Renewable and Biocompatible Lecithin as an Efficient Organocatalyst

Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Renewable and Biocompatible Lecithin as an Efficient Organocatalyst for Reductive Conversion of CO2 with Amines to Formamides and Methylamines Yue Hu,†,‡ Jinliang Song,*,† Chao Xie,†,‡ Haoran Wu,†,‡ Zhenpeng Wang,† Tao Jiang,*,† Lei Wu,§ Yu Wang,§ and Buxing Han*,†,‡

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/16/18. For personal use only.

†

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Zhejiang NHU Company Co., LTD, Xinchang 312500, People’s Republic of China S Supporting Information *

ABSTRACT: Transformation of carbon dioxide (CO2) into valuable chemicals is of great importance, and development of novel and efficient catalysts is crucial. Herein, we found that the renewable lecithin could be used as an efficient organocatalyst for the formylation and methylation of various amines with CO2 to corresponding formamides and methylamines via the construction of new C−N bonds using PhSiH3 as the hydrogen source, and satisfactory yields could be obtained. More importantly, the selectivity of the products could be easily controlled by the molar ratio of reactants (i.e., CO2, amines, and PhSiH3) and reaction temperature. In the catalytic cycle, formamides and methylamines were generated by converting the carbon (+4) in CO2 into +2 and −2 via 2-electron and 6electron reduction pathway, respectively. KEYWORDS: Transformation of carbon dioxide, Lecithin, C−N bond construction, Formylation and methylation, Renewable organocatalyst

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INTRODUCTION

growing research interest has been devoted to the application of hydrosilanes in formylation and methylation of amines with CO2, and various catalysts have been developed. Except for metal-based catalysts,22−27 organocatalysts are an important group of efficient catalysts when using hydrosilanes as the reductants, including 1,5,7-triazabicyclo[4.4.0]dec-5-ene,28 ionic liquids,29 N-heterocyclic carbenes,20 B(C6F5)3,30 and tetrabutylammonium fluoride,31 etc. Although these organocatalysts showed good performance for the formylation and/or methylation of amines with CO2 in the presence of hydrosilanes, their complex preparation processes and high toxicity limited their applications to some extent. Therefore, it is still highly desired to develop nontoxic and renewable organocatalysts for the reaction of amines and CO2 with hydrosilanes as the reductants. Nowadays, using naturally occurring compounds as renewable catalysts have achieved more and more attention. As a pioneer, glycine betaine has been successfully applied as a

Carbon dioxide (CO2) has been recognized as an abundant, low-cost and renewable C1 building block for the synthesis of diverse valuable chemicals.1−8 In this context, the combination of CO2 reduction and the construction of new C−N bonds with amines is one of the most promising strategies for CO2 transformation, and has gained significant attention because the products (formamides and methylamines) are important and versatile intermediates in organic synthesis.9−12 To get the formamides and methylamines from CO2 via formylation and methylation, CO2 (C4+) should be reduced with a reductant through a two-electron or six-electron reduction to form the corresponding C2+ (formamides) and C2− (methylamines) products, respectively.13−16 Generally, H2 is the more desired reductant. However, harsh reaction conditions must be employed in H2-based systems owing to the high bond dissociation energy of H2 and the thermodynamic stability of CO2.17−19 In comparison with H2, hydrosilanes, byproducts of the silicone industry,20 are mild, nontoxic, easy-to-handle, and stable to air and moisture,21 and have strong reduction ability under milder conditions due to the weaker and more polar H−Si bonds.13 In recent years, © XXXX American Chemical Society

Received: July 9, 2018 Revised: August 7, 2018 Published: August 10, 2018 A

DOI: 10.1021/acssuschemeng.8b03245 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

the formylation products (Table S2), suggesting the priority of PhSiH3. This phenomenon was mainly resulted from the steric effect of the substituted group on the Si atom,39 especially over the bulky lecithin. Subsequently, we further determined the activity of lecithin for the formylation of various N-methylanilines with CO2. For most of the examined N-methylanilines, good to excellent yields of the corresponding N-methylformanilides were obtained (Table 1). However, the reactivity was significantly affected by the substituent groups on the benzene ring. For example, the substrates with electron-withdrawing groups (Table 1, entries 6 and 7) showed much lower reactivity than those with electron-donating groups (Table 1, entries 3− 5). Electron-donating groups could increase the electron density of N atom via the benzene ring, and thus a higher nucleophilicity was achieved for the N atom, which was beneficial to attacking the silyl active species (Scheme 2) to form the desired product. In contrast, electron-withdrawing groups played the opposite effect. Meanwhile, para-substituted N-methylanilines (Table 1, entry 4) showed a relatively higher yield than the ortho-substituted ones (Table 1, entry 5), resulting from the steric hindrance effect from the methoxy group. Except for N-methylanilines, we found that lecithin could also be suitable for the formylation of aliphatic secondary amines, including diethylamine, dipropylamine, morpholine and indoline (Table 1, entries 8−11), and excellent yields (>95%) of the corresponding formamides were successfully achieved. Owing to the excellent results for the various secondary amines, the catalytic activity of lecithin for formylation of various primary amines using CO2 as the C1 source was explored (Table 2). Lecithin still showed high activity for the formylation of most of the examined primary amines, including aromatic amines (Table 2, entries 1−3) and aliphatic amines (Table 2, entries 5 and 6). However, 2,6-diisopropylaniline (Table 2, entry 4) could not be formylated even when the reaction time was prolonged to 24 h due to the steric hindrance effect of the substituent group. Generally, a competition between mono- and bis-formylated products for primary amines was found in reported catalytic systems because of the same reactivity of both N−H bonds.40 However, no bis-formylated products were detected in our lecithin-catalytic system caused by the higher bulkiness of lecithin (Scheme 1). Therefore, lecithin was a superior organocatalyst for the selective formylation of primary amines to produce mono-formylated products. In the process for the formylation of various amines, methylation products were found to be the main byproduct. Therefore, we attempted to utilize lecithin as a catalyst for the methylation of various secondary amines using CO2 as the C1 source. Effect of various reaction parameters, including the molar ratio of amine with PhSiH3, CO2 pressure, and reaction temperature, was first studied for the methylation of Nmethylaniline (Table S3). As expected, the activity increased with the reaction temperature (Table S3, entries 1−4), and 100 °C was the suitable reaction temperature. Meanwhile, the activity changed with different PhSiH3/N-methylaniline molar ratio (i.e., 4:1, 6:1, 2:1, 3:1), and 4:1 showed the best performance (Table S3, entries 4−7). Additionally, when CO2 pressure was increased from 0.3 to 0.5 MPa (Table S3, entry 8), the selectivity of N,N-dimethylaniline decreased, while more formylated product was generated, suggesting 0.3 MPa was the suitable pressure. Under the optimal reaction

sustainable organocatalyst for reductive functionalization of CO2 with various amines and hydrosilanes.32,33 Therefore, there is great potential to utilize naturally occurring compounds for the reaction of CO2 with amines. Lecithin, which is more widespread than glycine betaine, can be easily obtained from soybeans, eggs, and rapeseed. 34 As a biocompatible, renewable and nontoxic biochemical, lecithin has been widely used as a food additive and biosurfactant as well as a catalyst.35−38 Similar with glycine betaine, lecithin possesses a zwitterionic structure (Scheme 1), and thus, it may Scheme 1. Chemical Structure of Lecithin

be a potential catalyst for the formylation and methylation of amines using CO2 as the C1 source. However, no concern was paid on the utilization of lecithin as a catalyst in reductive functionalization of amines using CO2 as the C1 source. Herein, we reported the first work on the use of lecithin as an efficient and biodegradable organocatalyst for formylation and methylation of various amines with CO2 via the construction of new C−N bonds using phenylsilane (PhSiH3) as the reductant. By tuning the reaction conditions, formamides and methylamines were formed by converting the oxidation of carbon (C4+) in CO2 into +2 and −2, respectively.

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RESULTS AND DISCUSSION In our initial experiment, the formylation of N-methylaniline with CO2 and PhSiH3 was used as a model reaction to estimate the activity of lecithin. The reaction could hardly occur without any catalyst (Table 1, entry 1). In contrast, when 5 mol % lecithin was applied as the catalyst, the reaction could proceed efficiently under the same reaction conditions (Table 1, entry 2) with complete conversion of the N-methylaniline to afford N-methylformanilide (97% yield). Lecithin with 2.5 and 1 mol % amount could still catalyze the reaction (Table S1, entries 1 and 2), and the yields of N-methylformanilide could reach 94% and 92% in 12 and 18 h, respectively. These results suggested the catalytic role of lecithin. Furthermore, the molar ratio of PhSiH3 with N-methylaniline and the CO2 pressure could affect the activity of the formylation reaction. When the PhSiH3/N-methylaniline molar ratio was decreased to 1:1, the conversion of N-methylaniline and the product yield were 75% and 74% (Table S1, entry 3), respectively, which were lower than those with a PhSiH3/N-methylaniline molar ratio of 2:1. Meanwhile, the activity decreased with the decreasing CO2 pressure from 0.5 to 0.3 and 0.1 MPa (Table S1, entries 4 and 5). These results indicated the optimal reaction conditions for the formylation were 0.5 MPa CO2 and 2:1 PhSiH3/Nmethylaniline molar ratio over 5 mol % lecithin at room temperature. Additionally, the activity of other hydrosilanes (i.e., Ph2SiH2, Ph3SiH, and poly(methylhydrosiloxane)) was also examined, but unfortunately, none of them could provide B


Letter

ACS Sustainable Chemistry & Engineering Table 1. Formylation of Various Secondary Amines over Lecithina

a Reaction conditions: substrate, 0.5 mmol; PhSiH3, 1 mmol; CO2, 0.5 MPa; room temperature; lecithin, 5 mol % based on the substrate; CH3CN, 2 mL. bC = Conversion, Y = Yield. C and Y were determined by GC using biphenyl as the internal standard.

conditions (100 °C and 0.3 MPa CO2 with 4 equiv PhSiH3), methylation of various secondary amines could be conducted via a 6-electron reduction pathway (Table 3). To confirm the carbon source of the generated methyl group, a control

experiment was carried out. No N,N-dimethylaniline was formed when the reaction was conducted without using CO2, indicating that the methyl group came from the reduction of CO2. C


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Scheme 2. Reasonable Reaction Mechanism for Formylation and Methylation of Amines Using CO2 as the C1 Source over Lecithin

Table 2. Formylation of Various Primary Amines Catalyzed by Lecithina

a

Reaction conditions: substrate, 0.5 mmol; PhSiH3, 1 mmol; CO2, 0.5 MPa; room temperature; lecithin, 5 mol % based on the substrate; CH3CN, 2 mL. bC = Conversion, Y = Yield. C and Y were determined by GC using biphenyl as the internal standard.

more PhSiH3 (1:4:6), the formed silyl formates could be further reduced into methoxysilane (Figure S5). However, amines could not react with trimethoxyphenylsilane (Figure S5) directly. Therefore, the methylation happened through the reduction of the corresponding formamide at higher reaction temperature.41,42 Through these above discussions, we could conclude that more CO2 and lower reaction temperature were beneficial for the formylation reaction, while more PhSiH3 and higher reaction temperature resulted in the methylation being the main reaction. To clarify the specific role of lecithin on the formylation and methylation reaction, NMR analysis was performed to examine the interaction of lecithin with N-methylaniline, PhSiH3, and CO2. First, by comparing the 1H NMR spectra of Nmethylaniline and the mixture of N-methylaniline and lecithin,

Based on the above results, it was obvious that the product selectivity could be tuned by the reactant ratios and reaction temperature. In our system, 0.3 and 0.5 MPa CO2 was about 3 and 5 mmol, respectively. At room temperature, the formylation was the main reaction when the molar ratio of amine, PhSiH3 and CO2 was 1:2:10 at room temperature (Table S1), while more methylation was happened when the molar ratio of the three reactants was changed into 1:2:6 at room temperature (Table S1, entry 4), indicating more CO2 was helpful for the formylation. As detected by 1H NMR (Figure S3) and 13C NMR (Figure S4), the silyl formates were formed as the active species with a reactant molar ratio of 1:2:10 at room temperature, and the formed silyl formates were attacked by amines to generate the desired formamide. In contrast, when the reaction was conducted at 100 °C with D


Letter

ACS Sustainable Chemistry & Engineering Table 3. Methylation of Various Amines Catalyzed by Lecithina

a Reaction conditions: substrate, 0.5 mmol; PhSiH3, 2 mmol; CO2, 0.3 MPa; 100 °C; lecithin, 5 mol % based on the substrate; CH3CN, 2 mL. bC = Conversion, Y = Yield. C and Y were determined by GC using biphenyl as the internal standard.

we could find that the 1H signal of N−H in N-methylaniline shifted from 4.310 to 4.331 ppm in the mixture, and the 1H signal of N−CH3 had changed from a double peak (2.737 ppm, 2.749 ppm) in N-methylaniline to a single peak (2.743 ppm) in the mixture of N-methylaniline and lecithin (Figure S1). These results indicated the strong interaction between lecithin and N-methylaniline through the generation of an intermediate (I in Scheme 2), which could activate the N−H bond, and thus was beneficial for the reaction. Second, no difference was found between the 1H NMR spectra for PhSiH3 and the mixture of lecithin and PhSiH3, suggesting there was no direct interaction between lecithin and PhSiH3. However, the 1H signal of Si−H of PhSiH3 shifted from 4.192 to 4.204 ppm after the addition of N-methylaniline in the mixture lecithin and PhSiH3 (Figure S2), indicating an occurring interaction between these three compounds. We thought that the intramolecular interaction between anion and cation groups in lecithin could be weakened by the interaction between N-methylaniline and lecithin, which was helpful for the interaction between the Si−H in PhSiH3 and the cation group in lecithin via the formation of an intermediate (II in Scheme 2), and this process favored the insertion of CO2 into the Si−H to produce the active species for formylation and methylation. Moreover, no interaction was existed between CO2 and lecithin determined by the 1H and 13C NMR. Additionally, the two alkyl chains of lecithin played the role to increase the interaction between the reactants and the catalytic

sites on lecithin rather than a directly catalytic role considering the fact that dodecane or methyl stearate could not catalyze the formylation (Table S4, entries 1 and 2), while dodecane could improve the activity of Na3PO4 on the formylation (Table S4, entries 3 and 4). On the basis of the above investigations and previous reports,14,43,44 a possible reaction pathway for the lecithincatalyzed formylation and methylation of amines using CO2 as the C1 source was proposed (Scheme 2). At the first stage, the interaction between lecithin and amines could generate an intermediate (I), which could interact with PhSiH3 to form intermediate II to activate the Si−H bonds. Then, CO2 was inserted into the activated Si−H bond in PhSiH3, and the active silyl formates (III) were generated, which was the ratedetermining step for both reaction pathways.39 Finally, the N atom in the active amines interacted with the silyl formates to produce the corresponding formamide with more CO2 at room temperature (Path A), while the obtained formamide would be further reduced with more PhSiH3 at higher temperature (100 °C) to provide the corresponding methylated product (Path B).41,42 In addition, after the reaction, PhSiH3 was converted into the corresponding silanols {[Si]−OH} and siloxanes {[Si]−O−[Si]}.39,45

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CONCLUSIONS In conclusion, lecithin could efficiently catalyze the formylation and methylation of various amines with CO2 using PhSiH3 as E


Letter


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the hydrogen source. Different products could be generated by controlling the molar ratio of the reactants and the reaction temperature. Formamides were the main product via a 2electron reduction with a 1:2 molar ratio of amine and PhSiH3 and at room temperature and 1 MPa CO2 pressure, while methylanilines became the predominate product via a 6electron reduction with a 1:4 molar ratio of amine and PhSiH3 and at 0.3 MPa CO2 pressure and 100 °C. We postulate that the renewable and biodegradable lecithin has great potential of applications in the reductive functionalization of amines using CO2 as the C1 source.

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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/acssuschemeng.8b03245. Experimental section, effect of various reaction parameters, activity of different hydrosilanes, 1H NMR spectra for PhSiH3, lecithin, N-methylaniline, and their corresponding mixture, 13C NMR spectra for the formylation of N-methylaniline, and GC−MS spectra of methoxysilan (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*J. Song. E-mail: [email protected]. *T. Jiang. E-mail: [email protected]. *B. Han. E-mail: [email protected]. ORCID

Jinliang Song: 0000-0001-9573-600X Buxing Han: 0000-0003-0440-809X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2017YFA0403003), National Natural Science Foundation of China (21733011, 21673249), Key Research Program of Frontier Sciences of CAS (QYZDYSSW-SLH013), and Youth Innovation Promotion Association of Chinese Academy of Sciences (2017043).

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Renewable and Biocompatible Lecithin as an Efficient Organocatalyst

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