPEA. The NBO charges of the

complexes do exist,19 the oxidative addition mechanism could not be excluded yet. In this article, density functional theory (DFT) methods were used to elucidate the dominant mechanism for the Agcatalyzed hydrogenation of aldehydes with H2. The calculation results indicate that the base-assisted hydrogen activation mechanism (path A) is the most favored one. The ligand XPhos is not a good proton acceptor compared with the base DIEPA, resulting in high-energy ligand-protonated intermediates in path B. The energy demand of the oxidative addition mechanism is estimated to be much higher because of the instability of cationic AgIII hydride complexes, and therefore, path C is also excluded. In addition, it is found that XPhos tends to coordinate with the Ag center by both the phosphine and the π-bond of the isopropyl-substituted phenyl group. This phenomenon explains the superiority of XPhos in Ag-catalyzed hydrogenation reactions compared with the other bidentate and monodentate phosphine ligands in Li’s study.

2. COMPUTATIONAL DETAILS The hydrogenation of benzaldehyde catalyzed by the silver complex was chosen as the model reaction (Scheme 2). The full structures of all

Scheme 2. Model Reaction for Mechanistic Studies

these species were used in DFT calculations. All calculations were performed with Gaussian 09.20 Geometry optimizations were conducted in the solution phase with Truhlar’s M06 function21 and the SMD solvation model22 (solvent = water). The LANL2DZ basis set and the effective core potential implemented23 with an extra polarization function [ζ(f) = 1.611]24 was used to describe the Ag atom, while 6-31G(d) basis set was used for the rest of the atoms. Frequency analysis was conducted at the same level of theory to 6578


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Scheme 3. Gibbs Free Energies of Different Cationic Ag(I) Complexes (kcal/mol)

Figure 1. Molecular orbitals of complex 4 (isovalue = 0.02).

Figure 2. Structures of complex 4′ (left) and its noncovalent interaction plot (right). For clarity, hydrogen atoms were not shown in the left picture except the three ones closest to the aryl group. The green surface between the cyclohexyl and isopropyl-substituted phenyl indicates that their concovalent interaction is weak and approaches van der Waals interactions.

two hydrogen atoms are −0.279 and +0.224, respectively, indicating that this step is a heterolytic process. Halper previously proposed a four-membered transition state for Ag-catalyzed hydrogen activation,10e but we failed to locate the related transition state. This is possibly because that DIPEA interacts with the hydrogen atom with a sp3-hybridized orbital (from the nitrogen atom), and this orbital is required to face toward both the hydrogen atom and the Ag center in the fourmembered transition state. This orientation of the bulk DIPEA

will cause a repulsion between DIPEA and the catalyst and make the four-membered transition state disfavored. In addition to the synperiplanar transition state TS1, the anticlinal isomer TS1′ was also considered. The free energy of TS1′ is +29.2 kcal/mol, higher than that of TS1 by 9.8 kcal/mol. The reason for such observation is related to the electron deficiency of the cationic AgI, which favors the extra coordination of the electron-donating ligand (the ortho-aryl group). As shown in Figure 1, the molecular orbitals show that the ortho-aryl group 6579


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Figure 3. Energy profile of base-assisted hydrogen activation catalyzed by the cationic Ag(I) complex.

Figure 4. Structures of the key transition states in the base-assisted hydrogen activation catalyzed by the cationic Ag(I) complex (bond length in angstrom). For clarity, hydrogen atoms were omitted except the ones from the hydrogen molecule.

Scheme 4. Computational Results of Ligand-Assisted Hydrogen Activation Mechanisma

a

(a) Gibbs free energies of the intermediates in ligand-assisted hydrogen activation mechanism (kcal/mol). (b) NBO charge on the carbon atoms of isopropyl-substituted phenyl group in complex 5.

of the π-coordination of the substituted phenyl group for Agcatalyzed hydrogen activation. The hydride complex 6 is formed after TS1 with an energy decrease of 10.3 kcal/mol. Then the nucleophilic attack of hydride to benzaldehyde occurs through transition state TS2. The free energy of TS2 is +21.8 kcal/mol, slightly higher than that of TS1 by 2.4 kcal/mol. TS2 is not a four-membered transition state, whereas it is frequently proposed or located by

coordinates to Ag and in the synperiplanar isomers, while no such coordination exists in the anticlinal isomer. Thus, it is understandable that the synperiplanar transition state TS1 is more stable than TS1′. Interestingly, if the coordination of the aryl group of XPhos is absent, the related free energy barrier of the hydrogen activation step will be elevated from 19.4 to 22.8 kcal/mol (from 1′ to TS1′). This result reveals the importance 6580


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Scheme 5. Comparison of the Protonation of XPhos and the Anionic Ligand 1* in Ligand-Assisted Hydrogen Activation

AgIII−O intermediate also led to AgI complexes. These results suggest that AgIII hydride intermediates are not stable in the studied system. To estimate the energy demand of the insertion of AgI into the H−H bond, we conducted partial geometry optimizations by fixing the H−H bond at different lengths. The relative electronic energies of these restricted structures are shown in Figure 5. It is found that no maximum is met even when the H−H bond length has reached 1.600 Å, and the energy of this state has been higher than that of 5 by +47.4 kcal/mol.

theoretical methods in the hydrometalation of unsaturated compounds catalyzed by other transition metals (Figure 4).35 The Ag−O bond is as long as 3.467 Å in TS2. The unmatched chemical hardness of Ag+ and RO− might be responsible for this phenomenon.36 However, the alkoxide spontaneously coordinates to Ag after TS2 to form the intermediate 7. Thereafter, the protonation of 7 gives the product and regenerates the catalyst 1. The free-energy change of the whole catalytic cycle is −7.6 kcal/mol. 3.3. Ligand-Assisted Hydrogen Activation. The possibility of ligand-assisted hydrogen activation mechanism (path B) was investigated in this section. Three XPhos-protonated intermediates (8−10) might be formed after hydrogen activation (Scheme 4a). In 8−10, the proton locates at the ortho-, meta-, and para-position of the iPr-substituted phenyl group, respectively. 9 is the most stable one among these compounds, and its free energy is +45.3 kcal/mol. The reason for the lower free energy of 9 relative to 8 and 10 is that the meta-position is more electron-rich than the ortho- or parapositions (NBO charge has been shown in Scheme 4b). The high energies of the XPhos-protonated intermediates imply that XPhos is not a competent proton acceptor compared with the previously reported ligands (e.g., 1* in Scheme 5) in the ligand-assisted hydrogen activation process.14−16 The major difference between XPhos and these ligands is their charge. Because XPhos is neutral, the protonation of XPhos will generate a Wheland intermediate and thus disrupts the aromaticity of the substituted aryl group. By contrast, the protonation of the anionic ligand 1* will not cause this problem. In fact, the hydrogen activation process in the presence of 1* is calculated to be exergonic partially due to the formation of the aromatic pyridine ring.16a Furthermore, XPhos is also inferior to the typical base DIPEA in accepting a proton because there is no highly nucleophilic site on XPhos. As a result, 9 is less stable than 6 and protonated DIPEA by 36.2 kcal/mol. Note that RuPhos was also efficient in Li’s reaction and the oxygen atom in RuPhos could possibly act as a proton acceptor. Such mechanistic possibilities have been examined in our study, while the RuPhos-protonated Ag−H intermediate was found to be highly unstable with a free energy over 48 kcal/mol. Thus, similar to the aforementioned conclusions, the ligand-assisted hydrogen activation mechanism is also unfeasible for [RuPhosAg]+ catalytic systems (Scheme S1 of the Supporting Information). 3.4. Oxidative Addition. Finally the possibility of oxidative addition mechanism (path C) was considered. Though much effort has been devoted to locate the oxidative addition transition states, only AgI complexes were obtained because spontaneous H−H reduction always occurred during unrestricted geometry optimizations. Besides, the geometry optimizations of the subsequent transition state of CO bond insertion on AgIII hydride intermediates and the resulting

Figure 5. Gibbs free energies (kcal/mol) of hydride Ag complex with the different H−H bond lengths of molecule hydrogen (Å).

The relatively low electronic-withdrawing ability of hydrogen and the use of bidentate ligand XPhos are suggested to be responsible for the spontaneous reduction of the investigated AgIII hydride complexes. First, the known AgIII complexes usually include a reduction-resistant anion like F−, OH−, and SO42−.19 In contrast, hydride ligands are present in the concerned complexes. Because the electronegativity of H is lower than F and O, electron transfer from the hydride ligands to the AgIII center becomes more facile. Second, tetradentate ligands or the ligands with conjugative skeletons such as biguanide and pyridine are commonly used to stabilize the electron-deficient AgIII center in previous reports. Here the bidentate ligand XPhos is used in the cationic AgIII complexes. We speculate that XPhos is not sufficiently electron-donating to stabilize the AgIII center compared with these tetradentate ligands. Due to the two factors, the investigated AgIII hydride complexes are highly unstable and can undergo spontaneous reduction. For the same reason, the variation from AgI to AgIII complexes by restricted geometry optimization also causes a remarkable energetic increase. 6581


Organometallics 3.5. Favored Hydrogen Activation Mechanism. According to the above calculation results, hydride transfer (TS2) is the rate-determining step in the base-assisted hydrogen activation/hydrogenation of aldehydes pathway. The total freeenergy barrier of this pathway is 21.8 kcal/mol.37 The free energies of the intermediates in ligand-assisted hydrogen activation mechanism have been above 45 kcal/mol, indicating that this pathway is disfavored. In addition, the energy demand of oxidative addition of H2 on cationic AgI complex is estimated to be higher than 47 kcal/mol, thus we also excluded this pathway. Therefore, the base-assisted hydrogen activation is the favored mechanism for cationic-AgI-catalyzed hydrogen activation.

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REFERENCES

(1) (a) De Vries, J. G.; Elsevier, C. J. Handbook of Homogeneous Hydrogenation; Wiley-VCH: Weinheim, 2007. (b) Blaser, H. U.; Pugin, B.; Spindler, F. In Organometallics as Catalysts in the Fine Chemical Industry; Beller, M., Blaser, H. U., Eds.; Springer-Verlag: Berlin, 2012; Vol. 42, p 65. (c) Lin, G. Q.; You, Q. D.; Cheng, J. F. Chiral Drugs: Chemistry and Biological Action; John Wiley & Sons Inc.: Hoboken, NJ, 2011. (d) Verendel, J. J.; Pàmies, O.; Diéguez, M.; Andersson, P. G. Chem. Rev. 2014, 114, 2130. (e) Zuo, W.; Lough, A. J.; Li, Y. F.; Morris, R. H. Science 2013, 342, 1080. (f) Etayo, P.; Vidal-Ferran, A. Chem. Soc. Rev. 2013, 42, 728. (g) Chen, Q.-A.; Ye, Z.-S.; Duan, Y.; Zhou, Y.-G. Chem. Soc. Rev. 2013, 42, 497. (h) Magano, J.; Dunetz, J. R. Org. Process Res. Dev. 2012, 16, 1156. (i) Wang, D.-S.; Chen, Q.-A.; Lu, S.-M.; Zhou, Y.-G. Chem. Rev. 2012, 112, 2557. (j) Ager, D. J.; de Vries, A. H. M.; de Vries, J. G. Chem. Soc. Rev. 2012, 41, 3340. (k) Xie, J. H.; Zhu, S. F.; Zhou, Q. L. Chem. Rev. 2011, 111, 1713. (l) Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 2411. (m) Knowles, W. S.; Noyori, R. Acc. Chem. Res. 2007, 40, 1238. (2) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Nature 1965, 208, 1203. (b) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. J. Chem. Soc. A 1966, 1711. (3) Selected examples of Rh-catalyzed hydrogenation: (a) Dornan, P. K.; Kou, K. G. M.; Houk, K. N.; Dong, V. M. J. Am. Chem. Soc. 2014, 136, 291. (b) Prakash, O.; Sharma, K. N.; Joshi, H.; Gupta, P. L.; Singh, A. K. Organometallics 2014, 33, 2535. (c) Jiang, J.; Wang, Y.; Zhang, X. ACS Catal. 2014, 4, 1570. (d) Konrad, T. M.; Schmitz, P.; Leitner, W.; Franciò, G. Chem.Eur. J. 2013, 19, 13299. (e) RaseroAlmansa, A. M.; Corma, A.; Iglesias, M.; Sánchez, F. ChemCatChem 2013, 5, 3092. (f) Li, S.; Huang, K.; Zhang, J.; Wu, W.; Zhang, X. Chem.Eur. J. 2013, 19, 10840. (g) Huang, K.; Li, S.; Chang, M.; Zhang, X. Org. Lett. 2013, 15, 484. (4) Selected examples of Ru-catalyzed hydrogenation: (a) Zeng, M.; Li, L.; Herzon, S. B. J. Am. Chem. Soc. 2014, 136, 7058. (b) Zirakzadeh, A.; Groß, M. A.; Wang, Y.; Mereiter, K.; Weissensteiner, W. Organometallics 2014, 33, 1945. (c) Dai, X.; Cahard, D. Adv. Synth. Catal. 2014, 356, 1317. (d) Du, W.; Wang, Q.; Wang, L.; Yu, Z. Organometallics 2014, 33, 974. (e) John, J. M.; Takebayashi, S.; Dabral, N.; Miskolzie, M.; Bergens, S. H. J. Am. Chem. Soc. 2013, 135, 8578. (f) Patchett, R.; Magpantay, I.; Saudan, L.; Schotes, C.; Mezzetti, A.; Santoro, F. Angew. Chem., Int. Ed. 2013, 52, 10352. (g) Ortega, N.; Tang, D. -T. D.; Urban, S.; Zhao, D.; Glorius, F. Angew. Chem., Int. Ed. 2013, 52, 9500. (5) Selected examples of Ir-catalyzed hydrogenation: (a) Cheng, C.; Kim, B. G.; Guironnet, D.; Brookhart, M.; Guan, C.; Wang, D. Y.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2014, 136, 6672. (b) Liu, X.; Han, Z.; Wang, Z.; Ding, K. Angew. Chem., Int. Ed. 2014, 53, 1978. (c) Gruber, S.; Pfaltz, A. Angew. Chem., Int. Ed. 2014, 53, 1896. (d) Saleem, F.; Rao, G. K.; Kumar, A.; Mukherjee, G.; Singh, A. K. Organometallics 2014, 33, 2341. (e) Zhou, Y.; Liu, D.; Fu, Y.; Yu, H.; Shi, J. Chin. J. Chem. 2014, 32, 269. (f) Müller, M.-A.; Pfaltz, A. Angew. Chem., Int. Ed. 2014, 53, 8668. (g) Núñez-Rico, J. L.; Fernández-Pérez, H.; Vidal-Ferran, A. Green Chem. 2014, 16, 1153. (h) Song, S.; Zhu, S.-F.; Pu, L.-Y.; Zhou, Q.-L. Angew. Chem., Int. Ed. 2013, 52, 6072. (6) Selected examples of Pd-catalyzed hydrogenation: (a) Duan, Y.; Li, L.; Chen, M.-W.; Yu, C.-B.; Fan, H.-J.; Zhou, Y.-G. J. Am. Chem. Soc. 2014, 136, 7688. (b) Li, Z.; Li, J.; Liu, J.; Zhao, Z.; Xia, C.; Li, F.

ASSOCIATED CONTENT

S Supporting Information *

Text giving the complete ref 20. Energies and Cartesian coordinates of all calculated intermediates and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We thank the 973 Program (2012CB215306), the NSFC (Grants 21325208, 21172209, 21361140372, 21202006), SRFDP (Grant 20123402110051), FRFCU (Grant WK2060190025), CAS (Grant KJCX2-EW-J02), Fok Ying Tung Education Foundation, Anhui Provincial Natural Science Foundation (Grant 1308085QB38), ChinaGrid project funded by MOE of China, and the supercomputer center of Shanghai and USTC.

4. CONCLUSIONS Homogeneous Ag-catalyzed hydrogen activation/hydrogenation reactions demonstrate the potential of Ag catalysts in organic transformations, expanding the synthetic tools for hydrogenation reactions. To gain a deeper mechanistic insight, three possible mechanisms including base-assisted hydrogen activation, ligand-assisted hydrogen activation and oxidation addition were investigated with the aid of DFT methods. Computational results indicate that XPhos is not a good proton acceptor because the protonation of XPhos will disrupt the aromaticity of the aryl group. The resulting intermediates in the ligand-assisted hydrogen activation mechanism thus are energetically much higher. The oxidative insertion of AgI into the H−H bond is also estimated to be difficult due to the insufficiently electron-donating XPhos and relatively weak electron-withdrawing hydride ligands. By contrast, the baseassisted hydrogen activation with the following steps represents a favored mechanism. This mechanism mainly consists of heterolytic H−H bond cleavage, hydride transfer and protonation. The free energy of the hydride transfer transition state is slightly higher than that of the hydrogen activation transition state, and hydride transfer is the rate-determining step of the whole catalytic cycle. In addition, the ligand XPhos tends to coordinate to Ag by both the phosphine and the iPrsubstituted phenyl group. This unique coordination fashion is able to lower the energy barrier of hydrogen activation, making XPhos different from other less-efficient bidentate and monodentate phosphine ligands. In this context, the present study provided a systematic quantum-chemical study on the mechanism of homogeneous Ag-catalyzed hydrogen activation. The related results are expected to benefit mechanistic understanding and further development of organic transformations based on Ag-catalyzed hydrogenation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6582


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Organometallics

Article

G.; Zhang, Y.; Garcia-Fortanet, J.; Kinzel, T.; Buchwald, S. L. Science 2009, 325, 1661. (d) Partyka, D. V.; Updegraff, J. B., III; Zeller, M.; Hunter, A. D.; Gray, T. G. Organometallics 2009, 28, 1666. (34) (a) Konig, M.; Reith, L. M.; Monkowius, U.; Knor, G.; Bretterbauer, K.; Schoefberger, W. Tetrahedron 2011, 67, 4243. (b) Diebolt, O.; Fortman, G. C.; Clavier, H.; Slawin, A. M. Z.; Escudero-Adan, E. C.; Benet-Buchholz, J.; Nolan, S. P. Organometallics 2011, 30, 1668. (35) For four-membered transition states see: (a) Jiang, Y.-Y.; Li, Z.; Shi, J. Organometallics 2012, 31, 4356. (b) Hopmann, K. H. Organometallics 2013, 32, 6388. (c) Hong, X.; Liu, P.; Houk, K. N. J. Am. Chem. Soc. 2013, 135, 1456. (d) Fan, T.; Sheong, F. K.; Lin, Z. Organometallics 2013, 32, 5224. (e) Gridnev, I. D.; Kohrt, C.; Liu, Y. Dalton Trans. 2014, 43, 1785. (f) Xie, H.; Zhao, L.; Yang, L.; Lei, Q.; Fang, W.; Xiong, C. J. Org. Chem. 2014, 79, 4517. (36) (a) Pears, R. G. J. Am. Soc. Chem. 1963, 85, 3533. (b) Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512. (37) Some reviewers suggested the possibility of aldehyde-assisted and Ag-OH species catalyzed hydrogen activation mechanisms. We excluded these mechanisms for their high energy barriers. For more details please see Schemes S2 and S3 in the Supporting Information.

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Theoretical Study on Homogeneous Hydrogen Activation Catalyzed

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