DFT Studies on the Reaction Mechanism of 1,3-Conjugated Dienes

Article pubs.acs.org/Organometallics

DFT Studies on the Reaction Mechanism of 1,3-Conjugated Dienes Isomerization Catalyzed by Ruthenium Hydride Yu Chen, Mei-yan Wang,* Sheng Fang, Ting Wang, and Jing-yao Liu* Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *

ABSTRACT: The detailed reaction mechanism for the isomerization of 1,3-conjugated dienes catalyzed by the ruthenium hydride complex RuHCl(CO)(H2IMes)(PCy3) has been studied with the aid of density functional theory (DFT) calculations. Both cis and trans isomers of a 1,3conjugated diene were considered as the reactants. For each isomer, two catalytic cycles were calculated, which (respectively) generate a 1,3-hydride shift product or a 1,5-hydride shift product. Both catalytic cycles proceed via alkene migratory insertion into the Ru−H bond, σ-allyl ruthenium isomerization, and β-H elimination steps. Our computational study shows that the cis isomer of the model reactant reacts preferentially via the pathway leading to the 1,5-hydride shift product, consistent with the experimental results. The σ-allyl ruthenium isomerization step is found to be crucial for reaction regioselectivity. Strong binding of the CC bond to Ru is involved in the generation of the 1,5-hydride shift product. In addition, the steric effect of the bulky N-heterocyclic carbene ligand in ruthenium hydride RuHCl(CO)(H2IMes)(PCy3) was considered theoretically.

1. INTRODUCTION Olefin isomerization, an atom-economical chemical reaction,1−3 plays an important role in many important chemical processes, such as petrochemical refining4,5 and polymerization reactions.6 Recently, the olefin isomerization reaction has been highlighted as a very useful method to synthesize many desired targets.7 Catalysts containing a broad range of transition metals, including Cr,8 Fe,9,10 Co,11,12 Pd,13−15 Ni,13 Rh,16 Ir,17 Pt,18 Zr,19 and Ru,20−23 have been developed to facilitate olefin isomerization. Most such studies, however, focus on the migration of nonconjugated double bonds.8−21 Only a limited number of investigations consider the isomerization of conjugated dienes.22,23 The ruthenium hydride complex RuHCl(CO)(H2IMes)(PCy3) 1 (Scheme 1a) has been found to show high catalytic activity in olefin isomerization,24,25 although it has been pointed out that its activity is limited by slow phosphine loss, which curbs initiation.26 Recently, Diver et al. reported that 1 catalyzed the isomerization of 1,3-conjugated dienes into more highly substituted 1,3-conjugated dienes (Scheme 1a).23 Interestingly, their study showed that the isomerization reactions of a series of 1,3-conjugated dienes are highly regioselective. That is, only the products of a 1,5-hydride shift were generated; no 1,3hydride shift products were observed, although the latter products are commonly formed in olefin isomerization.27 The reaction mechanisms of olefin isomerization catalyzed by transition-metal complexes have been studied theoretically.28−31 In general, there are two accepted reaction mechanisms for olefin isomerization: alkyl mechanism and π-allyl mechanism. The alkyl mechanism is typically followed when a metal hydride precatalyst is employed,25,27 while the π-allyl mechanism is usual for nonhydride catalysts.27 For the isomerization of conjugated dienes catalyzed by complex 1, Diver et al. proposed a © XXXX American Chemical Society

Scheme 1. (a) Isomerization of 1,3-Conjugated Dienes Catalyzed by the Ruthenium Hydride Complex RuHCl(CO)(H2IMes)(PCy3) 1. (b) Mechanism Proposed by Diver et al. in ref 23

mechanism23 that involves, first, hydrometalation of the terminal CC bond of the 1,3-diene reactant, followed by the transit of the ruthenium atom across the diene framework via a πReceived: February 15, 2015

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Organometallics allylruthenium intermediate, and formation of the final product through β-H elimination (Scheme 1b). However, many fundamental issues remain unclear. These include the possible intermediates formed in the reaction process, the detailed reaction pathways, and the factors leading to the high regioselectivity of the products. To the best of our knowledge, no theoretical studies have yet been reported for the catalytic isomerization of conjugated dienes. Therefore, in this paper, we carried out a detailed density functional theory (DFT) study aimed at gaining deeper insight into the mechanism of diene isomerization by catalyst 1. To simplify the reaction, the model reactant (E)-2-ethylhexa-1,3-diene (R1 = H, R2 = R3 = CH3) 2 and catalyst 1 were employed (eq 2). We also performed some calculations on a small catalyst model, RuHCl(CO)L(PCy3) (L = 1,3-dimethylimidazol-2-ylidene) 1S, to investigate the steric effect on the regioselectivity of the reaction (eq 2). We expect that the present study will aid in understanding the mechanism of isomerization of conjugated dienes, and identifying the factors that influence product selectivity.

Scheme 2. Cycle I: 1,3-Hydride Shift; Cycle II: 1,5-Hydride Shift

β-H elimination to give complex D having the 1, 3-hydride shift product coordinated. Release of this product would regenerate the active species [Ru]−H. The other catalytic cycle (Cycle II) proceeds through the isomerization of σ-allyl ruthenium complex B to form π-allyl ruthenium complex E, which then transforms into another σ-allyl ruthenium complex F with a Ru−C4 σ bond. β-H elimination of F will give complex G, followed by release of the 1, 5-hydride shift product, to regenerate the active species [Ru]−H. There are two isomers of the reactant 2, trans-2 and cis-2 (Figure 1). It is found that cis-2 is only higher by 2.4 kcal/mol in

2. COMPUTATIONAL DETAILS Molecular geometries of all model complexes were optimized without constraints via DFT calculations using the M06L functional.32 This functional has been shown to be appropriate for studies of rutheniumcatalyzed reactions,30,33,34 and benchmark studies have confirmed its excellent accuracy in transition-metal systems.35 The effective core potential (ECP) of Hay and Wadt with double-ζ LANL2DZ36 was chosen to describe Ru, and for H, C, O, Cl, N, and P, the 6-31G** basis set was used. Frequency calculations were carried out at the same level of theory to identify all of the stationary points as transition states (one imaginary frequency) or as minima (zero imaginary frequencies) and to provide the thermal correction to free energies at 298.15 K and 1 atm. Intrinsic reaction coordinates (IRC)37 were calculated for all transition states to confirm that these structures indeed connect two relevant minima. In order to consider solvent effects, the single-point energy calculations were performed at the M06L level with the SDD38 basis set as well as polarization functions (ζf = 1.235)39 for Ru, and the 6311+G** basis set for all other atoms, using the continuum solvent model SMD.40 Toluene, which was used as the solvent in Diver’s experimental study,23 was adopted. The Gibbs free energy for each species on the potential energy profiles is taken as the sum of the thermal correction to free energies in gas phase and the single-point energy in solution. All calculations were performed with the Gaussian 09 software package.41

Figure 1. Gibbs energy profile calculated for the transformation between trans-2 and cis-2. The calculated relative free energies are given in kcal/mol.

energy than trans-2, and the transformation between them occurs easily via transition state TSTC, with an energy barrier of 6.6 kcal/mol. Thus, both isomers should be considered, respectively, to figure out which one participates in diene isomerization. Although trans-2 is more stable than cis-2, our results indicate that the favorable pathway is the one starting with cis-2. Therefore, the reaction pathways starting with cis-2 are described in detail. Isomerization of cis-1,3-Diene. For systems with a high degree of conformational flexibility, there is normally more than one possible structure for each intermediate and transition state. It is impossible to explore the whole potential energy surface, however, and a single plausible structure for each intermediate and transition state was, therefore, examined in the present study. The Gibbs energy profiles for the isomerization of cis-2 are shown in Figures 2−4. In this paper, unless otherwise stated, all the calculated energies on Gibbs energy profiles are given relative to catalyst 1 + trans-2. For catalyst 1, the phosphine ligand PCy3 first dissociates to generate an open site for reactant coordination. The dissociation of PCy3 gives the active species

3. RESULTS AND DISCUSSION Two possible reaction mechanisms for diene isomerization are shown in Scheme 2. In addition to the 1,5-hydride shift (Cycle II) proposed by Diver et al.,23 a mechanism for formation of the 1,3hydride shift product (Cycle I) is shown. Although a 1,3-hydride shift product was not experimentally observed, its formation mechanism is considered and compared to analyze the regioselectivity of the reaction. For entry into either cycle, the first step involves binding of the terminal CC bond of the 1,3diene reactant to the active species [Ru]−H, giving complex A. Subsequent migratory insertion generates σ-allyl ruthenium complex B, containing a Ru−C2 σ bond. From complex B, there are two reaction pathways. One catalytic cycle (Cycle I) involves B

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Figure 2. Gibbs energy profile calculated for the reaction of catalyst 1 with cis-2 to form agostic intermediate 4C. The calculated free energies relative to catalyst 1 + trans-2 are given in kcal/mol.

1′, which is higher than catalyst 1 by 14.8 kcal/mol. Coordination of the terminal CC bond of cis-2 to the active species 1′ generates π-coordinated complex 3C. Migratory insertion of the coordinated alkene into the Ru−H bond forms intermediate 4C through transition state TS34C (24.4 kcal/mol). Intermediate 4C is an agostic complex with a Ru···H length of 1.990 Å and a C···H length of 1.133 Å. From complex 4C, there are two reaction pathways (Figure 3). In one pathway (Figure 3a), by clockwise rotation of σ-allyl around the σ Ru−C bond, an agostic intermediate 5Ca with a Ru···H length of 1.934 Å and a C···H length of 1.143 Å is formed via transition state TS45Ca. Then, the β-H elimination takes place via transition state TS56Ca to give a π-coordinated complex 6Ca, followed by release of the 1,3-hydride shift product ProCa, to regenerate the active species 1′. The overall energy barrier for this reaction pathway is 29.9 kcal/mol (TS45Ca relative to catalyst 1 + trans-2). In the other pathway (Figure 3b), σ-allyl of intermediate 4C rotates around the σ Ru−C bond in an anticlockwise direction via TS45Cb to form intermediate 5Cb with binding of the CC bond to Ru. A facile isomerization proceeds via π-allyl transition state TS56Cb to give complex 6Cb, in which the CC bond weakly coordinates to Ru. The subsequent rotation of σ-allyl around the Ru−C bond generates intermediate 7Cb, which contains an agostic interaction with a Ru···H length of 1.974 Å and a C···H length of 1.138 Å. Comparing two pathways from intermediate 4C, it is found that TS45Cb is lower than TS45Ca by 1.9 kcal/mol, suggesting that the former pathway giving 7Cb is more favorable than the latter one generating the 1,3-hydride shift product ProCa. There are two reaction pathways from intermediate 7Cb (Figure 4). In one pathway (Figure 4a), the direct β-H elimination proceeds via TS78CbE, giving π-coordinated complex 8CbE. Release of the 1,5-hydride shift (E)-product ProCbE and coordination of PCy3 generate the catalyst 1en, which is the enantiomer of catalyst 1. The catalyst 1en can react with the enantiomer of cis-2 in the same manner. The other pathway (Figure 4b) first involves the rotation of the methyl substituent around the C−C bond, giving an agostic intermediate 7′Cb (with a Ru···H length of 1.969 Å and a C···H length of 1.141 Å). Subsequent β-H elimination forms π-coordinated complex

Figure 3. Gibbs energy profiles calculated for two reaction pathways from intermediate 4C. (a) Formation of the 1,3-hydride shift product ProCa. (b) Formation of agostic intermediate 7Cb. The calculated relative free energies are given in kcal/mol.

8CbZ, from which the catalyst 1en is generated by release of the 1,5-hydride shift (Z)-product ProCbZ and coordination of PCy3. Comparing two pathways from 7Cb, it is found that TS78CbE is 2.5 kcal/mol lower than TS7′8CbZ and ProCbE lies below ProCbZ by 1.4 kcal/mol, indicating that the reaction pathway generating the 1,5-hydride shift (E)-product ProCbE is kinetically and thermodynamically more favorable than that giving the 1,5-hydride shift (Z)-product ProCbZ. For the isomerization of cis-2, the rate-determining transition state of the most preferred reaction pathway is TS45Cb (28.0 kcal/mol). C

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obtained for cis-2 are also found for trans-2. For comparison, only the preferred isomerization pathways of cis-2 and trans-2 are shown in Figure 5. It is seen that both reactions proceed via a 1,5-hydride shift mechanism to give (E)-product ProCbE and ProTbE, respectively. The energy of the rate-determining transition states TS45Cb (28.0 kcal/mol) is lower than that of TS45Tb (31.7 kcal/mol), suggesting that cis-2 rather than trans-2 participates in diene isomerization to give the 1,5-hydride shift (E)-product, although trans-2 is more stable than cis-2. The Factors Affecting Reaction Regioselectivity. As discussed above, starting with cis-2 or trans-2, the pathway generating the 1,5-hydride shift product is more favorable than that giving the 1,3-hydride shift product. To analyze the factors affecting the regioselectivity of the reaction, the structures of four rate-determining transition states leading to the final products are shown in Figure 6a. Strong binding of the CC bond to Ru in TS45Cb is found with two Ru−C distances of 2.900 and 3.648 Å, while only a weak Ru···H interaction is observed in TS45Ca, where two Ru−H distances are 2.602 and 3.026 Å, respectively. Relatively strong binding of the CC bond to Ru is responsible for the lower energy of TS45Cb than TS45Ca. A similar feature can be found for TS45Tb and TS45Ta. Comparing both TS45Cb and TS45Tb leading to the 1,5-hydride shift products, TS45Cb is lower than TS45Tb by 3.7 kcal/mol. The lower energy of TS45Cb than TS45Tb can be attributed to stronger binding of the CC bond to Ru in TS45Cb (with a Ru−C distance of 3.648 Å) than that in TS45Tb (with a Ru−C distance of 3.909 Å). To study the effect of the bulky N-heterocyclic carbene ligand in catalyst 1 on reaction regioselectivity, we also employed a small catalyst model 1S (see eq 2), where mesityl (Mes) in catalyst 1 is replaced with methyl. The geometries and energies of the corresponding four rate-determining transition sates labeled as S-TS45Ca, S-TS45Cb, S-TS45Ta, and S-TS45Tb are shown in Figure 6b. It is seen that in this reaction system, S-TS45Ca (STS45Ta) is lower in energy than S-TS45Cb (S-TS45Tb), and as a result, the 1,3-hydride shift product becomes more favored over the 1,5-hydride shift product. Clearly, this product selectivity is inconsistent with that observed experimentally.23 To understand the original nature of the ligand effect, we compared the structures of the rate-determining transition states in both models. When using catalyst 1 bearing a bulky Mes group, it is found that there is large steric hindrance in TS45Ca and TS45Ta due to the presence of the bulky Mes group, while the influence of the steric hindrance in the TS45Cb and TS45Tb is relatively small. In other words, the steric effect caused by bulky Mes destabilizes TS45Ca and TS45Ta; thus, the reaction path involving TS45Cb/TS45Tb is the major channel, leading to the formation of the product ProCbE/ProTbE. However, the case is different when using catalyst 1S, since the large steric hindrance caused by Mes is removed from transition states S-TS45Ca and S-TS45Ta. STS45Ca and S-TS45Ta become more stable than S-TS45Cb and STS45Tb, respectively, and so the product selectivity is changed. These results indicate that the bulky Mes group in catalyst 1 has a significant influence on reaction regioselectivity and could not be simplified to a small group such as methyl.

Figure 4. Gibbs energy profiles calculated for the generation of the 1,5hydride shift (E)-product ProCbE (a) and (Z)-product ProCbZ (b). The calculated relative free energies are given in kcal/mol.

Note that, since the binding through the internal alkene fragment of cis-2 is also possible, intermediate 3′C formed by binding of the internal CC bond to the active species 1′ is located (see the Supporting Information). It is seen that the energies of 3′C and 3C are almost the same, 12.3 kcal/mol vs 12.2 kcal/mol. On the basis of the similar reaction mechanism discussed above, however, the generating nonconjugated product (named as Pro′) (see the Supporting Information) from the pathway via 3′C is found to lie higher in energy than reactant trans-2 by 3.5 kcal/mol and is less stable than product ProCbE by 8.8 kcal/mol, indicating that the reaction pathway via 3′C is thermodynamically less favorable than that via 3C. Thus, the pathways through 3′C have not been considered in this work. Isomerization of trans-1,3-Diene. The mechanism of the isomerization reaction starting with trans-2 has also been studied, and the detailed reaction pathways are given in the Supporting Information. The same trends that have been

4. CONCLUSION The reaction mechanism for the isomerization of 1,3-conjugated dienes catalyzed by ruthenium hydride RuHCl(CO)(H2IMes)(PCy3) 1 has been theoretically investigated by carrying out DFT calculations. The reactant (E)-2-ethylhexa-1,3-diene 2 with cis and trans isomers is adopted as the reactant model. Although trans-2 is lower in energy than cis-2, the transformation between D

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Figure 5. (a) The preferred reaction pathway calculated for the reaction of catalyst 1 with cis-2. (b) The preferred reaction pathway calculated for the reaction of catalyst 1 with trans-2. The Gibbs free energies relative to catalyst 1 + trans-2 are given in kcal/mol.

tively. Our calculation results show that, starting with cis-1,3diene, the reaction pathway giving the 1,5-hydride shift (E)product is kinetically and thermodynamically more favorable. The favored reaction pathway takes place as follows: First,

them is likely to occur and both isomers are considered in the calculations of the reaction pathway. We identified the reaction pathways generating the 1,3-hydride shift product, 1,5-hydride shift (E)-product, and 1,5-hydride shift (Z)-product, respecE

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Figure 6. (a) Four transition states TS45Ca, TS45Cb, TS45Ta, and TS45Tb bearing a bulky Mes group. The Gibbs free energies (in kcal/mol) are calculated relative to catalyst 1 + trans-2. (b) Four transition states S-TS45Ca, S-TS45Cb, S-TS45Ta, and S-TS45Tb having a small methyl group. The Gibbs free energies (in kcal/mol) are calculated relative to catalyst 1S + trans-2. Bond distances are given in Å.

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dissociation of the phosphine ligand from the ruthenium hydride 1 forms an open site for cis isomer coordination, and then migratory insertion of the terminal CC bond into the Ru−H bond generates an σ-allyl ruthenium complex containing an agostic Ru···H interaction, followed by several σ-allyl ruthenium isomerization steps. Finally, the β-H elimination gives the 1,5hydride shift (E)-product and coordination of the phosphine ligand regenerates the ruthenium hydride 1en for the next catalytic cycle. Strong binding of the CC bond to Ru and the steric effect of the N-heterocyclic carbene ligand in catalyst 1 may dictate the regioselectivity of the unique 1,5-hydride shift, not the 1,3-hydride shift, in good agreement with the experimental observation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-y.L.). *E-mail: [email protected] (M.-y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21373098 and 21203073). The authors are grateful to the Computing Center of Jilin Province for essential support.

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00127. The structures and Gibbs energies of 3C′ and Pro′, the detailed reaction pathways starting with trans-2, the sample input files, and IRC calculation (PDF) Cartesian coordinates for calculated structures (XYZ) F

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