Cobalt-based catalysis for carboxylative cyclization of propargylic amines with CO2 at atmospheric pressure

Journal of CO₂ Utilization 34 (2019) 404–410

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Cobalt-based catalysis for carboxylative cyclization of propargylic amines with CO2 at atmospheric pressure

T

⁎⁎

Zhi-Hua Zhoua, Shu-Mei Xiaa, Si-Yuan Huanga, Yu-Zhong Huanga, Kai-Hong Chena, , ⁎ Liang-Nian Hea,b, a b

State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 chemistry Cobalt catalysis Ligand effect Organobase 2-Oxazolinones

A cobalt-bicyclic guanidine catalytic system consisting of CoBr2 and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) for the carboxylative cyclization of terminal propargylic amines with CO2 was firstly developed in this work to produce 2-oxazolinones efficiently. The existence of induction period urged us to understand the reaction mechanism of cobalt catalysis. Investigation on the roles of CoBr2 and TBD was conducted using control experiments and density functional theory (DFT) calculation. TBD presumably acts as a base to activate propargylic amine for favorable CO2 capture, and a ligand to coordinate with CoBr2 via forming bulkier CoBr2(TBD) in the same time. This bulkier complex can enhance the O-nucleophility of the in situ formed carbamate intermediate and then promote subsequent intramolecular cyclization to generate 2-oxazolinone, accounting for the high activity of cobalt catalysis. This protocol enables the synthesis of various 2-oxazolinones from propargylic amines and CO2 under atmospheric pressure in good to excellent yields, representing a simple, cost-effective and practical route for CO2 fixation to 2-oxazolinones under mild conditions.

1. Introduction Carbon dioxide, one of main greenhouse gases, has the potential to become an ideal carbon source to prepare value-added chemicals because of its advantages of low price, abundance, non-toxicity and renewability [1–10]. In fact, recycling and utilization of CO2 is of great significance in the view of environmental and economic benefits. To date, multifarious methodologies with CO2 as C1 building block in organic synthesis have been discovered, affording carboxylic acids, carbonate, formamides, and other value-added products [11–18]. However, CO2 is in the highest oxidation state of carbon, making it be of thermodynamic stability and kinetic inertia. Hence, harsh reaction conditions such as high temperature and high CO2 pressure are often needed in most conversions involving CO2. Therefore, developing efficient processes for CO2 conversion especially under mild conditions is a meaningful but challenging task. 2-Oxazolinones [19] are important structure motifs in organic chemistry in view of their wide application as chiral auxiliary [20,21] and synthetic intermediates [22,23]. Among a large number of wellestablished methods for preparing 2-oxazolinones, carboxylative

cyclization of easily available propargylic amines with renewable CO2 is especially attractive because of high atom economy [24]. Many methodologies involving transition metal catalysis [25–37] and organocatalysis [38–46] have been developed to promote this reaction. Typically, dual catalytic systems combining transition metals such as silver [28,32], copper [34] and zinc [36] with organobases are capable of realizing this cyclization with high efficiency under mild conditions. Transition metals are believed to have the ability to coordinate and then activate C^C triple bond so as to lower down the activation energy in this reaction. Cobalt with unfilled 3d orbit owns the ability to coordinate with C^C triple bond, and the application of cobalt catalysis including π components in various reactions such as CeH functionalization [47,48], enyne reductive coupling [49,50] and cycloaddition of alkynes with enones [51] have been significantly developed in last decades. As one of cheap and abundant metals, cobalt complexes have been employed as cost-effective catalysts in a series of reactions including CO2 [52–55]. However, there is no report, to the best of our knowledge, about carboxylative cyclization of propargylic amines with CO2 involving the cobalt catalyst. In this context, developing efficient cobalt

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Corresponding author at: State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (K.-H. Chen), [email protected] (L.-N. He). https://doi.org/10.1016/j.jcou.2019.07.027 Received 22 February 2019; Received in revised form 21 June 2019; Accepted 22 July 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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(δH = 7.26 ppm, δC = 77.0 ppm) on Bruker Advance 400 MHz spectrometer. GC–MS data were obtained by using a Shimadzu GCMSQP2010. High resolution mass spectrometry was conducted using a Varian 7.0 T FTICR-MS by ESI technique.

catalysis for the reaction of propargylic amine with CO2 will undoubtedly broaden the application of cobalt and increase the possibility of CO2 utilization. On the other hand, organobases have been found to be important organocatalysts [56–62] or co-catalysts combining with transition metal catalysis [63–67] due to their excellent performance in organic reactions. The basicity and nucleophilicity are two of important criterions for organobase involved reactions [68,69]. In CO2 conversion, specifically, organobase is widely accepted to capture CO2 [28,60] and/ or activate substrates [34,59,65]. Structurally, organobases with electron-rich nitrogen atoms are capable of coordinating with transition metals [70–72]. However, the coordination of organobases as neutral and N-based donor ligands with transition metals is rarely considered [73–75] in most transition metal/organobase dual catalyst systems. Therefore, it is a significant and useful guidance for optimizing more efficacious catalysts to understand the exact role of the organobase in such catalyst systems. In view of the activation of C^C triple bond via cobalt salts and promotion of organobase in CO2 conversion, we envisaged the organobase assistance could further enhance the cobalt catalytic activity, thus allowing the fixation of CO2 into 2-oxazolinone to proceed smoothly. In fact, we found cobalt(II) compounds showed excellent performance for the cyclization of propargylic amines with CO2, and various 2-oxazolinones could be afforded with yields up to 99% in the presence of TBD as the cooperative catalyst. Investigating the role of organobase in this system showed TBD acted not only as a base to activate NeH bond of propargylic amine in favor of CO2 fixation, but also as a ligand to coordinate with metal center [76] (Scheme 1). Control experiments and density functional theory (DFT) calculations demonstrated that the coordinated cobalt by organobase showed higher activity than CoBr2 because of a more favorable intramolecular cyclization of carbamate intermediate.

2.2. General procedure for preparation of 2-oxazolinone To a 10 mL Schlenk tube with a stirring bar, CoBr2 (0.05 mmol, 10.9 mg), TBD (0.1 mmol, 13.9 mg), propargylic amine (0.5 mmol), THF (0.5 mL) was added successively. Then, the Schlenk tube was sealed and connected to a CO2 balloon (about 1 L). The reaction was carried out at 80 °C for 9 h. After finished, 1,3,5-trimethoxybenzene (20 mg) as internal standard was added into the mixture to determine the yield of 2-oxazolinone by 1H NMR analysis. The crude residue was further purified by chromatography on silica gel (with petroleum ether and ethyl acetate as eluents) to afford the desired 2-oxazolinone after the solvent was removed under vacuum. 3. Results and discussion 3.1. Optimization conditions To evaluate the cobalt catalysis for chemical fixation of CO2 with propargylic amines, the benchmark reaction of N-benzylprop-2-yn-1amine (1a) with CO2 was initially examined to afford 3-benzyl-5-methyleneoxazolidin-2-one (2a). As described in Table 1, 1a was completely recovered without any catalyst (entry 1). With CoBr2 as the catalyst, a 33% yield of 2a was afforded in THF at 60 °C for 21 h (entry 2) [77], indicating the likelihood of cobalt catalysis for this cyclization. Though organobases have been reported to show catalytic activity for this reaction especially under compressed CO2 conditions [38–40], TBD alone showed low efficiency in the given conditions (entry 3). Combining cobalt(II) salts such as CoCl2, CoBr2, CoI2, Co(OAc)2 with TBD as cooperative catalytic system was then investigated in view of the commonly exhibited high activity of metal-organic cooperative catalysis [78]. Gratifyingly, the obviously increased yields of the target product 2a were observed by employing cobalt(II) salts and TBD as the catalytic system (entries 4–7). Particularly, a 96% yield of 2a was afforded in the presence of CoBr2/TBD (entry 5). Generally, basicity [34,59,65] and/or nucleophilicity [28,60] of organobases have remarkable influence on the reactions involving CO2. Thus, several sterically hindered organobases [79] with different basicity (pKa value [80,81] in CH3CN ranges from 18.3 to 26.0) and nucleophilicity (nucleophilicity data [82–85] in CH3CN ranges from 15.3 to 18.8) were examined as co-catalyst in combination with CoBr2 (entries 5 and 8–10). Resultantly, the reaction efficiency increases in the

2. Experimental 2.1. Materials and methods CO2 (99.99% purity) was purchased from Liquefied Air (Tianjin) Co., Ltd. CoBr2 anhydrous (98% purity), TBD (98% purity) and anhydrous solvents (99.8% purity) were obtained from J&K Scientific Ltd. and used as received. Other commercial available reagents were used directly without further purification. Prepared substrates and products were isolated by chromatography performed on silica gel (200–300 mesh) with petroleum ether (60–90 °C) and ethyl acetate as solvents. FT-IR was recorded on a Bruker Tensor 27 FT-IR spectrophotometer with KBr pellets. NMR spectra were conducted in CDCl3

Scheme 1. Cobalt(II) catalysis for reaction of terminal propargylic amines with CO2. 405

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Table 1 Evaluation of cobalt catalysis.a

Table 2 Substrate scope of propargylic amines.a

Entry 1

Entry

Cobalt source

Organobase

Conversion of 1a (%)b

Yield of 2a (%)c

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

– CoBr2 – CoCl2 CoBr2 CoI2 Co(OAc)2 CoBr2 CoBr2 CoBr2 CoBr2 AgOAc CuI ZnCl2

– – TBD TBD TBD TBD TBD DABCO NEt3 DBU TBD TBD TBD TBD

0 > 99 22 > 99 > 99 > 99 > 99 43 86 > 99 > 99 91 > 99 > 99

0 33 6 59 96 93 78 22 60 76 > 99 58 80 86

Note: DABCO = 1,4-Diazabicyclo[2.2.2]octane, DBU = 1,8-Diazabicyclo[5.4.0] undec-7-ene, TBD = 1,5,7-Triazabicyclo[4.4.0]dec-5-ene. a Conditions: 1a (0.5 mmol, 72.6 mg), cobalt source (0.05 mmol), organobase (0.1 mmol), THF (0.5 mL), 60 °C, 21 h, CO2 balloon. b Determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard. c 80 °C, 9 h.

order of DABCO < NEt3 < DBU < TBD with CoBr2 as the cobalt source, being consistent with the enhancement of basicity. In other words, the organobase basicity has a more significant effect than their nucleophilicity on the cobalt-catalyzed cyclization, and TBD with strong basicity can facilitate this reaction. In addition, non-bulky amines such as piperidine and pyridine as organobases in combination with CoBr2 for the reaction of propargylic amine 1a and CO2 was examined, affording low yields of 2a (Table S2, see Supporting information), which may be because the weak basicity of piperidine and pyridine and their strong coordination with CoBr2 is not favorable to the activation of 1a, thus resulting in low efficiency. Solvent effect on the reaction was then examined (Table S1, see Supporting information). Solvents with lower polarity such as PhCH3 and THF (Table S1, entries 1 and 2) are more beneficial to the reaction than that with higher polarity such as CH3CN, CH3OH and DMSO (Table S1, entries 3–5) and cobalt catalysis performs best in the presence of THF as the solvent (Table S1, entry 2). Optimization of reaction conditions revealed that lowering temperature diminished the yield of 2a and equivalent conversion of 1a was achieved at 80 °C (Table S3, see Supporting information). The amount of CoBr2 could not be further reduced and the best ratio of CoBr2 to TBD is 1:2 (Table S3, see Supporting information). In addition, the reaction time can be shortened to 9 h at 80 °C (Table 1, entry 11). Performing the reaction of 1a with CO2 in air, only a 4% yield of 2a was obtained because of the low concentration of CO2 in air (Scheme S1, see Supporting information). After that, the activity of different metals was also evaluated under the optimized conditions. Although AgOAc could give a 91% of conversion for 1a, only moderate yield of 2a was obtained (entry 12). Compared with AgOAc, CuI and ZnCl2 exhibited higher activities but the yield of 2a was only 80% and 86%, respectively (entries 13 and 14). Clearly, CoBr2 is a better catalyst in this reaction.

Substrate

Product

Yield (%)b 99(88c)

2

99

3

98(88c)

4

96(84c)

5

90

6

74(70c)

7

91

8

37

9

trace(27d)

10

55

11

38(67e)

12

15(50d)

a Reactions were conducted with 1 (0.5 mmol), CoBr2 (0.05 mmol, 10.9 mg), TBD (0.1 mmol, 13.9 mg), THF (0.5 mL), CO2 balloon, 80 °C, 9 h. b Determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard. c Isolated yield. d 2 MPa CO2, 48 h. e 48 h.

3.2. Substrate scope After obtaining the optimized conditions, the scope and limitations of this approach were tested with various propargylic amines, and the results are summarized in Table 2. Obviously, most of aryl and alkyl substituted propargylic amines worked well when reacting with CO2 under the cobalt-catalyzed conditions. Halogens such as Cl and Br are compatible, and the corresponding 2-oxazolinones 2b and 2c were afforded in 99% and 98% yields, respectively (entries 2 and 3). The electronic effect of substituents on aryl rings was found to have an effect on the reaction outcome. Comparatively speaking, proaprgylic amines with electron-withdrawing substituents like halogen groups on the aryl rings (entries 2 and 3) exhibited higher reactivity than those with electron-donating substituents such as methyl, methoxyl and tbutyl groups (entries 4–6), which should be ascribed to an easier deprotonation of these substrates. When alkyl substituted propargylic amine, for example, N-(prop-2-yn-1-yl)cyclohexanamine (1g) was employed, the corresponding 2-oxazolinone 2g can be afforded in 91% yield (entry 7). However, the reaction of propargylic amine 1h bearing 406

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and yield of 2a increased rapidly within 6 h and almost full conversion of 1a with 94% yield of 2a was observed after 6 h. After that, the yield of 2a increased continually, and a complete conversion of 1a to 2a was achieved at 9 h. What should be mentioned is that no 2a and other byproducts were detected within 1 h when 14% of 1a was consumed, suggesting there should be an induction period at the initiate stage. This induction period in this reaction, which has not been reported in previously, spurs us to further investigate the role of CoBr2 and TBD in cobalt catalysis. Therefore, several one-pot two-step experiments were conducted to get deep insight into this reaction, as illustrated in eqs. 1–4. Firstly, propargylic amine 1a with CoBr2 was mixed at 80 °C for 1 h (eq. 1). As a result, a 51% conversion of 1a was observed (Fig. 2a, eq. 1). In this case, no by-product except 1a was detected via GCeMS (Fig. S4, see Supporting information). Further FT-IR and 1H NMR analysis indicated the coordination between CoBr2 and 1a may exist in this system (Fig. S5, S6, see Supporting information). Running the reaction for another 8 h, 2a was afforded only in 25% yield (Fig. 2b, eq. 1). Therefore, the catalytic activity will decrease if the cobalt compound coordinates with propargylic amine at first, probably due to the coordination between cobalt and nitrogen weakens the N-nucleophilicity of propargylic amine. Subsequently, TBD was stirred with 1a at first, followed by the addition of CoBr2 and CO2 as shown in eq. 2. Resultantly, a 96% yield of 2a was afforded in this case (Fig. 2b, eq. 2). Thus, the deprotonation of 1a by TBD may occur at the initial stage, subsequently cobalt activates the C^C triple bond of 1a. Another thing that attracted our interesting was that only 12% of propargylic amine 1a was consumed after stirring 1a with 20 mol% TBD for 1 h (Fig. 2a, eq. 2), being consistent with the result obtained in the induction period. For this result, the specific reason is unclear, but a reversible process may be reasonable. In addition, the similar result could also be obtained by using half amount of TBD (Fig. 2a, eq. 3). In view that bicyclic guanidine such as TBD can be used as ligands for metal compounds [70–72], therefore, a hypothesis was proposed that TBD may act as the base and ligand at the same time [86]. In this aspect, the ligand role was investigated. Thus, the prepared complex CoBr2(TBD) [87] was added into the resultant mixture after agitating 1a with 10 mol% TBD for 1 h (eq. 3). To our surprise, excellent yield of 2a was acquired (Fig. 2b, eq. 3). By contrast, only 50% of 2a was observed by adding CoBr2 instead of CoBr2(TBD) after the agitation of 1a with 10 mol% TBD for 1 h (Fig. 2b, eq. 4). Clearly, the interaction between Co and TBD can facilitate this reaction. Since TBD can coordinate with cobalt, the effect of organobases as ligands needs to be disclosed. Thus, CoBr2(TBD) was used as the catalyst instead of CoBr2 in the presence of 10 mol% TBD, and a comparable yield of 2a under standard conditions was obtained (Table 3, entry 1 vs. 2). These results further confirmed that CoBr2(TBD) has similar activity as standard conditions. However, when DABCO coordinated cobalt complex i.e. CoBr2(DABCO) [87] was used as catalyst, only about half of 1a was converted and the yield of 2a was 39% even in the presence of TBD (entry 3). Clearly, ligands can influence the activity of cobalt significantly, and TBD is a better ligand than DABCO. Therefore, the low efficiency of CoBr2/DABCO system (Table 1, entry 8) should be ascribed to the weak basicity of DABCO and the poor activity of CoBr2(DABCO). How does the coordination of TBD with CoBr2 affect this reaction? We thus tried to understand this question from the perspective of reaction mechanism. Firstly, based on previous reports [24–45] and our results, a possible catalytic cycle of cobalt catalysis was speculated as depicted in Scheme 2. At the first in induction period, propargylic amine is deprotonated, leading to the formation of intermediate I with the aid of TBD. After CO2 insertion, the key intermediate II with the activated C^C triple bond via CoBr2(TBD) is generated. Subsequent nucleophilic cyclization of intermediate II affords the alkenyl cobalt intermediate III. Finally, 2-oxazolinone is produced after a protodemetalation step. Since the ring-closing process of the carbamate

Scheme 2. Proposed catalytic cycle.

a flexible n-butyl group with CO2 gave a low yield of 3-butyl-5-methyleneoxazolidin-2-one (2h) (entry 8). Thus, substrate with flexible group on the nitrogen atom will hinder this process. Phenyl substituted propargylic amine 1i was then employed as the substrate, which exhibited low reactivity even with increased CO2 pressure and prolonged reaction time (entry 9), presumably being ascribed to its poor N-nucleophilicity for inferior fixation of CO2. Comparatively, propargylic amine 1j with a phenethyl substituent gave a moderate yield of 2j (entry 10). Besides, internal propargylic amines such as N-benzyl-3phenylprop-2-yn-1-amine (1k) and N-benzylbut-2-yn-1-amine (1l) also participated in this transformation, giving the corresponding 2k and 2l in moderate to good yields in the presence of longer reaction time and/ or enhanced CO2 pressure (entries 11 and 12). Harsh reaction conditions are needed because the increased steric hindrance at the terminal alkyne position may hinder the formation of intermediate II (Scheme 2), resulting in the inferior reactivity of internal propargylic amines. In a word, cobalt catalysis exhibits excellent performance for most of benzyl and alkyl substituted terminal propargylic amines, but low efficiency for phenyl substituted propargylic amine with weak N-nucleophility and sterically hindered internal propargylic amines. 3.3. Mechanism study To further understand this reaction, the time-course conversion of 1a into 2a was investigated. As shown in Fig. 1, the conversion of 1a

Fig. 1. Time dependence of conversion and yield. Reaction conditions: 1a (0.5 mmol, 72.6 mg), CoBr2 (0.05 mmol, 10.9 mg), TBD (0.1 mmol, 13.9 mg), THF (0.5 mL), 80 °C, CO2 balloon. 407

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Fig. 2. Results of control experiments (eqs. 1–4). Taking eq. 1 as an example to illustrate the reaction conditions. Step I: 1a (0.5 mmol, 72.6 mg), CoBr2 (0.05 mmol, 10.9 mg), THF (0.5 mL), 80 °C, 1 h; Step II: TBD (0.1 mmol, 13.9 mg), CO2 balloon, 80 °C, 8 h. Conversion and yield are determined by GC analysis with 1,3,5trimethoxybenzene as internal standard.

cyclization of intermediate II would be easier for CoBr2(TBD) as the possible catalytic species. In addition, the formation of hydrogen bonding between TBD and oxygen of carbamate intermediate can stabilize this intermediate, leading to a more negative energy of intermediate II. This can be understandable that the coordinated cobalt complex with increased steric hindrance and reduced electrophilicity may weaken the interaction between cobalt and carbamate intermediate, which thus renders the ring-closing step easier. Therefore, the efficiency of cobalt catalysis can be deemed that TBD, as a Brønsted base, can activate propargylic amine through deprotonation to facilitate CO2 fixation (Scheme 3a). Meanwhile, it can enhance the cobalt catalytic activity by working as a ligand to coordinate with CoBr2 with the formation of bulkier CoBr2(TBD), leading to activation of C^C triple bond, stabilization of carbamate intermediate and simultaneously improvement of the O-nucleophility of the carbamate intermediate (Scheme 3b).

Table 3 Investigation on catalytic activities of CoBr2(base).a

Entry

Cobalt source

TBD (mol%)

Conversion of 1a (%)b

Yield of 2a (%)b

1 2 3

CoBr2 CoBr2(TBD) CoBr2(DABCO)

20 10 10

> 99 > 99 58

99 93 39

a Reaction conditions: 1a (0.5 mmol, 72.6 mg), cobalt salt (10 mol%), TBD, CO2 balloon, THF (0.5 mL), 80 °C, 9 h. b Determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard.

intermediate is believed as the rate-determining step of this reaction [35,41,42,88], the effect of coordination between Co and TBD was further investigated by DFT calculations using intermediate II as the standard. As shown in Fig. 3, more negative charge of oxygen, when cobalt coordinates with TBD, which means that the intramolecular

4. Conclusions In summary, we firstly developed cobalt-based catalyst to promote the carboxylative cyclization of terminal propargylic amines with CO2 408

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Fig. 3. Optimized structures of intermediate II when Co coordinates with two bromide ions (a) and with one bromide ion and TBD (b) at the SMD-B3LYP/def2TZVP//B3LYP/6–31 G(d) + LANL2DZ(Co, Br) level. (H: light gray, C: dark gray, N: blue, O: red, Co: purple). [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

Scheme 3. Proposed activation modes involving a dual role of TBD as base (a) and ligand (b) in this work.

[16] [17]

under atmospheric pressure to prepare 2-oxazolinones. With the combination of CoBr2 and TBD, various 2-oxazolinones were obtained with yields up to 99%. The discovery of an induction period indicates the uniqueness of this cobalt catalysis. Through a combination of control experiments and DFT calculations, dual roles of TBD are proved in this reaction, which acts as a base to activate propargylic amine to facilitate the fixation of CO2 and as a ligand for CoBr2 in favor of the key intramolecular cyclization, thus accounts for the high efficiency. The method developed in this work broadens the application of cheap transition metal catalysis in CO2 conversion under mild conditions and simultaneously highlights the role of organobase as ligand in transition metal/organobase dual catalyst systems.

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Declaration of Competing Interest

[30] [31] [32] [33]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[34]

Acknowledgments

[35]

This work was financially supported by National Natural Science Foundation of China (21672119), and the China Postdoctoral Science Foundation (2018M641624).

[36] [37] [38]

Appendix A. Supplementary data

[39] [40] [41]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.07.027.

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