DBU Catalyst System for the Carboxylative Cyclization

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New Copper (I) /DBU Catalyst System for the Carboxylative Cyclization of Propargylic amines with Atmospheric CO2: An Experimental and Theoretical Study Yuling Zhao, Jikuan Qiu, Li Tian, Zhiyong Li, Maohong Fan, and Jianji Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01288 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Sustainable Chemistry & Engineering

New Copper(I)/DBU Catalyst System for the Carboxylative Cyclization of Propargylic amines with Atmospheric CO2: An Experimental and Theoretical Study Yuling Zhao,† JikuanQiu, † Li Tian,† Zhiyong Li,† Maohong Fan‡ and Jianji Wang*,†

†

Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine

Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, 46# East of Construction Road, Xinxiang, Henan 453007, P. R. China ‡

Department of Chemical and Petroleum Engineering, School of Energy Resources,

University of Wyoming, 1000 E. University Avenue, Laramie, Wyoming 82071, United States

AUTHOR INFORMATION Corresponding Author *Tel: +86-373-3325805. E-mail: [email protected]; Notes The authors declare no competing financial interest.

1

ACS Paragon Plus Environment


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ABSTRACT Carbon dioxide (CO2) is an abundant and renewable feedstock for the production of high-value chemicals. Herein, CuI and 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) were employed as an efficient catalyst system to synthesize 2-oxazolidinones through the coupling reaction of CO2 and propargylic amines. It was found that this cost-competitive catalyst system could efficiently catalyze the reaction within only 4 hours at atmospheric pressure CO2, and a wide range of substrates were suitable for this reaction. Various target products were obtained in excellent yields. Furthermore, the catalytic mechanism of CuI/DBU system was investigated by using density functional theory. It was shown that Copper(I) and DBU played synergistic role in activating both the C≡C triple bond and the amino group of propargylic amines in the reaction.

KEYWORDS: Carbon dioxide, propargylic amines, copper(I), density functional theory

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INTRODUCTION Carbon dioxide (CO2) is an abundant, ubiquitous, cheap, and nontoxic gas in nature. Many useful organic chemicals have been produced by using CO2 as a feed stock in the last decades. At present, significant efforts have been devoted to the development of technologies for transforming CO2 into various high-value chemicals1-9. However, CO2 transformation on a large scale is still a great challenge. The key issue is that harsh and severe reaction conditions are often required due to its high thermodynamic stability and kinetic inertness. Therefore, it remains important to develop new and efficient catalytic strategies in order to widen the scope of products that incorporate CO2 as raw material under mild conditions. 2-Oxazolidinones are important heterocyclic compounds and have many applications10,11. For example, they can be used as cholesteryl ester transfer protein inhibitor12, monoamine oxidase inhibitor13, chiral auxiliaries14, and antibacterial drugs15. Many synthesis routes have been reported to produce 2-oxazolidinones, such as the allylic C-H oxidation of N-Boc amines16, the formal [3+2] cycloaddition reaction17, and others18-20. In recent years, the synthesis of 2-oxazolidinones through coupling reaction between propargylic amines and CO2 has attracted considerable attention. Many efficient metal-free catalyst systems such as t-BuOI21, protic ionic liquid22, superbase23, NHC24,25 and guanidine-CO2 complex26 have been developed for this reaction. It is also reported that some metal catalyst systems such as silver 27-30, Pd31,32, Ru33 and N-heterocyclic carbine Au complexe34-36 can catalyze the reaction with high activities (Scheme 1a). In general, the reported metal catalyst systems require high reaction temperature and/or high CO2 pressure, and using expensive noble metals or complicated catalyst systems, while the substrate scope is not wide for the metal-free approaches. Therefore, searching for more efficient and less-expensive catalysts is of great importance. Recently, Cu(II)-substituted polyoxometalate-based ionic liquid catalysts37 (Scheme 1b) have been found to catalyze this reaction with excellent yield at atmospheric pressure. However, a long reaction time (20 hours) is required and the 3



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reactivity of the propargylic amines depends strongly on the nature of the R2 and R3 substituents (Scheme1b). Therefore, it still remains a challenge to expand the substrate scope for the economic and versatile synthesis of 2-oxazolidinones. Herein, we report our results by using cost-effective copper(I) salt and DBU to catalyze the coupling reaction of propargylic amines with CO2 at atmospheric pressure (Scheme 1c). It is found that the CuI/DBU catalyst system is robust, and highly efficient for the reactions with a wide range of substrates, and the target products have been obtained in excellent yields. Then the catalytic mechanism of CuI/DBU catalyst system is investigated by using density functional theory (DFT). Based on such calculations, a possible reaction mechanism is proposed. It should be noted that Cu(I)-catalyzed synthesis of oxazolidinones from CO2 by multi-component coupling had been published previously, and significant results were obtained38,39. However, the organic starting materials were different, so it was different with the present work.

Previous work

Ag, Pd, Ru and Au catalysts (a) O

high cost R1

HN R4 R3

R2

O +

CO2

N R4

R1 R2

R3

[nC7H15)4N]6[α-SiW11O39Cu], 60°C, 20h

(b)

R2 ≠ H, R3 = H This work (c)

R1

O HN R4 R2

R3

O

CuI, DBU, DMSO, 50°C, 4h R1

+ CO2 R2, R3 =H, aryl,alkyl

R2

N R4 R3

Scheme 1. Carboxylative cyclization of propargyl amines with CO2 by different strategies.

EXPERIMENTAL METHODS 4


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Chemicals. CO2 was supplied by Beijing Analytical Instrument Factory with a purity of 99.99%. All the solvents were commercially available and used as received unless otherwise indicated. CuX, CuO and CuCN were provided by J & K Scientific Ltd. DBU was purchased from Aladdin Company. Other bases were analytical grade and were supplied from Beijing Chemical Reagent Company. The deuterated solvents (CDCl3 and DMSO-d6) were obtained from Sigma-Aldrich. The propargylic amine substrates were synthesized by following the procedures described in literature22. General procedures for the carboxylation cyclization of propargylic amine with CO2. In a 20 ml Schlenk flask, propargylic amine (1.0 mmol), DBU (0.2 mmol), indicated amount of the catalyst, and DMSO (2 ml) were added. The flask was capped with a stopper and then sealed. The gas-exchanging process was conducted using the “freeze–pump–thaw” method. The reaction mixture was stirred at 50°C for 4 h under CO2 atmosphere (balloon). After the reaction was finished, the product was extracted with n-hexane, and the crude mixture was purified by silica gel column chromatography (EtOAc : petroleum ether = 1 : 20) to obtain the desired 2-oxazolidinone. Compound 2a: Light yellow oil; 1H NMR (600 MHz, CDCl3): δ = 7.594-7.579 (m, 2 H), 7.335-7.309 (m, 2 H), 7.212-7.184 (m, 1 H), 5.452 (s, 1 H), 3.205 (t, J = 7.92 Hz, 2 H), 1.686-1.634 (m, 2 H), 1.493 (s, 6 H), 1.371-1.334 (m, 2 H), 0.958 (t, J = 7.38 Hz, 3 H)ppm; 13C NMR (150 MHz, CDCl3): δ =154.19, 153.51, 133.68, 128.46, 128.28, 126.71, 100.38, 62.17, 40.41, 31.52, 27.61, 20.24, 13.75 ppm; HRMS (ESI, m/z) calcd. for C16H21NO2Na [M+Na]+: 282.1464, found: 282.1499; IR (selected absorbances): 2961, 2933, 1772, 1687, 1398, 1315, 1020 cm-1. Compound 2b: Colourless oil; 1H NMR (400 MHz, CDCl3): δ= 7.580 (d, J = 7.6 Hz, 2 H), 7.329 (t, J = 7.2 Hz, 2 H), 7.208 (t, J = 7.2 Hz, 1 H), 5.476 (d, J = 2.0 Hz, 1 H), 4.556-4.503 (m, 1 H), 3.583-3.507 (m, 1 H), 3.174-3.104 (m, 1 H), 1.645-1.512 (m, 2 H), 1.475 (d, J = 6.4 Hz, 3 H), 1.400-1.340 (m, 2 H), 0.960 (t, J = 7.2 Hz, 3 H), 0.902 (t, J = 7.32 Hz, 3 H) ppm;

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C NMR (100 MHz, CDCl3): δ= 154.80,

148.60,133.56, 128.49, 128.26, 126.80, 102.08, 54.65, 41.26, 29.36, 19.93, 19.74, 13.71 ppm; HRMS (ESI, m/z) calcd. for C15H19NO2Na [M+Na]+: 268.1308, found: 5



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268.1299; IR (selected absorbances): 2959, 2932, 1768, 1692, 1416, 1077, 1019, 694 cm-1. Compound 2c: Colourless oil; 1H NMR (600 MHz, CDCl3): δ= 7.592 (d, J = 1.2 Hz, 1 H), 7.579 (s, 1 H), 7.341-7.315 (m, 2 H), 7.219-7.194 (m, 1 H), 5.462 (d, J = 1.74 Hz, 1 H), 4.559-4.543 (m, 1 H), 3.641-3.590 (m, 1 H), 3.040-2.993 (m, 1 H), 1.962-1.913 (m, 1 H), 1.771-1.729 (m, 1 H), 1.629-1.529 (m, 3 H), 0.959 (t, J = 7.38 Hz, 3 H), 0.902 (t, J = 7.32 Hz, 3 H) ppm; 13C NMR (150 MHz, CDCl3): δ= 155.29, 146.61, 133.62, 128.48, 128.28, 126.73, 102.35, 59.00, 41.06, 29.23, 24.90, 19.93, 13.68 ppm; HRMS (ESI, m/z) calcd. for C16H21NO2Na [M+Na]+: 282.1464, found: 282.1454; IR (selected absorbances): 2960, 2932, 1769, 1686, 1441, 1406, 1039, 756 cm-1. Compound 2d: Light yellow oil; 1H NMR (600 MHz, CDCl3): δ= 7.581 (d, J = 7.26 Hz, 2 H), 7.326 (t, J= 7.62 Hz, 2 H), 7.203 (t, J = 7.44 Hz, 1 H), 5.465 (d, J = 1.74 Hz, 1 H), 4.524 (t, J = 1.8 Hz, 1 H), 3.633-3.582 (m, 1 H), 3.047-3.000 (m, 1 H), 1.886-1.826 (m, 1 H), 1.712-1.655 (m, 1 H), 1.622-1.520 (m, 2 H), 1.417-1.348 (m, 4 H), 0.958 (t, J = 7.32 Hz, 6 H) ppm; 13C NMR (150 MHz, CDCl3): δ=155.17, 147.00, 133.62, 128.48, 128.28, 126.75, 102.32, 58.40, 41.13, 34.43, 29.26, 19.92, 15.84, 13.91, 13.68 ppm; HRMS (ESI, m/z) calcd. for C17H23NO2Na [M+Na]+: 296.1621, found: 296.1612; IR (selected absorbances): 2960, 2933, 1769, 1643, 1405, 1091, 1048, 757, 694 cm-1. Compound 2e: Colourless oil; 1H NMR (600 MHz, CDCl3): δ= 7.591 (d, J = 7.56 Hz, 2 H), 7.328 (t, J = 7.44 Hz, 2 H), 7.210 (t, J = 8.76 Hz, 1 H), 5.486 (s, 1 H), 4.306 (s, 1 H), 3.679-3.629 (m, 1 H), 3.075-3.030 (m, 1 H), 2.159-2.126 (m, 1 H), 1.631-1.538 (m, 2 H), 1.375-1.341 (m, 2 H), 1.099 (d, J = 6.96 Hz, 3 H), 0.955 (t, J = 7.26 Hz, 3 H), 0.920 (d, J = 6.72 Hz, 3 H) ppm;

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C NMR (150 MHz, CDCl3):

δ=155.35, 145.02, 133.57, 128.47, 126.85, 104.20, 63.71, 41.37, 30.20, 29.18, 19.91, 17.52, 15.44, 13.68 ppm; HRMS (ESI, m/z) calcd. for C17H23NO2Na [M+Na]+: 296.1621, found: 296.1612; IR (selected absorbances): 2962, 2932, 1775, 1687, 1420, 1246, 1037, 754, 693 cm-1. Compound 2f: White solid; 1H NMR (600 MHz, CDCl3): δ = 7.518 (d, J = 7.2 Hz, 6


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2H), 7.450-7.397 (m, 3H), 7.343-7.279 (m, 4H), 7.184 (t, J = 7.6 Hz, 1H), 5.386 (d, J= 1.8 Hz, 1H), 5.248 (d, J = 1.8 Hz, 1H), 3.540-3.490 (m, 1H), 2.848-2.801 (m, 1H), 1.546-1.439(m, 2H), 1.37-1.26 (m, 2H), 0.880 (t, J = 7.2 Hz, 3H) ppm; 13C NMR (150 MHz, CDCl3):δ =155.04, 147.70, 137.35, 133.46, 129.37, 129.32, 128.44, 128.33, 127.81, 126.93, 104.54, 63.87, 41.62, 28.96, 26.92, 19.81, 13.62 ppm; HRMS (ESI, m/z) calcd. for C20H21NO2Na [M+Na]+: 330.1464, found: 330.1477; IR (selected absorbances): 2959, 2931, 1779, 1690, 1412, 1039, 943, 751, 693 cm-1. Compound 2g: Colourless oil; 1H NMR (600 MHz, CDCl3): δ = 7.588 (d, J = 1.08 Hz, 2 H), 7.330-7.305 (m, 2 H), 7.205-7.179 (m, 1 H), 5.440 (s, 1 H), 3.188 (t, J = 8.01 Hz, 2 H), 2.111-2.005 (m, 4 H), 1.900-1.853 (m, 4 H), 1.723-1.670 (m, 2 H), 1.396-1.346 (m, 2 H), 0.959 (t, J = 7.38 Hz, 3 H) ppm; 13C NMR (150 MHz, CDCl3): δ = 156.06, 154.25, 133.91, 128.45, 128.31, 126.62, 100.35, 71.82, 41.02, 39.67, 31.45, 25.63, 20.32, 13.75 ppm; HRMS (ESI, m/z) calcd. for C18H23NO2Na [M+Na]+: 308.1621, found: 308.1605; IR (selected absorbances): 2958, 2872, 1773, 1685, 1399, 1037, 978, 754, 693 cm-1. Compound 2h: White solid; 1H NMR (600 MHz, CDCl3): δ = 7.619 (d, J = 7.8 Hz, 2 H), 7.327 (t, J = 7.2 Hz, 2 H), 7.204 (t, J = 7.8 Hz, 1 H), 5.802 (s, 1 H), 3.182 (t, J = 8.4 Hz, 2 H), 1.858-1.811 (m, 6 H), 1.728-1.623 (m, 4 H), 1.386-1.348 (m, 2 H), 1.286-1.234 (m, 2 H), 0.954 (t, J = 7.2 Hz, 3 H) ppm; 13C NMR (150 MHz, CDCl3): δ = 152.42, 128.75, 128.44, 126.77, 103.71, 63.96, 40.24, 34.25, 31.71, 24.20, 21.61, 20.29, 13.79 ppm; HRMS (ESI, m/z) calcd. for C19H25NO2Na [M+Na]+: 322.1777, found: 322.1796; IR (selected absorbances): 2932, 2856, 1775, 1664, 1403, 1047, 754, 696 cm-1. Compound 2i: Colourless oil; 1H NMR (600 MHz, CDCl3): δ = 7.474 (d, J = 7.74 Hz, 2 H), 7.124 (d, J = 7.68 Hz, 2 H), 5.408 (s, 1 H), 3.182 (t, J = 7.80 Hz, 2 H), 2.327 (s, 3 H), 2.083-1.992 (m, 4 H), 1.865 (d, J = 3.9 Hz, 4 H), 1.705-1.666 (m, 2 H), 1.392-1.355 (m, 2 H ), 0.957 (t, J = 7.26 Hz, 3 H) ppm; 13C NMR (150 MHz, CDCl3): δ = 155.28, 154.38, 136.39, 131.05, 129.15, 128.22, 100.30, 71.78, 44.01, 39.65, 31.47, 25.61, 21.21, 20.34, 13.76 ppm; HRMS (ESI, m/z) calcd. for C19H25NO2Na [M+Na]+: 322.1777, found: 322.1767; IR (selected absorbances): 2959, 2872, 1771, 7



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1681, 1401, 1036, 752 cm-1. Compound 2j: Colourless oil; 1H NMR (600 MHz, CDCl3): δ = 7.510 (d, J = 7.20 Hz, 2 H), 7.130 (d, J = 7.80 Hz, 2 H), 5.769 (s, 1 H), 3.173 (t, J = 7.80 Hz, 2 H), 2.330 (s, 3 H), 1.842-1.782 (m, 7 H), 1.725-1.605 (m, 4 H), 1.394-1.331 (m, 2 H), 1.285-1.230 (m, 1 H ), 0.950 (t, J = 7.20 Hz, 3 H) ppm; 13C NMR (150 MHz, CDCl3): δ = 154.42, 151.68, 136.54,131.09, 129.13, 128.65, 128.20, 103.65, 63.91, 40.21, 34.26, 31.73, 24.22, 21.62, 21.21, 20.30, 19.93, 13.80 ppm; HRMS (ESI, m/z) calcd. for C20H27NO2Na [M+Na]+: 336.1934, found: 336.1924; IR (selected absorbances): 2962, 2868, 1809, 1698, 1231, 1147, 1052, 1014, 858, 765 cm-1. Compound 2k: Colourless oil; 1H NMR (600 MHz, CDCl3): δ = 7.561-7.538 (m, 2 H), 7.009 (t, J = 9.0 Hz, 2 H), 5.431 (s, 1 H), 4.508 (s, 1 H), 3.620-3.570 (m, 1 H), 3.042-2.996 (m, 1 H), 1.870-1.819 (m, 1 H), 1.694-1.638 (m, 1 H), 1.616-1.512 (m, 2 H ), 1.421-1.302 (m, 4 H), 0.956 (t, J = 7.32 Hz, 3 H) ppm;

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C NMR (150 MHz,

CDCl3): δ = 162.29, 160.56, 155.08, 146.62, 129.91, 129.86, 129.80, 129.78, 115.43, 115.29, 101.26, 58.36, 41.15, 34.39, 29.23, 19.91, 15.86, 13.90, 13.67 ppm; HRMS (ESI, m/z) calcd. for C17H22FNO2Na [M+Na]+: 314.1526, found: 314.2034; IR (selected absorbances): 2992, 1806, 1696, 1507, 1148, 1019, 852, 766, 694 cm-1. Compound 2l: Light yellow oil; 1H NMR (600 MHz, CDCl3): δ = 7.582 (d, J = 7.38 Hz, 2 H), 7.326 (t, J = 7.68 Hz, 2 H), 7.204 (t, J = 14.82 Hz, 1 H), 5.466 (d, J = 1.68 Hz, 1 H), 4.531-4.516 (m, 1 H), 3.618-3.568 (m, 1 H), 3.042-2.996 (m, 1 H), 1.884-1.825 (m, 1 H), 1.712-1.655 (m, 1 H), 1.638-1.588 (m, 1 H), 1.449-1.383 (m, 1 H), 1.349-1.310 (m, 7 H), 0.956 (t, J = 7.32 Hz, 3 H), 0.905-0.882 (m, 3 H) ppm; 13C NMR (150 MHz, CDCl3): δ = 155.16, 147.02, 133.63, 128.48, 128.28, 126.75, 102.30, 58.41, 41.42, 34.43, 31.41, 27.19, 26.37, 22.53, 15.84, 13.99, 13.91 ppm; HRMS (ESI, m/z) calcd. for C19H27NO2Na [M+Na]+: 324.2042, found: 324.2611; IR (selected absorbances): 2930, 2859, 1769, 1742, 1644, 1405, 1088, 1053, 756, 693 cm-1. Compound 2m: Light yellow oil; 1H NMR (400 MHz, CDCl3): δ = 7.582 (d, J = 7.60 Hz, 2 H), 7.348-7.256 (m, 4 H), 7.226-7.164 (m, 4 H), 5.445 (d, J = 1.60 Hz, 1 H), 4.493-4.471 (m, 1 H), 3668-3.594 (m, 1 H), 3.054-2.986 (m, 1 H), 2.703-2.619 (m, 2 H), 1.932-1.841 (m, 1 H), 1.741-1.558 (m, 5 H), 0.870 (t, J = 7.60 Hz, 3 H) ppm; 8


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C NMR (100 MHz, CDCl3): δ = 155.33, 146.51, 141.75, 133.58, 128.50, 128.43,

128.29, 126.80, 125.96, 102.43, 58.94, 41.07, 35.34, 28.39, 26.64, 24.85, 6.49 ppm; HRMS (ESI, m/z) calcd. for C22H25NO2Na [M+Na]+: 358.1777, found: 358.1766; IR (selected absorbances): 2934, 1775, 1690, 1420, 1269, 1099, 1036, 925, 751, 694 cm-1. Compound 2n: Colourless oil; 1H NMR (600 MHz, CDCl3): δ = 7.569 (d, J = 7.62 Hz, 2 H), 7.318 (t, J = 9.48 Hz, 2 H), 7.193 (t, J = 7.32 Hz, 1 H), 5.414 (s, 1 H), 4.542 (s, 1 H), 3.508-3.493 (m, 1 H), 1.967-1.789 (m, 6 H), 1.684 (d, J = 12.6 Hz, 1 H), 1.579 (d, J = 12.54 Hz, 1 H), 1.427-1.294 (m, 4 H), 1.202-1.389 (m, 1 H), 0.944 (t, J = 7.02 Hz, 3 H) ppm;

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C NMR (150 MHz, CDCl3): δ = 154.71, 147.43, 133.73,

128.46, 128.24, 126.65, 101.82, 58.76, 54.27, 36.79, 31.36, 29.99, 25.85, 25.83, 25.27, 15.56, 13.91 ppm; HRMS (ESI, m/z) calcd. for C19H25NO2Na [M+Na]+: 322.1777, found: 322.1764; IR (selected absorbances): 2974, 1824, 1698, 1214, 1055, 1006, 793, 693 cm-1. Compound 2o: Colourless oil; 1H NMR (400 MHz, CDCl3): δ = 7.396-7.305 (m, 3 H), 7.271 (d, J = 7.8 Hz, 2 H), 4.755-4.735 (dd, J = 2.40 Hz, J = 2.80 Hz, 1 H), 4.472 (s, 2 H), 4.251-4.232 (m, 1 H), 4.023(t, J = 7.2 Hz, 2 H) ppm; 13C NMR (100 MHz, CDCl3): δ = 155.66, 148.93, 134.96, 128.99, 128.27, 128.19, 86.81, 47.86, 47.23ppm; HRMS (ESI, m/z) calcd. for C11H11NO2Na [M+Na]+: 212.0682, found: 212.0674; IR (selected absorbances): 1772, 1677, 1424, 1280, 1236, 1082, 1054, 967, 876, 831, 752, 699 cm-1. Compound 2p: Colourless oil; 1H NMR (400 MHz, CDCl3): δ = 7.371-7.269 (m, 5 H), 4.570-4.536 (m, 1 H), 4.460 (s, 2 H), 3.959 (s, 2 H), 1.672 (d, J = 7.2 Hz, 3 H)ppm;

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C NMR (100 MHz, CDCl3): δ = 155.06, 141.67, 135.19, 128.93, 128.16,

97.60, 47.89, 47.10, 9.96 ppm; HRMS (ESI, m/z) calcd. for C12H13NO2Na [M+Na]+: 226.0838, found: 226.0854; IR (selected absorbances): 2921, 1773, 1713, 1478, 1423, 1264, 1058, 997, 753, 699 cm-1.

THEORETICAL METHODS 9



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All quantum chemical calculations were performed with the Gaussian 09 package using hybrid density functional theory B3LYP40. For O and N, the 6-311+G(d) basis set was used, while the other atoms (C, H) were described by the standard 6-31G(d,p) basis set41. For the evaluation of solvent effect of DMSO, self-consistent reaction field (SCRF) calculations based on the SMD salvation model were carried out. Vibrational frequency calculations were performed at the same level of theory to verify that a local minimum has no imaginary frequency and each transition state has only one single imaginary frequency. To obtain the relative free energies, single-point energy calculations for all of the species studied were made at the same level. Intrinsic reaction coordinate (IRC) calculations were also performed to make sure that each calculated transition state connects two relevant minima.

RESULTS AND DISCUSSION The

carboxylative

cyclization

of

propargylic

amines

with

CO2

to

2-Oxazolidinones The carboxylative cyclization of N-butyl-2-methyl-4-phenylbut-3-yn-2-amine (1a) was used as a model reaction to systematically study the influence of various parameters on the reaction outcome (Table 1). It was shown that in the absence of catalyst and base, the reaction did not occur (entry 1). Then the effect of catalyst was investigated at 50 °C for 6 h of reaction in DMSO. The product was only obtained in a low yield in the absence of a base or a copper catalyst (entries 2-3). To our delight, 96% NMR yield was obtained in the presence of 10 mol% of CuI catalyst and 1 equivalent (equiv.) of DBU (entry 4). This suggests that CuI and DBU had excellent synergistic effect on catalyzing the reaction. Encouraged by this result, we investigated the effect of base types and dosage on the yield of product. For this purpose, different kinds of bases such as NaOH, K2CO3, and trimethylamine (NEt3) were used in the reaction, but the yields were not satisfactory (entries 5-7). This result indicates that DBU was a superior base for the direct carboxylic cyclization of propargylic amine with CO2. Interestingly, an almost quantitative yield was also obtained when the amount of DBU 10


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was reduced to 0.2 equiv. (entry 8). Based on this observation, we investigated the effect of the solvent on the reaction in the presence of 0.2 equiv. DBU. It can be seen from Table 1 that yield of the product was strongly dependent on the reaction media, for example, the reaction in THF, C2H5OH and CH3CN gave a 58% to 67% yield (entries 9-11), while great improvement was achieved when the reaction was conducted in DMF (81%, entry 12). Among these solvents, DMSO was identified as the most efficient medium for the formation of 2a. With these good results at hand, we then examined the effect of reaction temperature (Figure1) and catalyst amount (Table 1) on the reaction. It was shown that the best results were obtained by using 10 mol% of catalyst at 50 °C. A higher amount of the catalyst (say 20 mol%) did not improve the yield further (entry 13), but using lower amount of catalyst (say 5 mol%) could decrease the yield from 97% to 63% (entry 14).

Table 1. Carboxylic cyclization of propargylic amine with CO2a HN

Bu

O O

Catalyst, Solvent, Base +

N Bu

CO2 0.1MPa, 50°C 2a

1a

Entry

Catalyst

Base

Solvent

Yield(%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14

----c ----c CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(10mol%) CuI(20mol%) CuI(5mol%)

----d DBU(1.0 equiv.) ----d DBU(1.0 equiv.) NaOH(1.0 equiv.) K2CO3(1.0 equiv.) NEt3(1.0 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.)

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO THF C2H5OH CH3CN DMF DMSO DMSO

0 38 46 96 57 71 84 97 58 62 67 81 97 63

11



15 16 17

CuCl(10mol%) CuBr(10mol%) CuO(10mol%)

DBU(0.2 equiv.) DBU(0.2 equiv.) DBU(0.2 equiv.)

DMSO DMSO DMSO

CuBr > CuCl (entries 8, 15-16). Additionally, Cu2O and CuCN could also promote the carboxylative cyclization to give the yields of 41% and 76%, respectively (entries 17 and 18). Thus, CuI was the best catalyst among the catalysts studied and was employed for the further investigations. Figure2 illustrates the dependence of yield on the reaction time. It can be seen that a yield of >96% was obtained for this reaction in only 4 h. Compared with the reported metal and metal-free catalysts in literature, the reaction time was much shorter with high yields (Scheme 1). As a result, the optimal reaction conditions were CuI (10 mol %) as the catalyst, DBU as the base, DMSO as the solvent, and the reaction time was 4 h.

100 90 80

Yield(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

70 60 50 40 30 20 20

30

40

50

60

70

Temperature (°C)

Figure 1. The effect of reaction temperature on the yield of product 2a. Typical reaction conditions were as follows: 1a (1.0 mmol), CuI (0.1mmol), DBU (0.2 equiv.), DMSO (2 ml), CO2 (0.1 MPa), 6 h. Yields were determined by 1H NMR spectroscopy using anisole as an internal standard. 12


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100

80

Yield(%)

Page 13 of 25

60

40

20

0 0

2

4

6

8

Time (h)

Figure 2. The reaction time dependence of the yield of 2a. Reaction conditions were as follows: 1a (1.0 mmol), CuI (0.1mmol), DBU (0.2 Equiv.), DMSO (2 ml), CO2 (0.1 MPa), 50 °C and different reaction times. Yields were determined by 1H NMR spectroscopy using anisole as an internal standard.

The reaction of CO2 with a range of different substituted propargylic amines was performed under the optimized reaction conditions by using CuI/DBU catalyst system, and the yields of the target products were summarized in Table 2. It was found that propargyl amines with various substituents were effective substrates to give the corresponding products in good to excellent yields. The catalyst worked well when R2 was hydrogen and R3 was an alkyl or aryl group (entries 2-6, 11-14). Even the R2 was an alkyl group, the reaction also went to completion within only 4 hours (entries 1, 7-10). This suggests that by using CuI/DBU catalyst system, the reactivity of the propargylic amines did not depend on the nature of the R2 and R3 substituents, and the substrate scope is much broader than that reported in literature37. Additionally, propargylic amines with different R1 and R4 groups were also studied, and excellent yields were obtained for the products (entries 9-16). These results confirm the versatility of our new catalytic strategy for the synthesis of 2-oxazolidinones.

Table 2. Scope of the propargylic amine substrates for the reaction under optimized conditionsa 13



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Page 14 of 25

O HN R4 R2 R3

R1

CuI, DBU, DMSO +

CO2

O

0.1MPa, 50°C, 4h

R1

1

Entry

Substrate

Yield(%)b

Product HN

Bu

O N Bu

O

1

92 2a

1a HN

Bu

O N Bu

O

2

Bu

O N Bu

O

3

98 2c

1c HN

99 2b

1b HN

2

N R4 R2 R3

O

Bu

N Bu

O

4

96 1d HN

2d O

Bu

N Bu

O

5

1e HN

90 2e

O

Bu O

N Bu

6

93 1f

2f

14


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HN

O

Bu

N Bu

O

7 1g HN

2g O

Bu

N Bu

O

8 1h HN

87

91 2h

O

Bu

N Bu

O

9

89 1i HN

2i

Bu

O N Bu

O

10

2j

1j HN

76

O

Bu O

N Bu

11

97 F

F

1k

2k

HN

O

12

93

N

O

1l

2l O

HN O

13

N

92 2m

1m

15



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Page 16 of 25

O

HN

O

14

N

94 2n

1n O

15

O

H N

N

98

1o

2o O

16

O

H N

97

N

1p

2p

a

Reaction condition: propargylic amine (1mmol), CuI (10 mol% based on 1), DBU (0.2 Equiv.), DMSO (2 ml), CO2 (99.999%, 0.1MPa), 4 h, 50°C. bIsolated yield.

Reaction Mechanism To gain insight into the reaction mechanism for the formation of 2-oxazolidinones, a kinetic isotope effect (KIE) study was performed (Scheme 2). It was shown that the value of kH/kD for the catalyzed reaction of CO2 with 1 equiv of 1p and 1equiv of 1p-d was 1.2 by using the product 2p and 2p-d. This suggests that the cleavage of N−H bond of propargylic amine substrates was not involved in the rate-determining step.

D HN

+

1p

CuI, DBU, DMSO N

+

1p-d

0.1 MPa, 50oC,4h KH/KD=1.2

O

O O

CO2

N

2p

O

D

N

2p-d

Scheme 2. Kinetic isotope effect study for the formation of 2-oxazolidinone 2p.

16


Page 17 of 25

To further investigate the reaction mechanism catalyzed by CuI/DBU in detail, we performed DFT calculations where solvation in DMSO was taken into account by using the SMD salvation model. In the DFT calculations, simplified propargylamine N-methyl-3-phenylprop-2-yn-1-amine

and

base

1,4,5,6

-

tetrahydro

-1-methylpyrimidine were, respectively, used as the representative substrate molecule and DBU in order to reduce the computing complexity. From the reaction shown in Table 1, the amine proton of a given propargylamine substrate migrates to one of the two alkyne carbons after the reaction. Thus deprotonation and protonation are expected to occur in the reaction process.

O N R4 R1 R2

O R3

R3 N

6 O O

N R3

R4 N R1 R2

HN R4 R1 R2 1 R4 N H R1 R2

N

CO2

HN + N CuI

R3

N +

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O TS5-6

O

R4 N H R1 R2

R3 + N H

N

CuI

O O R3

N +

TS2-3

N R4 R1 R2

O

CuI

O

5

R3 CuI

O O C N R4 R1 R2

R3

N

O O C N R4 R1 R2 CuI

TS4-5

R3

N R4 R1 R2

3

4

Scheme 3. Detailed mechanism for the carboxylative cyclization reactions of propargylamines with CO2 catalyzed by CuI/DBU catalyst system.

17


+ N H

N


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Free energy profiles of the pathway for the carboxylative cyclization of propargyl amines with CO2 catalyzed by CuI/DBU catalyst system.

Through our calculations, it was found that there are three steps in the reaction mechanism: the deprotonation of propargylamine by DBU base, the intramolecular cyclization catalyzed by CuI, and the proto-demetallation. The detailed mechanism and free energy profiles were illustrated in Scheme 3 and Figure 3 respectively. Firstly, the hydrogen atom of amino group in propargylic amine (1) can be captured by DBU which makes the CO2 electrophilic attack on the amino group more favorable. This step via transition state TS2–3 has a free energy barrier of 81.1 kJ/mol (Figure 3). After this step, a [DBUH]+ cation and carbamate anion (3) are formed (Scheme 3). In the subsequent attack of CuI on the C≡C bond of propargylic amine, the species 4 with lower energy is formed. The next step is ring formation from the carbamate ion species 4. A five-membered ring (5) via TS4‑5 is formed with the help of CuI catalyst. The free energy barrier of this step is 43.2 kJ/mol (Figure 3). In the last proto-demetallation step via transition state TS5–6, the target product is formed by robbing the hydrogen proton of [DBUH]+ cation, and DBU and CuI are regenerated. Figure 3 suggests that the proto-demetallation step with a free energy barrier of 103.8 18


Page 18 of 25

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kJ/mol is the rate-determining step. The above deuterium experiment indicates that the deprotonation of amino group in propargylic amine was not involved in the rate-determining step, which is consistent with the calculation results (Figure 3). Based on these results, it is reasonable to conclude that deprotonation of propargylic amine by DBU base can assist the formation of carbamate, while the copper (I) is able to activate the C≡C triple bond of propargylic amine in the reaction. Such roles of DBU and copper (I) played in this reaction is supported by the results reported in literatures42-44. It is known from the experimental results that the reactions of CO2 with propargylic amines to form 2-oxazolidinones could also proceed with a lower yield in the absence of DBU base or CuI catalyst (Table 1), but the reaction could not occur without DBU and CuI. This suggests that both DBU base and CuI catalyst can catalyze this reaction to a different extent, respectively. To explore the nature of this phenomenon, the mechanism for the cycloaddition of propargylic amine with CO2 in the presence of DBU base or CuI catalyst was studied by using DFT method. The detailed mechanism and free energy profiles for the reaction in the presence of DBU were illustrated in Scheme S1 and Figure S1. It is clearly showed that H atom of the cation [DBUH]+ attacks the C ≡ C bond synchronously when the amino group undergoes electrophilic attack by CO2 molecule with the help of DBU (via TS2‑3). The target product is formed via transition state TS3–6 with a free energy barrier of 150.4 kJ/mol. The overall free energy barrier for the cycloaddition is 226.8 kJ/mol in the presence of DBU base, which is higher than that for the reaction catalyzed by CuI/DBU catalyst system. This suggests that the intramolecular cyclization is more favorable in the presence of CuI catalyst. Therefore, the reaction rate was reduced in the absence of CuI catalyst, and a lower yield was obtained in the experiments. For the reactions between CO2 and propargylic amines in the presence of only CuI catalyst, the mechanism and free energy profiles were shown in Scheme S2 and Figure S2, respectively. It can be seen that propargylic amine itself could also work as a base to assist with the formation of carbamate (via TS2‑3’). The free energy barrier of this step is 94.7 kJ/mol, which is higher than that of the reaction with DBU as a 19



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base (81.1 kJ/mol). This demonstrates that the deprotonation ability of DBU is stronger than that of propargylic amine. The subsequent intramolecular cyclization and proto-demetallation steps are similar with that of the reaction with CuI/DBU catalyst system, and the free energy barriers are 43.2 (TS4-5) and 65.7 kJ/mol (TS5-6’), respectively. In order to compare the free energy barriers more conveniently, the free energy profiles for the above three pathways of CO2 reaction were shown in Figure 4. It is obvious that the reaction with CuI/DBU catalyst system has a lowest overall free energy barrier among the three pathways, resulting in the highest reaction rate. By comparing the other two pathways, we find that the free energy barrier for the reaction with DBU is higher than that for the reaction with CuI, but the experimental product yields are not significantly different (38% versus 46%). This indicates that other factors such as dynamic influence may also play important roles in this practical reaction process. Therefore, the theoretical calculation results are only consistent with the experimental ones qualitatively. In addition, these results confirm that DBU could efficiently assist with the carbamate formation and the C≡C triple bond of propargylic amine could be activated by CuI catalyst.

Figure 4. Free energy profiles for the pathways of the carboxylative cyclization of propargyl amines with CO2 catalyzed by different catalyst systems.

20


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CONCLUSION In conclusion, CuI catalyst and DBU base have excellent synergistic effect in promoting

the

reactions

between CO2 and

propargylic

amines to form

2-oxazolidinones. The reactions can proceed at atmospheric pressure with excellent yields. A wide range of substrates have been extended which confirms the versatility of

the

new

CuI/DBU

catalyst

system

for

the

production

of

2-oxazolidinones.Theoretical studies reveal that both the CuI catalyst and DBU base are crucial in catalyzing the reaction. The DBU promotes the deprotonation and proto-demetallation steps by capturing and providing proton, while CuI catalyst activates the C≡C triple bond of propargylic amine in the reaction. The novel CuI/DBU catalyst system has some obvious advantages such as simple, cheap, wide substrate range, short reaction time and high yields, which makes it possible to have potential in applications. Further studies regarding the recycling of catalyst system for this type of reaction are now in progress.

ASSOCIATED CONTENT Supporting Information Further details including the optimized geometries of the transition states and characterization of products are available. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21403060 and 21133009) and Program for Innovative Research Team in Science and Technology in University of Henan Province (16IRTSTHN002).

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For Table of Contents Use Only New Copper(I)/DBU Catalyst System for the Carboxylative Cyclization of Propargylic amines with Atmospheric CO2: An Experimental and Theoretical Study

Yuling Zhao,† JikuanQiu, † Li Tian,† Zhiyong Li,† Maohong Fan‡ and Jianji Wang*,†

HN R4 R1 R2

R3

CuI/DBU, DMSO

O R3

+

O C

N R4 R1 R2

50°C, 4h

0.1MPa Atmospheric pressure High efficiency and fast transformation Cost-competitive and versatility catalytic system

CuI catalyst and DBU base have excellent synergistic effect in promoting the sustainable reaction between CO2 and propargylic amine.

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DBU Catalyst System for the Carboxylative Cyclization

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