Open AccessCCS ChemistryRESEARCH ARTICLE1 Jun 2021

Visible-Light-Driven Anti-Markovnikov Hydrocarboxylation of Acrylates and Styrenes with CO2

    Light-driven carbon dioxide (CO2) capture and utilization is one of the most fundamental reactions in Nature. Herein, we report the first visible-light-driven photocatalyst-free hydrocarboxylation of alkenes with CO2. Diverse acrylates and styrenes, including challenging tri- and tetrasubstituted ones, undergo anti-Markovnikov hydrocarboxylation with high selectivities to generate valuable succinic acid derivatives and 3-arylpropionic acids. In addition to the use of stoichiometric aryl thiols, the thiol catalysis is also developed, representing the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. The UV–vis measurements, NMR analyses, and computational investigations support the formation of a novel charge-transfer complex (CTC) between thiolate and acrylate/styrene. Further mechanistic studies and density functional theory (DFT) calculations indicate that both alkene and CO2 radical anions might be generated, illustrating the unusual selectivities and providing a novel strategy for CO2 utilization.

    Introduction

    Photosynthesis is one of the most fundamental reactions in nature. For more than one century, chemists have been mimicking nature and searching for highly efficient visible-light photochemistry to realize novel chemical transformations in an environmentally friendly way.1 As diverse organic compounds cannot be excited by visible light, external photocatalysts are indispensable for most cases.25 Compared with widely investigated photocatalysis, visible-light-driven charge-transfer complex (CTC) or electron donor and acceptor (EDA) complex in the absence of external photocatalyst is less developed, although it is more economical and sustainable for industry.6 Notably, the light-driven CTC of thiols/disulfides and alkenes has attracted a great deal of attention in organic and polymer chemistry.79 For example, Wang reported a novel visible-light-driven CTC of alkenes and disulfides with alkenes as electron donors and disulfides as electron acceptors (Figure 1b).9 While few examples of CTCs of thiolates and (hetero)arenes have been reported,1013 to the best of our knowledge, the CTCs of thiolates and acrylates/styrenes have not been reported.

    Figure 1

    Figure 1 | (a) Selected examples for valuable succinic acid and 3-arylpropionic acid derivatives. (b–d) Visible-light-driven CTCs between alkenes and disulfides or thiolates and acrylates/styrenes. CTC, charge-transfer complex; 4-PE-HE, 4-phenethyl Hantzsch ester.

    Carbon dioxide (CO2), a well-known greenhouse gas, has gained considerable attention as an ideal one-carbon building block in chemical transformations due to its abundance, low cost, and sustainability.1423 Hydrocarboxylations of alkenes with CO2, especially transition-metal-catalyzed hydrocarboxylations, represent efficient strategies to generate bioactive and synthetically useful carboxylic acids.24 Recently, in addition to UV-light photocatalysis,16,2527 visible-light photocatalysis has emerged as an intriguing strategy to realize novel organic transformations with CO2.17,18,22,2842 Notably, Iwasawa pioneered this strategy by realizing the hydrocarboxylation of alkenes with Markovnikov regioselectivity via visible-light photoredox/Rh dual catalysis.27,41 Besides Markovnikov selectivity, König et al.35 also realized a novel hydrocarboxylation of styrenes with anti-Markovnikov regioselectivity26,4345 via photoredox/nickel dual catalysis. In these cases, both photocatalysts and transition-metal catalysts are indispensable. Moreover, CO2 is proposed to undergo reaction with highly reactive organometalic intermediates or a low-valent transition-metal catalyst, both of which are two-electron processes. In contrast, it is still highly challenging for visible-light-driven single-electron activation of CO2 to generate its corresponding radical anion, which is reactive and with selectivity that is hard to control.

    Anti-Markovnikov hydrocarboxylation of acrylates with CO2,46 which is in high demand to provide valuable succinic acid derivatives for pharmaceuticals (Figure 1a),47 in the organic and polymer chemistry industry,48 remains an unresolved challenge, we wondered whether we could realize this in an economical, efficient, and selective way. We envisioned that the electron-rich thiolates and electron-deficient acrylates/styrenes might form the CTC to facilitate the single-electron reduction of acrylates/styrenes or CO2 to generate radical anions, thus promoting anti-Markovnikov hydrocarboxylation of acrylates/styrenes (Figure 1c). We recognized, however, that such a strategy faced many challenges. For example, the light-driven thiol–ene reaction, oligmerization, and/or polymerization with C–S bond formation are competitively efficient.7,8,49 The thiocarboxylation33 and transesterification of acrylates may also occur, thus further complicating the reaction mixture. Herein, we report our success in realizing the first anti-Markovnikov selective hydrocarboxylation of acrylates and styrenes with CO2 under visible-light-driven photocatalyst-free conditions, enabled by a novel CTC between thiolate and acrylate/styrene (Figure 1d).

    Experimental Section

    Experimental methods

    To an oven-dried Schlenk tube (10 mL) equipped with a magnetic stir bar is added the alkene (0.2 mmol, 1.0 equiv for nonliquid substrates). Then the tube is moved into a glovebox and charged with NaOtBu (0.5 mmol, 2.5 equiv). The tube is sealed, evacuated, and backfilled with CO2 three times. Subsequently, the tube is opened and N-Methyl-2-pyrrolidinone (NMP) (2 mL) is added, followed by thiophenol (0.4 mmol, 2.0 equiv), tBuOH (0.4 mmol, 2.0 equiv), and alkene (0.2 mmol, 1.0 equiv for liquid substrates) via syringe under CO2. Once added, the resulting mixture is degassed by using a freeze-pump-thaw procedure (two times). Then the Schlenk tube is backfilled with CO2 and sealed at atmospheric pressure of CO2 (1 atm). The reaction is stirred and irradiated with a 30 W blue light-emitting diode (LED) lamp (1 cm away, with cooling fan to keep the reaction temperature at 25 °C and the reaction region located in the center of LED lamp) for 24 h. The resulting mixture is diluted with 3 mL EtOAc and quenched by 1.5 mL 2 N HCl, then stirred for 5 min. The reaction mixture is extracted by EtOAc six times, and the combined organic phases are concentrated in vacuo. The residue is purified by silica gel flash column chromatography (petroleum ether/EtOAc/AcOH 10/1/0.4%∼3/1/0.4%) to give the pure desired product. Further details may be found in the Supporting Information.

    Computational methods

    All the density functional theory (DFT) calculations are carried out with the Gaussian 0950 series of programs. DFT method ωB97XD51 with a standard 6–31+G(d) basis set is used for geometry optimizations. The solvent effects are considered by an universal solvation model based on solute electron density (SMD)52. Harmonic vibrational frequency calculations are performed for all of the stationary points to confirm them as local minima or transition structures, and to derive the thermochemical corrections for the enthalpies and free energies. The large basis set 6–311+G(d,p) is used to calculate the single-point energies to give more accurate energy information.

    Results and Discussion

    At the beginning of the research, we hypothesized that the addition of a proton source and the use of sterically hindered thiols might inhibit the side reactions, including thiocarboxylation and thiol–ene reactions. After systematic screening (Table 1), the selective hydrocarboxylation of methacrylate 1a proceeded in the presence of the easily available and bulky thiol, 2,4,6-triisopropylthiophenol, as well as NaOtBu and tBuOH to give the desired product 2a in 73% yield (Entry 1). The resulting chemo- and regioselectivities were both excellent; only very low yields of 2a′ (8% yield) and 2a″ (<5% yield) were obtained. Neither the Markovnikov-type hydro- nor thiocarboxylation was observed. Control experiments confirmed the essential roles of thiol, light, base, and CO2 (Entries 2–5). In the absence of tBuOH, 2a was obtained in a much lower yield (Entry 6). Using iPrOH instead of tBuOH gave a lower yield (Entry 7). The use of less sterically hindered thiols, such as 4-tert-butylthiophenol, resulted in poorer chemoselectivity (Entries 8–10). Other bases and solvents were tested, but they gave worse results (Entries 11 and 12).

    Table 1 | Optimization of the Reaction Conditions

    if1.eps
    Entry Alteration Yield/2a Yield/2a′ Yield/2a″
    1 None 76% (73%) 8% <5%
    2 Without ArSH N.D. N.D. N.D.
    3 Without light N.D. 34% N.D.
    4 Without NaOtBu N.D. 33% N.D.
    5 Under N2 instead of CO2 N.D. N.D. N.D.
    6 Without tBuOH 42% 6% 6%
    7 iPrOH instead of tBuOH 65% 12% <5%
    8 p-tBuC6H4SH as ArSH 64% 24% <5%
    9 p-tBuC6H4SH, FeCl3 (5 mol %) Trace 5% 66%
    10 Entry 9, tBuOH (2 equiv) Trace 16% 76%
    11 K2CO3 instead of NaOtBu 46% 21% <5%
    12 DMSO instead of NMP 33% 16% <5%

    Reaction conditions: 1a (0.2 mmol), 2,4,6-triisopropylthiophenol (ArSH, 0.4 mmol), NaOtBu (0.5 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h. NMR yields are determined by crude 1H NMR with CH2Br2 as internal standard. Yield of isolated product is provided in parenthesis. DMSO, dimethyl sulfoxide; LED, light-emitting diode; N.D., not detected; RT, room temperature.

    With satisfactory conditions (Condition A: Table 1, Entry 1) in hand, we aimed to investigate the scope of acrylates (Figure 2). A diversity of 2-methylacrylates bearing tertiary ( 1a 1e), secondary ( 1f 1h), and primary ( 1i and 1j) alkyl substituents in the ester moiety all underwent hydrocarboxylation under these conditions with high chemo- and regioselectivity. Besides methyl, other kinds of primary ( 1k 1o), secondary ( 1p 1r), and tertiary ( 1s) alkyl groups, as well as aryl groups ( 1t 1w), were well tolerated at the α-position of tert-butyl acrylates. Excellent chemoselectivity was also observed in the hydrocarboxylation of 2-allylacrylate 1o. Furthermore, β-substituted acrylates also provided the corresponding products 2x 2ba in good yields. Notably, the challenging tetrasubstituted acrylates with high steric hindrance, such as 1ca and 1da, also gave the desired products in moderate yields accompanied by recovery of the starting material (in 28% for 1ca and 44% yield for 1da, respectively) without any products of reduction, thiol–ene reaction and thiocarboxylation detected. Moreover, some acrylates ( 1ea 1ia) with bioactive motifs, including isoborneol, L-(–)-menthol, α-terpineol, 4-carvomenthenol, and β-cholesterol, were applicable in this transformation, providing the corresponding acids in moderate to good yields.

    Figure 2

    Figure 2 | Substrate scope of acrylates and styrenes. Condition A: Table 1, Entry 1, isolated yields. Condition B: 3 (0.2 mmol), p-tBuC6H4SH (0.4 mmol), NaOtBu (0.5 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h, isolated yields. Condition C: 1 or 3 (0.2 mmol), p-tBuC6H4SH (0.02 mmol), 4-PE-HE (0.6 mmol), NaOtBu (0.4 mmol), tBuOH (0.4 mmol), NMP (2 mL), 1 atm of CO2, 30 W blue LED, RT, 24 h, isolated yields. aThe diastereoselectivity (d.r.) values are about 1∶1 by crude NMR. b3 mL of NMP was used. cd.r. = 3∶1. dd.r. = 1.8∶1. eThe Z/E ratio of acrylates 1 is 1∶1. fd.r. = 5∶1. gd.r. > 19∶1, cis-isomer (see Supporting Information for more details). h3 equiv of ArSH and 3.5 equiv of NaOtBu were used, 48 h. iGram scale. j2 equiv of p-tBuC6H4SNa was used instead of p-tBuC6H4SH, NaOtBu, and tBuOH. kThe ratio (95∶5) of 4p and 4a is determined by ultra-performance liquid chromatography (UPLC). LED, light-emitting diode; RT, room temperature; N.D., not detected; 4-PE-HE, 4-phenethyl Hantzsch ester.

    As 3-arylpropionic acids also are valuable compounds (Figure 1a),35,4345 we further investigated the visible-light-driven photocatalyst-free anti-Markovnikov hydrocarboxylation of styrenes (Figure 2). Under slightly modified reaction conditions (Condition B: p-tBuC6H4SH is used instead of 2,4,6-triisopropylthiophenol in Condition A), diverse styrenes bearing either electron-neutral ( 3a 3c, 3i, and 3m) or electron-donating ( 3d, 3f, and 3j) groups on the arene moiety all reacted smoothly to afford the desired products 4 in moderate to good yields. A diversity of functional groups and heteroarenes ( 3g and 3ba) were tolerated well. The electron-withdrawing CF3 group ( 3h) could also be compatible to give desired product in 31% yield when employing p-tBuC6H4SNa. Notably, o-allyloxystyrene 3l underwent the reaction smoothly to afford 4l with excellent chemoselectivity. 3-Chlorostyrene 3q provides desired product 4q in good yield with very little dehalogenation (95∶5 ratio 4q: 4a). The styrenes bearing mono-, di-, and trisubstituted arenes as well as α-substitution all were suitable substrates under our protocol. A bioactive product 4ia, an antidiabetic GPR40 agonist, was generated in 74% yield through the hydrocarboxylation of 3ia. More challenging substrates, such as trisubstituted alkene ( 3ma), could also give the desired product in 50% yield. Notably, the gram-scale reaction of 3a went smoothly to give 4a in 78%. Interestingly, a dihydrobenzothiazole could replace the aryl thiol for the generation of 4a in 68% under otherwise identical reaction conditions (see Supporting Information for more details).

    Given that the aryl thiolates act as the electron donors to form disulfides as main byproducts and are not incorporated into the desired products, we hypothesized whether we could realize a thiol catalysis in the presence of stoichiometric reductant. If successful, to the best of our knowledge, such a strategy would represent the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. However, significant challenges remained due to facile side reactions of aryl thiols with alkenes, including thiol–ene reaction and thiocarboxylation, which would consume the catalyst and thus terminate the catalytic cycle. Moreover, competitive reduction of alkenes, CO2 and disulfides by the external reductant might be difficult to control. Therefore, we strove to realize this organocatalytic strategy (see Supporting Information for more details). Fortunately, we found that 4-phenethyl Hantzsch ester (4-PE-HE)53 was an ideal reductant to promote the thiol catalysis, providing the desired product 3a in 70% yield (Figure 2). Under the catalytic reaction conditions (Condition C), diverse acrylates, such as those bearing different ester moieties ( 1a, 1f, 1h, and 1i), α-alkyl ( 1k, 1r, and 1s), α-phenyl ( 1t), and β-ethyl ( 1y) substitution, all were suitable substrates. Moreover, bioactive derivatives of acrylates, such as 1ha and 1ia, could also undergo such a transformation smoothly. Besides acrylates, styrenes bearing mono- ( 3b, 3d, 3e, 3i, and 3m) and disubstituted ( 3r and 3x) arenes delivered the desired products 4 smoothly. The successful application of α-aryl ( 3ca 3ea) and α-methyl ( 3ja) styrenes in this catalytic hydrocarboxylation was also realized.

    To gain more insights into the reaction mechanism, we tested various control experiments (Figure 3). When 2,2,6,6-tetramethyl-1-piperinedinyloxy (TEMPO) was added to the reaction mixture, the desired product 2a was not detected (Figure 3a). “Radical clock” experiments resulted in exclusive formation of the corresponding ring-opening products (Figure 3b). These results suggested the formation of radical intermediates. In addition, some reductive coupling products of 3a, the reduction products of 3aa, and the reductive ring-opening products of 3la were also detected under N2 or CO2 (Figures 3c3e). Cyclic voltammetry (CV) test demonstrated that the reaction mixture is highly reductive (see Supporting Information for more details). Electron paramagnetic resonance (EPR) spectroscopy further supported the reduction of alkenes to generate alkyl radicals in this system (Figures 3f and 3g). The experiments using 13C-labeled CO2 generated the 2a, sodium formate, and sodium oxalate, all with 13C-labeling (Figure 3h). Moreover, both sodium formate and sodium oxalate were detected under the reaction conditions in the absence of alkenes (Figure 3i), which suggested that CO2 could be also reduced in such highly reductive conditions.

    Figure 3

    Figure 3 | (a–i) Mechanistic studies. EPR, electron paramagnetic resonance; HRMS, high-resolution mass spectra; LED, light-emitting diode; N.D., not detected; RT, room temperature; TEMPO, 2,2,6,6-tetramethyl-1-piperinedinyloxy.

    To probe our hypothesis of formation of CTC, we conducted the UV–vis absorption spectra tests. A slight bathochromic shift was observed when we tested the mixture of p-tBuC6H4SNa with either 1a or 3a (Figures 4a and 4b). When we used a more electron-deficient 4-(trifluoromethyl)styrene, the bathochromic shift was more significant (Figure 4c). To seek more evidence for the CTC formation, we carried out 19F-NMR experiments, mixing a certain amount of 4-(trifluoromethyl)styrene with increasing amounts of p-tBuC6H4SNa. The chemical shift of the CF3 group distinctly moved downfield (Figure 4d), indicating the increased electron density on the 4-(trifluoromethyl)styrene via electron donation from strongly reductive thiolates (−2.66 V vs Ag/Ag+; see Supporting Information for more details) to styrenes. All these results indicated the formation of CTC between electron-rich thiolates and electron-deficient styrenes.

    Figure 4

    Figure 4 | UV–vis measurements and DFT calculations on the CTC between thiolate and acrylate/styrene. (a) UV–vis measurement of 1a. (b) UV–vis measurement of 3a. (c) UV–vis measurement of 4-(trifluoromethyl)styrene. (d) Correlation between the chemical shift of the CF3 group of 4-(trifluoromethyl)styrene and the relative p-tBuC6H4SNa concentration by 19F-NMR. (e) The energy of HOMO of CTC between thiolate and 1a is −6.45 eV. (f) The energy of LUMO of CTC between thiolate and 1a is +0.86 eV. (g) ESP surface of singlet state of CTC. (h) ESP surface of triplet state of CTC. CTC, charge-transfer complex; DFT, density functional theory; ESP, electrostatic potential; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.

    Besides the experimental evidence, we further investigated the formation of CTC between thiolate and acrylate 1a using DFT calculations at the ωB97XD/6–31+G(d) level of theory. The energy gap was reduced as 7.31 eV (Figures 4e and 4f) compared with 1a and thiolates ( Supporting Information Figures S18a–S18d), which might arise from the electron donation from the highest occupied molecular orbital (HOMO) of thiolate to the lowest unoccupied molecular orbital (LUMO) of 1a. The reduced HOMO–LUMO gap explained well the bathochromic shift of absorption of CTC. Moreover, DFT calculations of electrostatic potential (ESP) surface of singlet and triplet states showed that the electron donation from thiolates to 1a take place in the excited state (Figures 4g and 4h). Although CO2 might also act as an electron acceptor, the DFT calculations indicated the formation of CTC between thiolate, and CO2 might be less favored because of its higher energy gap (8.06 eV) and excited energy (65.5 kcal/mol; Supporting Information Figures S21 and S23) than that of thiolate–alkene CTC (7.31 eV; 56.5 kcal/mol; Supporting Information Figure S23) in our reaction system.

    Based on the experimental and computational results, we proposed possible pathways for the hydrocarboxylation of 3a (Figure 5a). Irradiation of the CTC 5 might generate styrene radical anion 6 (Path a) or CO2 radical anion 7 (Path b) as well as thiol radical 8. Both reactions of 6 with CO2 and 7 with 3a could generate the same intermediate 9, which might undergo hydrogen atom transfer (HAT) to produce carboxylate 10. Final protonation would lead to desired product 4a. Besides the desired hydrocarboxylation to give 4a, hydrothiolation and thiocarboxylation are also possible via the intermediates 11 and 12, which might be generated from the reaction of 8 and 3a or 9.

    Figure 5

    Figure 5 | (a–c) Proposed mechanism and corresponding DFT calculations. CTC, charge-transfer complex; DFT, density functional theory.

    To probe the possibility of two such productive pathways (Figure 5a, Paths a and b), we did further DFT calculations. The CTC at triplet state could drive the single electron transfer (SET) process to styrene or CO2 resulting styrene radical anion 6 (−4.0 kcal/mol) or CO2 radical anion 7 (−9.7 kcal/mol) respectively. This suggested that CO2 reduction might be more thermodynamically favorable (Figure 5b). Moreover, the spin density distribution in styrene 3a and acrylate 1a radical anion complexes was also calculated with higher spin density in β-position, which explained the high regioselectivity (see Supporting Information for more details).

    We further investigated the issue of chemoselectivity by DFT calculations (Figure 5c). The radical addition of 3a with 7 through transition state TS-1 afforded the carboxylated radical intermediate 9 with an energy barrier of 11.6 kcal/mol, leading to the hydrocarboxylation. However, the corresponding energy barrier for the radical addition of 3a with thiol radical, which leads to the hydrothiolation (via TS-2), was up to 13.2 kcal/mol. The generated radical intermediate 11 was also 9.9 kcal/mol more endergonic than 9. Therefore, the process of hydrocarboxylation was thermodynamically and kinetically more favorable than hydrothiolation. In addition, an alternative radical addition of 3a with 7 through TS-1-iso resulted in Markovnikov hydrocarboxylation. However, calculated results showed that Markovnikov selectivity was kinetically unfavorable due to its high energy barrier (16.2 kcal/mol) in comparison with anti-Markovnikov selectivity. Starting from intermediate 9, a distinct higher energy barrier of thiocarboxylation ( TS-4, 17.5 kcal/mol) was observed, which was inclined to conduct a HAT process for hydrocarboxylation with lower energy barrier ( TS-3, 10.9 kcal/mol). Further DFT calculations for different HAT process, thiol catalysis and dihygrobenzothiazole-involved case were also investigated in our reaction system for clarity (see Supporting Information for more details).

    Conclusion

    We have discovered a novel CTC between thiolate and acrylate/styrene, which is supported by UV–vis measurements, NMR tests, and computational investigations. Based on the CTC, the first visible-light-driven photocatalyst-free anti-Markovnikov hydrocarboxylation of acrylates and styrenes with CO2 is realized to generate valuable succinic acid derivatives and 3-arylpropionic acids, featuring unusual and excellent regio- and chemoselectivities. Moreover, this system is also general and practical for diverse acrylates and styrenes with different substitution modes, including the challenging tri- and tetrasubstituted ones. Notably, aryl thiol can be used as a catalyst, representing the first visible-light-driven organocatalytic hydrocarboxylation of alkenes with CO2. Further mechanistic studies and DFT calculations indicate that both alkene and CO2 radical anions might be generated in the reaction mixture. As this photocatalyst-free and transition-metal-free photochemistry show great potential for application in pharmaceuticals, organic chemistry, and the polymer industry, further investigations and applications of this system are underway in our laboratory.

    Supporting Information

    Supporting Information is available.

    Conflict of Interest

    The authors declare the following competing financial interest: A Chinese patent on this work is pending with the number (201911022392.0).

    Acknowledgments

    Financial support was provided by the the National Natural Science Foundation of China (nos. 21822108, 21822303, 21801176, 21772129, and 21772020), the Fok Ying Tung Education Foundation (no. 161013), Sichuan Science and Technology Program (nos. 2019YJ0379 and 20CXTD0112), and the Fundamental Research Funds for the Central Universities.

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