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Iron-catalyzed olefin synthesis by direct coupling of alkenes with alcohols: A DFT investigation
Computational and Theoretical Chemistry 1143 (2018) 36–42
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Iron-catalyzed olefin synthesis by direct coupling of alkenes with alcohols: A DFT investigation ⁎
T
⁎
Wei Fenga, Yanwei Sunb, Huiling Liua, , Kejin Xuc, , Xuri Huanga a
Institute of Theoretical Chemistry, Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun 130023, China The First Middle School of Linxi, Chifeng City, Inner Mongolia County 025250, China c Changchun University of Chinese Medicine, Changchun 130117, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: DFT Mechanism Iron-catalyst Alkenes alcohol coupling reaction
The mechanism of the iron-catalyzed olefin synthesis reaction was investigated using the density functional theory (DFT) calculations. The catalytic performances of the FeCl3 and FeCl2 in different spin states for the olefin synthesis reaction were studied in detail. Both of the two reactions are found to proceed predominantly on high spin state potential energy surfaces, sextet and quintet respectively. The whole catalytic cycle can be divided into three stages: the dehydration step, the electrophilic addition step, and the deprotonation step. The electrophilic addition transition state has the highest energy in the whole catalytic cycle, which is consistent with the experimental prediction. According to the results, (E)-alkene is the main product in both the FeCl3 and the FeCl2 catalyzed olefin synthesis reaction. The geometry, mechanism, and the overall reaction barriers of the olefin synthesis reaction are very similar when FeCl2 and FeCl3 are used as catalysts. The reaction barrier of FeCl2 catalyst is lower than that of FeCl3, showing that FeCl2 may be more effective in this kind of olefin synthesis reaction.
1. Introduction Over the years, transition metal catalysis has been important and widely used in many aspects of chemistry, especially in CeC bondforming reactions as a fundamental and powerful tool [1–5]. However, it is a critical challenge for organic synthetic chemists to use inexpensive and environmentally benign transition metal catalysts to develop the CeC bond formation processes. Considerable efforts have been devoted to the iron catalyst due to its sustainable, low-cost, and environmentally friendly characteristics [6–8]. As a consequence, a series of efficient iron catalyzed organic transformations have been extensively explored, and significant progress has been made in this field [9]. More technically feasible, economical, and environmentally friendly methods for the formation of CeC bond are still in great demand in both academia and industrial research areas. Most of the general approaches for the construction of CeC bonds are inevitably accompanied with the generation of stoichiometric amounts of wastes. The CeC bond formation by direct coupling of alcohols with other partners such as alkynes, active methylene, and aromatics compounds might be very attractive since it is cost-efficient and environmentally benign [10–12]. All these reactions generally proceed by the addition of carbon cations
⁎
to multiple bonds and subsequent deprotonation. The coupling of alcohols with alkynes has attracted great attention as an effective method for the formation of CeC bond with the concomitant loss of water. However, alcohols are seldom used as alkylating agents on account of the poor leaving ability of the hydroxyl group. So, although alcohols and alkenes as cheap and versatile building blocks have been extensively used in organic synthesis, only few strategies for the direct coupling of alcohols with alkenes to construct sp3-sp2 CeC bonds are reported up to date [13–17]. In 2011, Liu et al. reported a highly efficient FeCl3·6H2O and TsOH catalyzed olefin synthesis reaction via the direct coupling of alcohols with simple alkenes [10]. Environmentally friendly low-cost iron is used as catalyst in this reaction, meanwhile, water is generated as the only side product. Therefore, it is interesting to investigate the mechanism of the FeCl3·6H2O and TsOH catalyzed olefin synthesis reaction. Liu et al. carried out a competing kinetic isotope effect (KIE) experiment and proposed a mechanism (as shown in Scheme 1) for the reaction. Nevertheless, some details of the reaction mechanism and an entire catalytic cycle of iron-catalyzed reaction are not easily accessible by experiment. We select one typical experimental work of Liu et al. as an example to elucidate the mechanism of FeCl3·6H2O and TsOH catalyzed olefin
Corresponding author. E-mail addresses: [email protected] (H. Liu), [email protected] (K. Xu).
https://doi.org/10.1016/j.comptc.2018.09.011 Received 17 May 2018; Received in revised form 7 September 2018; Accepted 26 September 2018 Available online 27 September 2018 2210-271X/ © 2018 Elsevier B.V. All rights reserved.
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Scheme 1. Mechanistic proposal for the FeCl3·6H2O and TsOH catalyzed olefin synthesis reaction [10].
Scheme 2. FeCl3·6H2O and TsOH catalyzed olefin synthesis reactions of 1-(4-methyphenyl)-1-ethanol with styrene.
Fig. 1. Mechanism details for the FeCl3-catalyzed olefin synthesis by direct coupling of alkenes with alcohols, proposed from the present calculations.
contribute more efficient environmentally friendly catalysts to the olefin design and synthesis, and also provide useful information for further reaction development in this expanding area of research.
synthesis reaction. The experiment is depicted in Scheme 2, where the reaction was carried out in dichloromethane (DCM) at 318.15 K using FeCl3·6H2O and TsOH as catalysts, and the yield was 81%. Density functional theory (DFT) calculation is applied to this FeCl3 and TsOH catalyzed olefin synthesis reaction between 1-(4-methyphenyl)-1ethanol (R1) and styrene (R2). It is conjectured that the FeCl2 catalyst may have similar catalytic effect with FeCl3 when a further insight mechanism is gained. So the same theoretical calculations are performed on the FeCl2-catalyzed reaction. Knowledge gained from the mechanism investigation of the FeCl3- and FeCl2-catalyzed will
2. Computational details The theoretical calculations were performed with the Gaussian 09 program [18], using the polarizable continuum model (PCM), and the dichloromethane (DCM) as solvent. All of the structures were fully optimized based on the density functional theory (DFT) method with 37
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optimized geometries of the substrate-catalyst complex IM1, the transition state TS1, and the intermediate IM2 in sextet state are depicted schematically in Fig. 3 along with the selected key geometry parameters. The substrate R1 binds via an OeH interaction to the TsOH, resulting in an H1eO1 distance of 1.414 Å. The O3 atom of TsOH coordinates to the Fe center with the FeeO3 distance of 1.974 Å. Thus, the two weak interactions keep IM1 stable. Fig. 3 shows the catalytic dehydration process between the 1-(4methyphenyl)-1-ethanol (R1) and TsOH. The C1eO1 distance in TS1 (2.126 Å) is elongated by 0.675 Å compared to IM1 (1.469 Å), indicates that the C1eO1 bond is breaking. Due to the strong electrophilicity of Lewis acid, FeCl3 can interact with hydroxyl group to facilitate its leaving from 1-(4-methyphenyl)-1-ethanol (R1) to form a water molecule. From IM1 to IM2, the distance of the FeeO1 is shortened from 3.625 to 2.201 Å. This means that the new formed water molecule gets closer to the Fe center. Therefore, the formation of the Fe–O1 weak bond promotes the C1eO1 bond cleavage. Fig. 2 shows that the energy of activation for this step is calculated to be 11.1 [13.8] kcal/mol and the relative energy for the IM2 intermediates is 2.3 [5.1] kcal/mol respect to IM1. In IM2, the bond lengths of the H1eO1 and H1eO2 are 0.994 and 1.732 Å, respectively. After the cleavage of the C1eO1 bond, the C1 becomes a carbocation. It can be further confirmed by NBO analysis. The NBO charge on C1 of IM2 has a positive charge of 0.22, larger than the corresponding C on the OH removed 1-(4-methylphenyl)-1-ethanol ion. The natural population atomic (NPA) charges calculated at the B3LYP/B1 level are listed in Table 1. It is seen that the negative charge around O1 and O2 increased from IM1 to IM2, meanwhile the positive charge around C1 increased too (form 0.09 to 0.15). The changes of charge around O1, O2, and C1 illustrated that the C1 becomes a carbocation after the C1eO1 bond broken.
the B3LYP hybrid function [19–22]. Mixed basis sets were used for structure optimization and frequency analysis: the LANL2DZ basis set was applied for Fe atom [23–25], and the 6-31G* basis set was employed for C, H, O, S, and Cl atoms [26,27]. This kind of basis set is widely applied in the mechanistic studies of transition-metal catalyzed reactions and it is denoted as B1 in this work. Frequency analyses were utilized on the optimized structures to distinguish possible transition states from local minimum states. The stationary points on the potential energy surface at respective levels were characterized by all real vibrational frequencies, while the transition states were characterized by the only imaginary frequency. Whether the transitions states connected the correct reactants and products were firstly identified by the mode of the imaginary frequency. To confirm the transition state connects designated intermediates, the intrinsic reaction coordinate (IRC) calculations [28–30] were performed. To obtain more accurate energies, single-point calculations were performed at the same function with larger basis set 6-311+G** [31,32] for C, H, O, S, and Cl, and LANL2TZ [33] for Fe on the basis of the optimized geometries. This kind of basis set is denoted as B2. In order to prove the B3LYP hybrid function is suitable for the system, M062X/B2 single-point energies calculations have also been done on the sextet state FeCl3·6H2O and TSOH catalyzed reaction surface [34]. Natural bond orbital (NBO) [35] calculation was performed on the optimized structures for population analysis. 3. Results and discussion 3.1. Computational results for the FeCl3-catalyzed olefin synthesis reactions between 1-(4-methyphenyl)-1-ethanol and styrene In this section, we will discuss the detailed mechanism of FeCl3·6H2O and TsOH catalyzed olefin synthesis reactions between 1(4-methyphenyl)-1-ethanol and styrene. The whole catalytic cycle is outlined in Fig. 1. The DFT-computed energy profiles in quartet and sextet states are shown in Fig. 2. According to Fig. 2, the reaction primarily proceeds on the sextet-state energy surface. So the sextet spin state is described in detail during the following sections, and it is chosen to illustrate the mechanism of the olefin synthesis reaction. The energies predicted by the M062X/B2 level are also listed in Fig. 2 in brackets.
3.1.2. Electrophilic addition and proton transfer In the next step, the styrene (R2) inserts into the catalytic cycle. This step contains two possible channels, denoted as channels A and B, which are different in the approach mode of the R2 (Fig. 4). Along these two channels, the corresponding (E)- and (Z)-alkenes are obtained, respectively. In channel A, styrene interacts with IM2 from an eclipsed conformation direction, and forms complex IM3a. Once IM3a is formed, the addition reaction is initiated by electrophilic attacking of C1 to C3 to form the new carbonium ion intermediate IM4a. This step proceeds via transition state TS2a with a barrier of 4.3 [3.1] kcal/mol, lying 4.5 [7.2] kcal/mol lower in energy than IM3a. As shown in Fig. 4, the distance between C1 and C3 in TS2a is 2.350 Å, implying that the
3.1.1. Catalytic dehydration In the first step of the olefin synthesis reaction, the substrate R1 was activated by the catalyst FeCl3 with the coordination of the TsOH. The
Fig. 2. Energy profiles for the FeCl3·6H2O and TsOH catalyzed reaction in the DCM solution at the B3LYP/B2 level. The values in brackets indicate the relative energies obtained at the M062X/B2 level, with geometries optimized at the B3LYP/B1 level. The solid line represents the channel A and the dashed line represents the channel B.
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Fig. 3. Fully optimized structures of the substrate-catalyst complex IM1, the transition state TS1, and the intermediate IM2 in sextet-spin state. Bond lengths are in Å.
According to the Energetic Span Model (ESM) [36], all the reaction transition states and intermediates in a cycle catalyst reaction will affect the total reaction rate. The transition state with the highest Gibbs free energy and the intermediate with the lowest Gibbs free energy play a decisive role. For FeCl3, the overall reaction energy differences for the channels A and B are 21.9 [14.7] and 23.2 [19.0] kcal/mol, respectively. In both channels, the electrophilic addition is the rate-limiting state. The difference in the overall reaction barrier between the two channels is 1.3 [4.3] kcal/mol, suggesting that the channel A is energetically more favorable than the channel B. The energy of (E)-alkene is 5.9 [3.9] kcal/mol lower than that of (Z)-alkene indicates that the (E)-alkene is more stable. Estimation turnover frequency (TOF) according to the ESM is a method to measure the catalytic cyclic reaction. We calculated the TOFs of channel A and channel B by using the AUTOF program [37–39] applying the energetic span model. According to the degrees of TOF control (ΧTOF Values), the determining intermediate is 6 IM1 (ΧTOF = 0.96) and the demining transition state is 6TS2a (ΧTOF = 0.99) in channel A. While in channel B, the determining intermediate is 6 IM1 (ΧTOF = 0.72) and the demining transition state is 6TS2b (ΧTOF = 0.86). The TOF of channel A is −4.31 * 107 h−1 and that of channel B is −1.48 * 1010 h−1. Although the value may not be accurate, the dominant product can be judged from it. This result means that the (E)alkene is obtained as the major product in this iron-catalyzed olefin synthesis reaction, and it is in agreement with the experimental prediction.
Table 1 Natural population atomic charges in species involved in all elementary steps. Species
IM1
TS1
IM2
IM3a
TS2a
IM4a
TS3a
IM5a
Fe O1 O2 O3 C1 H1 C3 C4
0.86 −0.77 −0.95 −0.94 0.09 0.54 / /
0.85 −0.87 −1.00 −0.95 0.14 0.54 / /
0.88 −0.94 −1.00 −0.97 0.15 0.53 / /
0.88 −0.93 −1.00 −0.96 0.12 0.53 −0.42 −0.21
0.88 −0.93 −1.00 −0.96 0.03 0.53 −0.45 −0.11
0.86 −0.94 −1.00 −0.96 −0.24 0.53 −0.52 0.16
0.88 −0.93 −0.98 −0.96 −0.26 0.54 −0.46 0.02
0.87 −0.93 −0.95 −0.94 −0.29 0.54 −0.20 −0.28
C1eC3 bond is forming. The new formed C1eC3 bond causes the extension of the C3eC4 bond from 1.367 to 1.462 Å and the shrink of the C4eC5 bond from 1.448 to 1.397 Å. The C1eC3 bond has formed in IM4a with the length of 1.562 Å. As shown in Table 1, the charge around C1 changes from 0.12 to −0.24 due to the formation of the C1eC3 bond. After the electrophilic addition, the C4 becomes a carbocation evidenced by its charge changes from −0.21 to 0.16. The subsequent step is a deprotonation process, which proceeds via the transition state TS3a. The final barrier of 5.1 [4.9] kcal/mol is required to release the product and regenerate the catalyst. The final step is exothermic by 6.2 [6.1] kcal/mol. In TS3a, the distances between Ha and C3, Ha and O4 are 1.363 and 1.311 Å, respectively, suggesting that the O4eHa bond is forming and the C3eHa bond is rupturing. The unique imaginary frequency 1193i cm−1 corresponds to the normal mode of the transferring of Ha to O4. From IM4a to IM5a, the distance of the C3eC4 is shortened from 1.462 Å to 1.350 Å, and the charges around C3 and C4 are changed to −0.20 and −0.28, respectively. The changes of distance and charge suggest that the C3eC4 bond changes into a CeC double bond when the Ha migrates from C3 to O4 atom. The length of the O4eHa bond is 1.002 Å in IM5a, indicating that the O4eHa bond has been formed. Complete proton transfer yields the final (E)-alkene and regenerates the catalyst for the next catalytic cycle. Alternatively, along the channel B, styrene (R2) interacts with IM2 from a staggered direction and forming the intermediate IM3b. Once IM3b is formed, C1 adds to C3 forming the intermediate IM4b via a transition state TS2b. The energy barrier for the electrophilic addition in channel B is calculated to be 9.6 [7.0] kcal/mol. Then it is followed by fast deprotonating to give the corresponding (Z)-alkene.
3.2. The FeCl2-catalyzed olefin synthesis reaction in dichloromethane solution We considered the pathway for the FeCl2-catalyzed olefin synthesis reaction. The optimized geometries of intermediates and transition states of the FeCl2-catalyzed reaction in quintet state are given in Figure S1 (Supporting Information). The FeCl2-catalyzed reaction is similar with FeCl3 and the mechanism also includes three basic steps: (1) dehydration, (2) electrophilic addition, and (3) deprotonation. The calculated PCM energy surfaces in dichloromethane (DCM) at the B3LYP/B2 level on the quintet and triplet states for the FeCl2 catalyzed reaction are shown in Fig. 5. The 5IM1-3IM1 energy gap is about 36.1 kcal/mol, a little larger than that of FeCl3 catalytic system. All of the computed relative energies in triplet state are much higher than 39
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Fig. 4. Optimized geometries and main structural data for the electrophilic addition and proton transfer processes steps at sextet-spin state. Bond lengths are in Å.
those of the quintet state. Thus, the quintet-spin state plays an essential role in the FeCl2 catalyzed reaction. That is to say both FeCl3 and FeCl2 prefer a high spin catalytic system. On the quintet potential energy surface of FeCl2, two channels have also been located and leading to (Z)-alkene and (E)-alkene respectively. The electrophilic addition transition state also has the highest energy. In channel A, it is 15.1 kcal/ mol, which is 6.8 kcal/mol lower than that of the FeCl3 at the same level, while in channel B, it is 24.0 kcal/mol, almost the same as that of the FeCl3 (23.2). In order to estimate which catalyst is more suitable for the reaction, the TOF were calculated for the FeCl2 catalytic system. The result shows that for channel A the determining intermediate is 5 IM2 (ΧTOF = 0.82) and the demining transition state is 5TS2a (ΧTOF = 0.98), TOF = 5.32 * 106. While for Channel B, the determining intermediate is 5IM2 (ΧTOF = 0.58) and the demining transition state is 5 TS2b (ΧTOF = 0.99), TOF = −2.28 * 107. Compare with the FeCl3
catalytic system, FeCl2 catalyst may be more effective in this olefin synthesis reaction.
4. Conclusion The mechanism of the olefin synthesis reaction catalyzed by FeCl2 and FeCl3 has been investigated theoretically. Our calculations show that two reactions are composed of three steps: (1) dehydration of 1-(4methyphenyl)-1-ethanol (R1) to obtain the carbocation intermediate IM2, (2) electrophilic addition between IM2 and styrene (R2) to yield the new carbocation intermediate IM4, (3) deprotonating to give the corresponding (E)-alkene or (Z)-alkene and regenerate the catalysts for the next catalytic cycle. The sextet and quintet potential surfaces respectively play essential role in the FeCl3 and FeCl2 catalyzed olefin synthesis reaction. The electrophilic addition transition state has the 40
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Relative Energies (kcal/mol) 3TS2
3
TS1 43.3
Triplet
46.2
3
IM3 41.1
3
IM1 36.1
39.9 3IM4
33.9
3
IM2 26.5 IM3b 18.5
5IM3a
5
TS1 7.4
Quintet
IM1 0.0
3IM5
28.9
5TS2b
8.6
5TS3b
24.0
5
5
3TS3
20.5
5
TS2a 15.1
5
IM4b 9.4
5
IM4a 6.8
5IM2
5TS3a
12.3
5
IM5b 10.0
5IM5a
-1.0
-1.1
Fig. 5. Energy profile for the FeCl2 and TsOH catalyzed reaction in the DCM solution at the B3LYP/B2 level.
highest energy in the whole catalytic cycle. For FeCl3 catalyzed reaction, according to the energetic span model, channel A is more favorable than channel B and the (E)-alkene is the major product in the reaction. Furthermore, FeCl2 catalyst may be more effective than FeCl3 in this olefin synthesis reaction. These theoretical studies of the iron-catalyzed olefin synthesis reaction are anticipated to deliver valuable information for the development of iron catalysts and new-catalyzed reactions, as well as in designing new green catalysts.
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Contents lists available at ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Iron-catalyzed olefin synthesis by direct coupling of alkenes with alcohols: A DFT investigation ⁎
T
⁎
Wei Fenga, Yanwei Sunb, Huiling Liua, , Kejin Xuc, , Xuri Huanga a
Institute of Theoretical Chemistry, Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun 130023, China The First Middle School of Linxi, Chifeng City, Inner Mongolia County 025250, China c Changchun University of Chinese Medicine, Changchun 130117, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: DFT Mechanism Iron-catalyst Alkenes alcohol coupling reaction
The mechanism of the iron-catalyzed olefin synthesis reaction was investigated using the density functional theory (DFT) calculations. The catalytic performances of the FeCl3 and FeCl2 in different spin states for the olefin synthesis reaction were studied in detail. Both of the two reactions are found to proceed predominantly on high spin state potential energy surfaces, sextet and quintet respectively. The whole catalytic cycle can be divided into three stages: the dehydration step, the electrophilic addition step, and the deprotonation step. The electrophilic addition transition state has the highest energy in the whole catalytic cycle, which is consistent with the experimental prediction. According to the results, (E)-alkene is the main product in both the FeCl3 and the FeCl2 catalyzed olefin synthesis reaction. The geometry, mechanism, and the overall reaction barriers of the olefin synthesis reaction are very similar when FeCl2 and FeCl3 are used as catalysts. The reaction barrier of FeCl2 catalyst is lower than that of FeCl3, showing that FeCl2 may be more effective in this kind of olefin synthesis reaction.
1. Introduction Over the years, transition metal catalysis has been important and widely used in many aspects of chemistry, especially in CeC bondforming reactions as a fundamental and powerful tool [1–5]. However, it is a critical challenge for organic synthetic chemists to use inexpensive and environmentally benign transition metal catalysts to develop the CeC bond formation processes. Considerable efforts have been devoted to the iron catalyst due to its sustainable, low-cost, and environmentally friendly characteristics [6–8]. As a consequence, a series of efficient iron catalyzed organic transformations have been extensively explored, and significant progress has been made in this field [9]. More technically feasible, economical, and environmentally friendly methods for the formation of CeC bond are still in great demand in both academia and industrial research areas. Most of the general approaches for the construction of CeC bonds are inevitably accompanied with the generation of stoichiometric amounts of wastes. The CeC bond formation by direct coupling of alcohols with other partners such as alkynes, active methylene, and aromatics compounds might be very attractive since it is cost-efficient and environmentally benign [10–12]. All these reactions generally proceed by the addition of carbon cations
⁎
to multiple bonds and subsequent deprotonation. The coupling of alcohols with alkynes has attracted great attention as an effective method for the formation of CeC bond with the concomitant loss of water. However, alcohols are seldom used as alkylating agents on account of the poor leaving ability of the hydroxyl group. So, although alcohols and alkenes as cheap and versatile building blocks have been extensively used in organic synthesis, only few strategies for the direct coupling of alcohols with alkenes to construct sp3-sp2 CeC bonds are reported up to date [13–17]. In 2011, Liu et al. reported a highly efficient FeCl3·6H2O and TsOH catalyzed olefin synthesis reaction via the direct coupling of alcohols with simple alkenes [10]. Environmentally friendly low-cost iron is used as catalyst in this reaction, meanwhile, water is generated as the only side product. Therefore, it is interesting to investigate the mechanism of the FeCl3·6H2O and TsOH catalyzed olefin synthesis reaction. Liu et al. carried out a competing kinetic isotope effect (KIE) experiment and proposed a mechanism (as shown in Scheme 1) for the reaction. Nevertheless, some details of the reaction mechanism and an entire catalytic cycle of iron-catalyzed reaction are not easily accessible by experiment. We select one typical experimental work of Liu et al. as an example to elucidate the mechanism of FeCl3·6H2O and TsOH catalyzed olefin
Corresponding author. E-mail addresses: [email protected] (H. Liu), [email protected] (K. Xu).
https://doi.org/10.1016/j.comptc.2018.09.011 Received 17 May 2018; Received in revised form 7 September 2018; Accepted 26 September 2018 Available online 27 September 2018 2210-271X/ © 2018 Elsevier B.V. All rights reserved.
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Scheme 1. Mechanistic proposal for the FeCl3·6H2O and TsOH catalyzed olefin synthesis reaction [10].
Scheme 2. FeCl3·6H2O and TsOH catalyzed olefin synthesis reactions of 1-(4-methyphenyl)-1-ethanol with styrene.
Fig. 1. Mechanism details for the FeCl3-catalyzed olefin synthesis by direct coupling of alkenes with alcohols, proposed from the present calculations.
contribute more efficient environmentally friendly catalysts to the olefin design and synthesis, and also provide useful information for further reaction development in this expanding area of research.
synthesis reaction. The experiment is depicted in Scheme 2, where the reaction was carried out in dichloromethane (DCM) at 318.15 K using FeCl3·6H2O and TsOH as catalysts, and the yield was 81%. Density functional theory (DFT) calculation is applied to this FeCl3 and TsOH catalyzed olefin synthesis reaction between 1-(4-methyphenyl)-1ethanol (R1) and styrene (R2). It is conjectured that the FeCl2 catalyst may have similar catalytic effect with FeCl3 when a further insight mechanism is gained. So the same theoretical calculations are performed on the FeCl2-catalyzed reaction. Knowledge gained from the mechanism investigation of the FeCl3- and FeCl2-catalyzed will
2. Computational details The theoretical calculations were performed with the Gaussian 09 program [18], using the polarizable continuum model (PCM), and the dichloromethane (DCM) as solvent. All of the structures were fully optimized based on the density functional theory (DFT) method with 37
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optimized geometries of the substrate-catalyst complex IM1, the transition state TS1, and the intermediate IM2 in sextet state are depicted schematically in Fig. 3 along with the selected key geometry parameters. The substrate R1 binds via an OeH interaction to the TsOH, resulting in an H1eO1 distance of 1.414 Å. The O3 atom of TsOH coordinates to the Fe center with the FeeO3 distance of 1.974 Å. Thus, the two weak interactions keep IM1 stable. Fig. 3 shows the catalytic dehydration process between the 1-(4methyphenyl)-1-ethanol (R1) and TsOH. The C1eO1 distance in TS1 (2.126 Å) is elongated by 0.675 Å compared to IM1 (1.469 Å), indicates that the C1eO1 bond is breaking. Due to the strong electrophilicity of Lewis acid, FeCl3 can interact with hydroxyl group to facilitate its leaving from 1-(4-methyphenyl)-1-ethanol (R1) to form a water molecule. From IM1 to IM2, the distance of the FeeO1 is shortened from 3.625 to 2.201 Å. This means that the new formed water molecule gets closer to the Fe center. Therefore, the formation of the Fe–O1 weak bond promotes the C1eO1 bond cleavage. Fig. 2 shows that the energy of activation for this step is calculated to be 11.1 [13.8] kcal/mol and the relative energy for the IM2 intermediates is 2.3 [5.1] kcal/mol respect to IM1. In IM2, the bond lengths of the H1eO1 and H1eO2 are 0.994 and 1.732 Å, respectively. After the cleavage of the C1eO1 bond, the C1 becomes a carbocation. It can be further confirmed by NBO analysis. The NBO charge on C1 of IM2 has a positive charge of 0.22, larger than the corresponding C on the OH removed 1-(4-methylphenyl)-1-ethanol ion. The natural population atomic (NPA) charges calculated at the B3LYP/B1 level are listed in Table 1. It is seen that the negative charge around O1 and O2 increased from IM1 to IM2, meanwhile the positive charge around C1 increased too (form 0.09 to 0.15). The changes of charge around O1, O2, and C1 illustrated that the C1 becomes a carbocation after the C1eO1 bond broken.
the B3LYP hybrid function [19–22]. Mixed basis sets were used for structure optimization and frequency analysis: the LANL2DZ basis set was applied for Fe atom [23–25], and the 6-31G* basis set was employed for C, H, O, S, and Cl atoms [26,27]. This kind of basis set is widely applied in the mechanistic studies of transition-metal catalyzed reactions and it is denoted as B1 in this work. Frequency analyses were utilized on the optimized structures to distinguish possible transition states from local minimum states. The stationary points on the potential energy surface at respective levels were characterized by all real vibrational frequencies, while the transition states were characterized by the only imaginary frequency. Whether the transitions states connected the correct reactants and products were firstly identified by the mode of the imaginary frequency. To confirm the transition state connects designated intermediates, the intrinsic reaction coordinate (IRC) calculations [28–30] were performed. To obtain more accurate energies, single-point calculations were performed at the same function with larger basis set 6-311+G** [31,32] for C, H, O, S, and Cl, and LANL2TZ [33] for Fe on the basis of the optimized geometries. This kind of basis set is denoted as B2. In order to prove the B3LYP hybrid function is suitable for the system, M062X/B2 single-point energies calculations have also been done on the sextet state FeCl3·6H2O and TSOH catalyzed reaction surface [34]. Natural bond orbital (NBO) [35] calculation was performed on the optimized structures for population analysis. 3. Results and discussion 3.1. Computational results for the FeCl3-catalyzed olefin synthesis reactions between 1-(4-methyphenyl)-1-ethanol and styrene In this section, we will discuss the detailed mechanism of FeCl3·6H2O and TsOH catalyzed olefin synthesis reactions between 1(4-methyphenyl)-1-ethanol and styrene. The whole catalytic cycle is outlined in Fig. 1. The DFT-computed energy profiles in quartet and sextet states are shown in Fig. 2. According to Fig. 2, the reaction primarily proceeds on the sextet-state energy surface. So the sextet spin state is described in detail during the following sections, and it is chosen to illustrate the mechanism of the olefin synthesis reaction. The energies predicted by the M062X/B2 level are also listed in Fig. 2 in brackets.
3.1.2. Electrophilic addition and proton transfer In the next step, the styrene (R2) inserts into the catalytic cycle. This step contains two possible channels, denoted as channels A and B, which are different in the approach mode of the R2 (Fig. 4). Along these two channels, the corresponding (E)- and (Z)-alkenes are obtained, respectively. In channel A, styrene interacts with IM2 from an eclipsed conformation direction, and forms complex IM3a. Once IM3a is formed, the addition reaction is initiated by electrophilic attacking of C1 to C3 to form the new carbonium ion intermediate IM4a. This step proceeds via transition state TS2a with a barrier of 4.3 [3.1] kcal/mol, lying 4.5 [7.2] kcal/mol lower in energy than IM3a. As shown in Fig. 4, the distance between C1 and C3 in TS2a is 2.350 Å, implying that the
3.1.1. Catalytic dehydration In the first step of the olefin synthesis reaction, the substrate R1 was activated by the catalyst FeCl3 with the coordination of the TsOH. The
Fig. 2. Energy profiles for the FeCl3·6H2O and TsOH catalyzed reaction in the DCM solution at the B3LYP/B2 level. The values in brackets indicate the relative energies obtained at the M062X/B2 level, with geometries optimized at the B3LYP/B1 level. The solid line represents the channel A and the dashed line represents the channel B.
38
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Fig. 3. Fully optimized structures of the substrate-catalyst complex IM1, the transition state TS1, and the intermediate IM2 in sextet-spin state. Bond lengths are in Å.
According to the Energetic Span Model (ESM) [36], all the reaction transition states and intermediates in a cycle catalyst reaction will affect the total reaction rate. The transition state with the highest Gibbs free energy and the intermediate with the lowest Gibbs free energy play a decisive role. For FeCl3, the overall reaction energy differences for the channels A and B are 21.9 [14.7] and 23.2 [19.0] kcal/mol, respectively. In both channels, the electrophilic addition is the rate-limiting state. The difference in the overall reaction barrier between the two channels is 1.3 [4.3] kcal/mol, suggesting that the channel A is energetically more favorable than the channel B. The energy of (E)-alkene is 5.9 [3.9] kcal/mol lower than that of (Z)-alkene indicates that the (E)-alkene is more stable. Estimation turnover frequency (TOF) according to the ESM is a method to measure the catalytic cyclic reaction. We calculated the TOFs of channel A and channel B by using the AUTOF program [37–39] applying the energetic span model. According to the degrees of TOF control (ΧTOF Values), the determining intermediate is 6 IM1 (ΧTOF = 0.96) and the demining transition state is 6TS2a (ΧTOF = 0.99) in channel A. While in channel B, the determining intermediate is 6 IM1 (ΧTOF = 0.72) and the demining transition state is 6TS2b (ΧTOF = 0.86). The TOF of channel A is −4.31 * 107 h−1 and that of channel B is −1.48 * 1010 h−1. Although the value may not be accurate, the dominant product can be judged from it. This result means that the (E)alkene is obtained as the major product in this iron-catalyzed olefin synthesis reaction, and it is in agreement with the experimental prediction.
Table 1 Natural population atomic charges in species involved in all elementary steps. Species
IM1
TS1
IM2
IM3a
TS2a
IM4a
TS3a
IM5a
Fe O1 O2 O3 C1 H1 C3 C4
0.86 −0.77 −0.95 −0.94 0.09 0.54 / /
0.85 −0.87 −1.00 −0.95 0.14 0.54 / /
0.88 −0.94 −1.00 −0.97 0.15 0.53 / /
0.88 −0.93 −1.00 −0.96 0.12 0.53 −0.42 −0.21
0.88 −0.93 −1.00 −0.96 0.03 0.53 −0.45 −0.11
0.86 −0.94 −1.00 −0.96 −0.24 0.53 −0.52 0.16
0.88 −0.93 −0.98 −0.96 −0.26 0.54 −0.46 0.02
0.87 −0.93 −0.95 −0.94 −0.29 0.54 −0.20 −0.28
C1eC3 bond is forming. The new formed C1eC3 bond causes the extension of the C3eC4 bond from 1.367 to 1.462 Å and the shrink of the C4eC5 bond from 1.448 to 1.397 Å. The C1eC3 bond has formed in IM4a with the length of 1.562 Å. As shown in Table 1, the charge around C1 changes from 0.12 to −0.24 due to the formation of the C1eC3 bond. After the electrophilic addition, the C4 becomes a carbocation evidenced by its charge changes from −0.21 to 0.16. The subsequent step is a deprotonation process, which proceeds via the transition state TS3a. The final barrier of 5.1 [4.9] kcal/mol is required to release the product and regenerate the catalyst. The final step is exothermic by 6.2 [6.1] kcal/mol. In TS3a, the distances between Ha and C3, Ha and O4 are 1.363 and 1.311 Å, respectively, suggesting that the O4eHa bond is forming and the C3eHa bond is rupturing. The unique imaginary frequency 1193i cm−1 corresponds to the normal mode of the transferring of Ha to O4. From IM4a to IM5a, the distance of the C3eC4 is shortened from 1.462 Å to 1.350 Å, and the charges around C3 and C4 are changed to −0.20 and −0.28, respectively. The changes of distance and charge suggest that the C3eC4 bond changes into a CeC double bond when the Ha migrates from C3 to O4 atom. The length of the O4eHa bond is 1.002 Å in IM5a, indicating that the O4eHa bond has been formed. Complete proton transfer yields the final (E)-alkene and regenerates the catalyst for the next catalytic cycle. Alternatively, along the channel B, styrene (R2) interacts with IM2 from a staggered direction and forming the intermediate IM3b. Once IM3b is formed, C1 adds to C3 forming the intermediate IM4b via a transition state TS2b. The energy barrier for the electrophilic addition in channel B is calculated to be 9.6 [7.0] kcal/mol. Then it is followed by fast deprotonating to give the corresponding (Z)-alkene.
3.2. The FeCl2-catalyzed olefin synthesis reaction in dichloromethane solution We considered the pathway for the FeCl2-catalyzed olefin synthesis reaction. The optimized geometries of intermediates and transition states of the FeCl2-catalyzed reaction in quintet state are given in Figure S1 (Supporting Information). The FeCl2-catalyzed reaction is similar with FeCl3 and the mechanism also includes three basic steps: (1) dehydration, (2) electrophilic addition, and (3) deprotonation. The calculated PCM energy surfaces in dichloromethane (DCM) at the B3LYP/B2 level on the quintet and triplet states for the FeCl2 catalyzed reaction are shown in Fig. 5. The 5IM1-3IM1 energy gap is about 36.1 kcal/mol, a little larger than that of FeCl3 catalytic system. All of the computed relative energies in triplet state are much higher than 39
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Fig. 4. Optimized geometries and main structural data for the electrophilic addition and proton transfer processes steps at sextet-spin state. Bond lengths are in Å.
those of the quintet state. Thus, the quintet-spin state plays an essential role in the FeCl2 catalyzed reaction. That is to say both FeCl3 and FeCl2 prefer a high spin catalytic system. On the quintet potential energy surface of FeCl2, two channels have also been located and leading to (Z)-alkene and (E)-alkene respectively. The electrophilic addition transition state also has the highest energy. In channel A, it is 15.1 kcal/ mol, which is 6.8 kcal/mol lower than that of the FeCl3 at the same level, while in channel B, it is 24.0 kcal/mol, almost the same as that of the FeCl3 (23.2). In order to estimate which catalyst is more suitable for the reaction, the TOF were calculated for the FeCl2 catalytic system. The result shows that for channel A the determining intermediate is 5 IM2 (ΧTOF = 0.82) and the demining transition state is 5TS2a (ΧTOF = 0.98), TOF = 5.32 * 106. While for Channel B, the determining intermediate is 5IM2 (ΧTOF = 0.58) and the demining transition state is 5 TS2b (ΧTOF = 0.99), TOF = −2.28 * 107. Compare with the FeCl3
catalytic system, FeCl2 catalyst may be more effective in this olefin synthesis reaction.
4. Conclusion The mechanism of the olefin synthesis reaction catalyzed by FeCl2 and FeCl3 has been investigated theoretically. Our calculations show that two reactions are composed of three steps: (1) dehydration of 1-(4methyphenyl)-1-ethanol (R1) to obtain the carbocation intermediate IM2, (2) electrophilic addition between IM2 and styrene (R2) to yield the new carbocation intermediate IM4, (3) deprotonating to give the corresponding (E)-alkene or (Z)-alkene and regenerate the catalysts for the next catalytic cycle. The sextet and quintet potential surfaces respectively play essential role in the FeCl3 and FeCl2 catalyzed olefin synthesis reaction. The electrophilic addition transition state has the 40
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Relative Energies (kcal/mol) 3TS2
3
TS1 43.3
Triplet
46.2
3
IM3 41.1
3
IM1 36.1
39.9 3IM4
33.9
3
IM2 26.5 IM3b 18.5
5IM3a
5
TS1 7.4
Quintet
IM1 0.0
3IM5
28.9
5TS2b
8.6
5TS3b
24.0
5
5
3TS3
20.5
5
TS2a 15.1
5
IM4b 9.4
5
IM4a 6.8
5IM2
5TS3a
12.3
5
IM5b 10.0
5IM5a
-1.0
-1.1
Fig. 5. Energy profile for the FeCl2 and TsOH catalyzed reaction in the DCM solution at the B3LYP/B2 level.
highest energy in the whole catalytic cycle. For FeCl3 catalyzed reaction, according to the energetic span model, channel A is more favorable than channel B and the (E)-alkene is the major product in the reaction. Furthermore, FeCl2 catalyst may be more effective than FeCl3 in this olefin synthesis reaction. These theoretical studies of the iron-catalyzed olefin synthesis reaction are anticipated to deliver valuable information for the development of iron catalysts and new-catalyzed reactions, as well as in designing new green catalysts.
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