DFT Studies on the Mechanism of Copper-Catalyzed

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DFT Studies on the Mechanism of Copper-Catalyzed Boracarboxylation of Alkene with CO2 and Diboron Shujuan Lin and Zhenyang Lin* Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

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S Supporting Information *

ABSTRACT: DFT calculations have been carried out to study the reaction mechanism of copper-catalyzed three-component boracarboxylation of alkene with CO2 and diboron. The competing reaction pathways involving two components have also been investigated for comparison. Through this theoretical study deep mechanistic insight on this multicomponent reaction is provided.

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INTRODUCTION Utilization of CO2 as a C1 resource in chemical synthesis has attracted extensive research interest, because CO2 is abundantly available, nontoxic, inexpensive, and renewable.1,2 In this regard, considerable efforts have been devoted toward the development of various methodologies for converting CO2 into value-added chemical products.2−4 Transition-metalcatalyzed carboxylation using CO2 is among the most straightforward approaches for CO2 conversion.4 In the past decades, lots of protocols have been established to achieve carboxylation of various organic substances using CO2 as carboxylative agent.4 Many of these reactions couple CO2 with some unsaturated compounds (such as alkenes,5 alkynes,6 allenes,7 aldehydes,8 etc.) to prepare carboxylic acids or esters. However, reports of hetero(element)carboxylation, which gives difunctionalized products with incorporation of heteroatom, remain rather limited.9,10 This is likely due to that there exist more possible competing reactions, especially in multicomponent reaction systems.10 Recently, Popp and co-workers reported copper-catalyzed boracarboxylation of vinyl arenes with carbon dioxide and bis(pinacolato)diboron (eq 1).11 This boracarboxylation reaction involves three components and is of highly regioselectivity and efficiency:

Scheme 1. Two-Component Reactions Reported in the Literature

using CO2 (Scheme 1c) normally needs some reducing agent.16 Therefore, why the undesired products derived from competing two-component reactions can be avoided motivates us to conduct a comprehensive mechanistic study on the boracarboxylation reaction shown in eq 1. A very recent DFT study mainly focused on the ligand effect of this boracarboxylation reaction.17 Here, we will pay our special attention to the very fundamental issue of how the high regioselective and efficient boracarboxylation reaction is achieved when competing two-component reactions are also possible. In this work, we carried out DFT calculations on the mechanism of this copper-catalyzed boracarboxylation reaction. Scheme 2 shows a plausible mechanism proposed by the authors who reported the reaction.11 The precatalyst [ICyCuCl] is first converted to a copper alkoxide (A) by reacting with NaOtBu, from which metathesis with the

Interestingly, under the similar reaction conditions, reactions involving two components have also been reported in the literature. For example, CO2 could be reduced to CO by a Nheterocyclic carbene (NHC)-ligated copper−boryl complex (Scheme 1a).12 As shown in Scheme 1b, reaction between styrene and diboron could give diverse products under a copper−NHC catalyst.13−15 Reductive carboxylation of styrene © XXXX American Chemical Society

Received: September 16, 2018

A

DOI: 10.1021/acs.organomet.8b00680 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

olefin into Cu−B in the main reaction pathway. Starting from the Cu−alkyl intermediate (C), in addition to the CO2 insertion in the main pathway leading to the formation of the copper carboxylate intermediate (D), there are 4 possible competing pathways that involve only two components and require investigation (Scheme 3): (1) metathesis with B2pin2 leading to a diboration product, (2) further insertion of olefin leading to polymer product(s), (3) β-hydrogen elimination, and (4) decupration. Generation of the Cu−Boryl Bond A → B. As reported, B2pin2 is readily to be activated by a Cu(I) complex in the present of NaOtBu to generate the Cu−boryl species. Generally, the copper−boryl species is considered to be formed through the σ-bond metathesis between B2pin2 and the copper−alkoxy intermediate. Given the excess amount of NaOtBu in the current experimental conditions, B2pin2 could selectively be trapped by NaOtBu delivering a Lewis acid−base adduct Na[tBuO→B2pin2], enhancing nuclophilicity in the B(sp2) center.18 Thus, the copper−boryl species could alternatively be produced from the transmetalation between this sp2−sp3 diboron adduct and the copper(I) complex. Following this line of thought, we calculated the energy profile for the formation of copper−boryl species B, which is shown in Figure 1. Overall, formation of the Cu−boryl species is very

Scheme 2. Plausible Mechanism

diboron substrate B2pin2 generates Cu−boryl species B. The subsequent olefin insertion into the Cu−boryl bond leads to the formation of a Cu−alkyl species (C). Next, CO2 insertion into the Cu−C bond in the Cu−alkyl species affords copper complex D. Metathesis between copper complex D and NaOtBu gives the boracarboxylation product and regenerates copper tert-butoxide species A. In addition to the catalytic cycle shown in Scheme 2, we will also investigate relevant twocomponent reactions for comparison. Through the detailed comparison, we hope to provide a mechanistic understanding of how the boracarboxylation reaction is achieved in the presence of the possible competing two-component reactions.

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RESULTS AND DISCUSSION As mentioned above, the mechanism for boracarboxylation (shown in Scheme 2) is initialized by the generation of a copper−boryl species, followed by olefin borylcupration and then insertion of CO2 to the Cu−alkyl intermediate. Scheme 3 Scheme 3. Main Reaction Pathway Together with Possible Competing Reaction Pathways

Figure 1. Energy profile calculated for the formation of copper−boryl species. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.

thermodynamically and kinetically favored. In the literature, transmetalation between the copper−alkoxyl species (A) and B2 pin2 regenerating the copper−boryl species (B) has been well-studied and was also found to be very facile.13c Olefin versus CO2 Insertion into the Cu−Boryl Bond. The energy profiles calculated for the olefin and CO2 insertion processes are presented and compared in Figure 2, and the optimized structures for selected intermediates and transition states are given in Figure 3. There exist two different types of insertion for an asymmetric alkene like styrene, and the insertion pattern determines the regioselectivity of the boracarboxylation products. In our calculations, both the 1,2and 2,1-insertions are evaluated, and the results are shown in the right-hand side of Figure 2. The insertion process starts with the coordination of styrene on the Cu center, forming η2alkene intermediate B1 (or B1′). In the B1 intermediate (Figure 3), the η2-alkene coordination leads to a significant

shows a version of the expanded mechanisms including the possible two-component competing pathways. Copper−boryl complex (B) is regarded as a catalytic active species in the current system. Formation of the copper−boryl complex (A → B) is discussed first in this section. Starting from the copper boryl intermediate (B), insertion of CO2 into Cu−B, which leads to CO2 borylation, would compete against insertion of B


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Organometallics

Figure 2. Energy profiles calculated for the olefin and CO2 insertion processes into the Cu−B bond in copper−boryl species B. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Figure 3. Optimized geometries for selected intermediates and transition states in the olefin and CO2 insertion processes. The distances are given in angstroms (Å).

bending in the L−Cu−B moiety. Then, the coordinated alkene undergoes migratory insertion into the Cu−B bond giving the β-boryl Cu−alkyl species. The barrier of the olefin migratory insertion is 14.8 kcal/mol (TSB1−C) for the 2,1-insertion, while that is 24.5 kcal/mol (TSB1′‑C′) for the 1,2-insertion. The insertion products, β-borylalkyl species (C or C′), are thermodynamically very stable, indicating the insertion processes are irreversible.

The results clearly show that the 2,1-insertion is more favored over the 1,2-insertion, in agreement with our previous study.19 The Cu−B bond in the copper−boryl species (B in Scheme 3 and Figure 2) is of strong nucleophilicity and serves as a nucleophile, delivering its σ-bonding electrons to the π* orbital of the coordinated styrene to break the alkenic π bond during the migratory insertion process. The observed preference of 2,1-insertion is related to the fact that the C


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Organometallics

Figure 4. Energy profile calculated for the main reaction pathway for the copper−alkyl species. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.

can isomerize through a rotation along the C(acyl)−O(Cu) single bond to another more stable intermediate D1. Finally, decupration occurs in D1 with a very small barrier (1.3 kcal/ mol) and regenerates the active catalyst species (Figure 4). Overall, for the main reaction pathway, the rate-determining step is the CO2 insertion into the Cu−C bond, which is in agreement with the previous study.17 Before continuing our discussion of the other competing reactions, it is interesting to compare the CO2 insertion into Cu−boryl (left-hand side of Figure 2) and Cu−alkyl bonds (Figure 4). The CO2 insertion into the Cu−boryl bond is exergonic by 3.4 kcal/mol and needs a barrier of 18.4 kcal/mol (TSB3‑E), while the CO2 insertion into the Cu−alkyl bond is exergonic only by 0.1 kcal/mol and the barrier is 25.1 kcal/mol (TSC‑D). The insertion into the Cu−C(sp3) bond is less favored, consistent with the previous finding.12b The Cu−boryl bond is more electron-rich due to the lower electronegativity of the boron atom, and as a result, the Cu−boryl bond is more nucleophilic and reactive. Figure 5a shows the energy profile calculated for the transmetalation reaction between the Cu−alkyl species (C) with the diboron reagent giving a diboration product. As mentioned above, the diboron reagent could be trapped by the alkoxide forming Lewis acid−base adduct Na[tBuO→B2pin2]. Herein, we used the Na[MeO→B2pin2] adduct as the substrate in our calculations. The calculated barrier for the transmetalation reaction between the copper−alkyl and Na[MeO → B2(OCH2CH2O)2] is extremely high (TSC1−B), indicating this transmetalation reaction is prohibitively unfavored. Given that the Na+ cation could be stabilized by the THF solvent molecule(s), we included one and two THF molecules in the calculations for this transmetalation process. The calculated barriers are similarly prohibitively high (Figure S1). The results suggest that THF molecule(s) exert the same effect on every species along the potential energy profile.

electron-withdrawing phenyl substituent makes the terminal carbon more electrophilic and accessible for the nucleophilic attack by the Cu−B bond. The CO2 insertion into Cu−B leading to CO2 borylation is potentially competitive. The energy profile calculated for the CO2 insertion into the Cu−B bond is given in the left-hand side of Figure 2. The CO2 coordination step leads to an η2CO2 intermediate B2 via the TSB−B2 transition state, which has a barrier of 14.7 kcal/mol. The η2-CO2 coordination in B2 is featured by a slightly bent L−Cu−B moiety and a slightly lengthened CO bond (Figure 3). From the η 2-CO 2 intermediate, the CO2 migratory insertion into the Cu−B bond gives intermediate E. Overall, the insertion of CO2 into the copper−boryl bond is exothermic by 3.4 kcal/mol, and the barrier is 18.4 kcal/mol. Comparing the olefin and CO2 insertions into the copper− boryl species, we see that the favorable olefin 2,1-insertion is much more favored, a result closely related to the fact that the coordination CO2 on the Cu center is energetically unfavorable, which is very different from the olefin coordination. Possible Reaction Pathways for the Cu−Alkyl Species. As mentioned above, the Cu−alkyl species (C in Scheme 3) is generated through insertion of styrene into the copper−boryl bond. Figures 4 and 5 show the calculated energy profiles for the main reaction pathway and other possible competing reaction pathways, respectively, of the Cu− alkyl species shown in Scheme 3. The optimized structures for selected transition states are given in Figure 6. The main reaction pathway involves CO2 insertion into the Cu−alkyl species (C). This step has a kinetic barrier of 25.1 kcal/mol (C → TSC‑D in Figure 4) and gives a copper− carboxylate intermediate (D). In TSC‑D, the Cu−alkyl σ bond is lengthened (2.14 Å, Figure 6) when compared to that in C (1.95 Å, Figure 3). The copper−carboxylate intermediate (D) D


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Figure 6. Optimized geometries for the transition states shown in Figures 4 and 5. The distances are given in angstroms (Å).

Direct decupration to give the boration product is unlikely to occur, because the present experimental conditions are lack of the proton source11 and the decupration product (sodium− alkyl species) is thermodynamically unstable (Scheme 3). Overall, the other possible competing reactions with Cu−alkyl species were found to be less favored than the CO2 insertion that gives the boracarboxylative product.

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CONCLUSIONS, FINDINGS, AND IMPLICATIONS In this work, we carried out DFT study on the mechanism of the copper-catalyzed boracarboxylation reaction of styrene using diboron and CO2. The main boracarboxylation reaction starts by generation of a Cu−boryl species, followed by sequential styrene insertion and CO2 insertion and finally an exchange of alkoxide for the carboxylate product. Under the reaction conditions, the Cu−boryl species was generated with almost no barrier. This Cu−boryl species preferentially reacted with styrene over carbon dioxide, suppressing a direct B2Pin2−CO2 two-component reaction. The alkyl intermediate, which was derived from styrene insertion, preferentially reacted with carbon dioxide over B2 pin2, another molecule of styrene, β-H elimination, and decupration, preventing from occurring the other twocomponent reactions, diboration, dehydrogenative borylation, polymerization, and hydroboration of styrene. Two very important findings are as follows: (1) Copper boryls preferentially undergo insertion of alkene versus carbon dioxide, while copper alkyls behave opposite, making to the main boracarboxylation reaction pathway dominant. (2) Excess NaOtBu greatly suppresses the transmetalation of copper alkyls with B2 pin2, minimizing the chance of giving alkene diboration. The significant implication of the first finding mentioned above is as follows. If insertion of CO2 is favored over insertion of the olefin substrate molecule, then the three-component reaction becomes not possible. This is because the CO2 insertion product having the Cu−O−C(O)−E (here E is the

Figure 5. Energy profiles calculated for other possible competing reaction pathways for the copper−alkyl species (C), including (a) metathesis with Na[MeO → B2(OCH2CH2O)2], (b) migratory insertion of styrene, and (c) β-H elimination. The relative free energies and electronic energies (in parentheses) are given in kcal/ mol.

The styrene can also sequentially insert into the Cu−C(sp3) bond of the Cu−alkyl species forming a polymeric product finally. The calculated energy profile is shown in Figure 5b. The migratory insertion of styrene into the Cu−alkyl species (C) needs a barrier of 40.2 kcal/mol (TSC2−F) to give F. The kinetic barrier for the generation of branched product is calculated to be 44.5 kcal/mol. In TSC2−F, the Cu−alkyl σ bond is significantly lengthened (2.45 Å, Figure 6) when compared to that in C (1.95 Å, Figure 3). The styrene insertion into the Cu−C(sp3) bond is clearly much less favored than CO2 insertion. The β-hydride elimination is also possible to take place for the Cu−alkyl species (C), and the barrier is 25.6 kcal/mol (TSC‑G in Figure 5c). The product (G) is thermodynamically unstable than the Cu−alkyl species (C), indicating this process is reversible. E


Organometallics

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boryl ligand) easily undergoes metathesis with the boron reagent, leading to diboration of CO2. The recently reported three-component reactions of alkynes/aldehydes with diboron and carbon dioxide9a,10a manifest the favorable insertion of the substrate molecules alkyne/aldehyde over CO2 into a Cu−B bond:

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00680. Calculations with the full ligand; relative energies of selected key intermediates and transition states with the full ligand; Energy profiles of C→ B considering the THF; Kinetic and thermodynamic data for the reactions of CO2/styrene insertion into L-Cu-E species (PDF) Cartesian coordinates for all calculated structures (XYZ)

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On the basis of the first finding, we would also like to make the predictions as shown in Scheme 4. Our calculations (see

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Scheme 4. Theoretical Prediction Regarding the Following Analogous Three-Component Reactions Using Different Boron Reagents

Zhenyang Lin: 0000-0003-4104-8767 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Research Grants Council of Hong Kong (HKUST 16303614, 16304416 and 16304017).

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REFERENCES

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the Supporting Information) indicate the following: (i) Insertion of CO2 into Cu−Ar is more favorable than that of olefin, leading to our prediction that the three-component reaction (a) is not possible. (ii) Insertion of olefin into Cu−H is more favorable than that of CO2, leading to our prediction that the three-component reaction (b) would occur. (iii) Insertions of olefin and CO2 into Cu−Si have comparable barriers, leading to our prediction that with proper reaction conditions the three-component reaction (c) may be possible.

COMPUTATIONAL DETAILS

Molecular geometries for all of the species studied in this work were fully optimized at the DFT level employing the hybrid B3LYP functional.20 The 6-311G* Wachters−Hay basis set was used for Cu. The 6-311G** Pople basis set was used for B, C, and O in the CO2 moiety, and C in the alkenic unit of styrene together with H involved in β-H elimination processes. All of the other atoms were described by the 6-31G* basis set. Vibrational frequency calculations were performed at the same level of theory to verify the characteristics of the stationary points, for which the local minima have no imaginary frequency and transition states have only one imaginary frequency. All of the transition states were further confirmed by intrinsic reaction coordinate (IRC) calculations that such structures could connect two relevant minima.21 To reduce the calculation cost, we use 1,3-dimethylimidazol-2ylidene to model the experimentally used ligand ICy (ICy = 1,3dicyclohexyl-1,3-dihydro-2H-imidazaol-2-ylidene). That is, the cyclohexyl substituents at the N atoms in the ICy ligand are replaced by methyl groups. Correspondingly, the tertiary butyl group in base NaOtBu and the methyl group in diboron substrate B2 pin2 are replaced by CH 3 and H, respectively. Thus, NaOMe and B2(OCH2CH2O)2 are used in our calculations. This simplification has been used to give reliable results according to our previous studies.22 Selected important intermediates and transition states were also tested using the experimentally employed substituents to validate the simplification. The results of these testing calculations are presented in the Supporting Information, suggesting that the simplification in the model is acceptable. All of the DFT calculations were performed with the Gaussian 09 software package.23 F


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DFT Studies on the Mechanism of Copper-Catalyzed

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