Waste Recycling: Ionic Liquid-Catalyzed 4-Electron Reduction of CO2

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Waste Recycling: Ionic Liquid-Catalyzed 4-Electron Reduction of CO2 with Amines and Polymethylhydrosiloxane Combining Experimental and Theoretical Study Xiao-Ya Li, Su-Su Zheng, Xiao-Fang Liu, Zhi-Wen Yang, Tian-You Tan, Ao Yu, and Liang-Nian He ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02352 • Publication Date (Web): 27 May 2018 Downloaded from http://pubs.acs.org on May 27, 2018

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ACS Sustainable Chemistry & Engineering

Waste Recycling: Ionic Liquid-Catalyzed 4-Electron Reduction of CO2 with Amines and Polymethylhydrosiloxane Combining Experimental and Theoretical Study Xiao-Ya Li†‡, Su-Su Zheng§‡, Xiao-Fang Liu†, Zhi-Wen Yang†, Tian-You Tan†, Ao Yu*§, and ‖ Liang-Nian He*†, †

State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China §

Applied Chemistry and Engineering Research Institute, College of Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China ‖

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China

E-mail: [email protected]; [email protected]

ABSTRACT: The acetate-based ionic liquids (ILs) e.g. [nBu4N]OAc have been developed for challenging 4-electron reduction of CO2 with amines and hydrosilane to afford aminals. Notably, polymethylhydrosiloxane (PMHS), a cheap, byproduct of silicone industry, also works well as a reductant. Furthermore, an alternative pathway is rationally proposed via thorough density functional theory (DFT) study. In addition, the ILs paly a surprisingly significant role through decreasing activation free energy. More importantly, the essence of being kinetically favorable with 4-electron reduction and thermodynamically favorable with 6-electron reduction is unveiled by DFT study, which guides us to successfully get 6electron reductive products of CO2, i.e. methylamine. In a word, this work represents upgrading usage of both carbon and silicon wastes to valuable chemicals via IL catalysis. KEYWORDS:

Carbon

dioxide

chemistry, Computational

INTRODUCTION CO2 is a nontoxic, abundant and renewable C1 source, and the conversion of CO2 into value-added chemicals and fuels has attracted extensive attention.1-3 Recently, the reductive functionalization of CO2 which combines both CO2 reduction and C-C, C-O or C-N bond formation has boomed, as this alternative method significantly broadens the range of chemicals accessible from CO2.4-8 Among these, reductive C−N bond-forming reactions have been intensively investigated, producing versatile chemicals and energy-storage materials e.g. formamides and/or methylamines.4,6 Numerous metal-based systems9-17 have been developed for reductive functionalization of CO2 with amines to generate formamides and/or methylamines. Following the seminal work in 2012 by Cantat on the organocatalytic reduction of CO2 into formamides using amines and hydrosilane,18 diverse efficient organocatalysts e.g. Verkade’s superbases,19 NHCs,20,21 NHOs,22 NHP-H,23,24 B(C6F5)3,25,26 TBAF27 and betaine28,29 have proved to be competent for the reductive functionalization of CO2. Among these reports, most attention focused on 2and/or 6-electron reduction of CO2, delivering C+II species e.g. formamide and/or C-II methylamine. However, specific termination at 4-electron reduction level of CO2 to af-

chemistry,

Formaldehyde,

Ionic

liquids,

Reduction

ford the C0 species would be interesting and promising, and still remains a challenge,30 probably due to its thermodynamic instability. Only a few reports have showed selective reduction of CO2 to C0 or C0 function (i.e. methylene).31-38 In this regard, Cantat et al. have described the TBD-catalyzed 4-electron reduction of CO2 with amines and PhSiH3 to access aminals for the first time.30 Subsequently, Fe-based 39 and betaine28,29 catalysts are also attempted to obtain aminals with 9-BBN or PhSiH3 (Scheme 1). It is worth mentioning that other catalytic systems including Co 40, Ru 41 or TBD42 have been developed for selective 4-electron reduction of CO2 coupled with C-O or C-C bond construction. In spite of these progresses, there is no doubt that developing a novel, efficient, and sustainable metal-free catalytic system for the synthesis of formaldehyde derivatives e.g. aminals, which can serve as important chiral ligands43 or electron donors44 from CO2 and silicon waste polymethylhydrosiloxane (PMHS) as a cheap, environmental-friendly reductant is still highly desirable. Ionic liquids (ILs) have been widely used in organic synthesis45,46 especially in promoting CO2 conversion.47,48 What is worth mentioning, ILs have shown excellent performance for 2-electron reductive functionalization of CO2 to formic acid level.9, 49-52 Nevertheless, the challenging 4-electron reductive functionalization of CO2 with ILs

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catalysis has never been reported. Considering the designablity of ILs, we envisioned that the kinetics of CO2 reduction can be controlled through modulating the ILs structure so as to terminate CO2 reduction at 4-electron level. In addition, inspired by nucleophile-promoted rearrangement of PMHS,53,54 the ILs containing nucleophilic site may transform PMHS with low reactivity to in situ generate more active reductant species, e.g. MeSiH3, leading to enhanced hydride-donating capacity. Herein, we would like to report the acetate-based ILs e.g. [nBu4N]OAc serves as an excellent catalyst for 4-electron reductive functionalization of CO2 with various amines and hydrosilane especially silicon waste PMHS under mild conditions to form aminals. To the best of our knowledge, this work offers the first IL catalysis for 4electron reduction of CO2 with amine and PMHS (Scheme 1). Scheme 1. 4-Electron Reduction of CO2 with Amines to Produce Aminals

RESULT AND DISCUSSION

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1b in up to 87% yield (entry 9). Notably, this is the only example for 4-electron reduction using PMHS, thus we have successfully reached upgrading usage of carbon and silicon wastes to obtain value added chemicals through IL catalysis. Furthermore, the reaction did not proceed without CO2 or catalyst (Supporting Information). Condition optimization results including the temperature, solvent and time are listed in Table S2. Table 1. Catalyst Screening for 4-Electron Reduction of CO2 with N-Methylaniline 1a to Obtain Aminal 1ba Entry

Catalyst

Hydrosilane

Conv.

Yield

(%)b

(%)b

1

[BMIm]OAc

Ph2SiH2

78

75

2

[DBUH]OAc

Ph2SiH2

90

90

3

[nBu4N]OAc

Ph2SiH2

99

98

4

[nBu4N]Cl

Ph2SiH2

0

0

5

[nBu4N]Br

Ph2SiH2

0

0

6

[nBu4N]NO3

Ph2SiH2

0

0

7c

[nBu4N]OAc

Ph2SiH2

92

90

8

[nBu4N]OAc

PMHS

92

67

9e

[nBu4N]OAc

PMHS

90

87

a

To start our initial investigation, the benchmark reaction of CO2 with N-methylaniline 1a and Ph2SiH2 was performed at 50 oC. As listed in Table 1, several acetate-based ILs including [BMIm]OAc, [DBUH]OAc and [nBu4N]OAc were examined. To our delight, those three acetate-based ILs were all effective for this 4-electron reduction of CO2 with 1a (entries 1-3) and [nBu4N]OAc showed the best activity affording aminal product, namely, N,N'-dimethylN,N'-diphenylmethanediamine 1b within 4 h in almost quantitative yield. Whereas, the other ILs with the identical cation were not active (entries 4-6). These results imply that the acetate anion could play a critical role in promoting this reaction. As the amount of [nBu4N]OAc was decreased to 1 mol%, 1b yield could still retain at 90% (entry 7). Subsequently, various hydrosilanes were investigated (Table S1 in Supporting Information). It should be noted that PMHS, as a cheap, environmental-friendly and byproduct of the silicone industry,55 is a suitable reducing agent for potential large-scale production (entry 8). However, examples for reductive functionalization of CO2 with PMHS as reductant were quite rare due to its low reactivity.20,56,57 In the light of nucleophile-promoted rearrangement of PMHS,53 the acetate-based ILs in this work could transform PMHS to more active reductant, e.g. MeSiH3. Control experiment confirmed the formation of one kind of gaseous reductant from PMHS in this IL catalytic system (see Supporting Information). Delightedly, the results showed that employing 9 equiv. PMHS could deliver

Conditions: 1a (1 mmol, 108 μL), catalyst (10 mol% relative to 1a), hydrosilane (6 mmol Si-H), CO2 (closed, approx. 1 mmol relative 1a), CH3CN (2 mL), 50 oC, 4 h. bDetermined by 1H NMR using 1,3,5-trimethyoxybenzene as an internal standard. c[nBu4N]OAc (1 mol%).ePMHS (9 mmol, 0.54 g), THF (2 mL) as solvent. With the optimized reaction conditions in hand, we then evaluated the scope of this IL-catalyzed 4-electron reductive protocol with various amines and Ph2SiH2 as reductant shown in Scheme 2. Secondary aromatic amines, bearing either electron-donating or electronwithdrawing groups on the phenyl ring, afforded the corresponding aminals in 70%-75% yields (2b-4b). Moreover, this 4-electron reductive protocol was also applicable to cyclic amines such as 1,2,3,4- tetrahydroquinoline and indoline, forming the aminal analogues 5b and 6b in 97% and 85% yields, respectively. Additionally, the substrate bearing a heteroaryl ring gave the desired product in excellent yield (7b). Notably, this protocol was also applicable to aliphatic amines such as morpholine and piperidine to furnish the corresponding aminals with moderate yields (8b and 9b). Furthermore, the formation of the heterocycle was favored via an intramolecular cyclization for the diamino substrate i.e. N,N’dipheneylethylendiamin (10a) to generate 1,3diphenylimidazolidine (10b) in 90% yield. Furthermore, we also investigated the applicability with PMHS as reductant (Scheme 2). Interestingly, the silicon waste PMHS


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ACS Sustainable Chemistry & Engineering even showed better performance than Ph2SiH2 and the desired products. The exceptional reactivity of PMHS could be due to the IL-promoted rearrangement of PMHS to MeSiH3.54

sence of acetate. Overall, the free-energy barrier of TS1 is higher compared to TS1a and TS1b with acetate-catalyzed, which indicate the acetate could play an important role in promoting this reaction.

Scheme 2. The 4-Electron Reduction of CO2 with Amines to Form Aminals

Reduction of silyl formate to formaldehyde (step II): It was proposed that the aminal is obtained through two successive nucleophilic attacks of the amine towards C0 bis(silyl)acetal.28,30 However, our computational results indicate that it is not a feasible pathway for yielding the aminal (see Figure S5 of Supporting Information). On the other hand, Wang et al have revealed that formaldehyde generated from the dissociation of bis(silyl)acetal should be inevitable for the NHC-catalyzed conversion of CO2 into methanol through DFT study.23,58 Generation of formaldehyde in our catalytic system was also verified by the control experiment (Scheme S1, eqn.1, Supporting Information). Therefore, an alternative mechanism through formaldehyde pathway seems to be rational and reliable (see Figure S6 in Supporting Information).

Reaction conditions: aamine (1 mmol), [nBu4N]OAc (10 mol%, 30.2 mg), Ph2SiH2 (2.5 mmol, 465 μL), CH3CN (2 mL), CO2 (closed, approx. 1 mmol per amine), 50 oC, 4 h; Yields were determined by 1H NMR using 1,3,5trimethyoxybenzene as an internal standard. The isolated yields are given in parentheses. bPMHS (9 mmol, 0.54 g), THF (2 mL) as solvent. cPMHS (6 mmol, 0.36 g), THF (2 mL) as solvent.

C-N bond formation (step III): The possible pathway for the formation of aminal is shown in Figure 1. To our surprise, the calculation showed that thermodynamically more stable product methylamine could also be obtained theoretically. In the whole pathway, aminomethyl formate IM6 is the key intermediate to realize the formation of aminal and methylamine, respectively. On the one hand, IM6 reacts with 1a to yield the 4-electron reductive product aminal 1b. Notably, converting IM6 to product 1b is endothermic by 0.6 kcal mol-1, illustrating that this step is reversible. On the other hand, IM6 can react with PhSiH2OH to give the 6-electron reductive product methylamine 1c (shown in Figure 1, green line).

THEORETICAL STUDY Given that the detailed mechanism and origination of selectivity for 4-electron reductive functionalization of CO2 is still unclear, a DFT mechanistic study was therefore performed. The pathway consists of three steps, including 2-electron reduction of CO2 to the silyl formate (step I), reduction of the silyl formate to formaldehyde (step II) and the C-N bond formation (step III). These three steps will be described in order as below. 2-electron reduction of CO2 to silyl formate (step I): The computed potential energy profile for the silyl formate formation is presented in Figure S3 (see Supporting Information). Firstly, acetate attacks PhSiH3 through nucleophilic addition, forming the active hypervalent silicon complexes Si-1a, which are more active than PhSiH3 to donate the hydride. Then CO2 inserts into the Si-H bond of Si-1a, which affords another hypervalent silicon intermediate IM1a via TS1a. Finally, IM1a gives silyl formate IM1 by releasing acetate, the first catalytic cycle is established with the acetate regeneration. In order to explore the promoting effect of acetate, we calculated the energy of transition state TS1 for CO2 direct insertion in the ab-

Figure 1. The Potential Energy Surface for the Formation of Aminal.

From above theoretical study, there are two different pathways to generate aminal and methylamine; in other words, the two pathways are competitive. For being kinetically favorable with aminal formation, through tuning



CO2 amount the aminal could revert to the key intermediate IM6, which will be transformed into thermodynamically stable product methylamine. Therefore, we envisaged that increasing the amount of CO2 may render further conversion of aminal into methylamine, thus exclusively delivering N-methylated product. Further exploration via tuning the amount of CO2 was conducted as shown in Table S3 (see Supporting Information). We were pleased to find that increasing the amount of CO2 to 20 equiv. relative to 1a or more (balloon of CO2, >20 equiv.), quantitative yield of the methylamine reached (entries 56). As a consequence, the CO2 amount controls the selectivity of products, that is, aminal and methylamine. Additionally, control experiments were further conducted (Scheme S1, eqn. 2 and eqn. 3), suggesting that the conversion of aminal to methylamine requires the CO2involving species (e.g. formic acid), which corroborates our computed mechanism and hypothesis. Furthermore, the 1H NMR spectra of the in situ catalytic reaction is given in Figure S2 of Supporting Information, hinting the aminal 1b may be possible intermediate in this Nmethylation reaction.

DFT computations, a novel alternative pathway via successive formation of formaldehyde, iminium and aminomethyl formate IM6 as intermediates is therefore rationally proposed. Furthermore, the essence of being kinetically favorable with 4-electron reduction process and thermodynamically favorable with 6-electron reduction has been unraveled to elucidate the excellent selectivity of aminal. It is the first time to realize 4-electron reduction of CO2 in the presence of amines and PMHS with IL catalysis. Therefore we realized upgrading usage of both carbon and silicon waste to obtain value-added chemicals via IL catalysis. Scheme 3. Proposed Reaction Mechanism for [nBu4N]OAc-Catalyzed Reductive Functionalization of CO2 to Selectively Form aminal and Methylamine

PROPOSED MECHANISM On the basis of our experimental results, DFT study and previous reports,28,30,42 a possible reaction mechanism was proposed, as demonstrated in Scheme 3. Firstly, the Si-H bond of hydrosilane is activated by acetate through oxygen nucleophilic interaction towards silicon, forming the active hypervalent silicon species Si-1a, which delivers hydride to CO2 affording the pentavalent silicon intermediate IM1a. Notably, the insertion of CO2 into Si-H bond establishes equilibrium: silyl acetate IM1' + HCOO‾ ⇌ IM1a ⇌ silyl formate IM1 + CH3COO‾ (see Supporting Information). Subsequently, IM1 is further reduced by Si1a to obtain the C0 bis(silyl)acetal IM2, which is dissociated into an important intermediate formaldehyde IM3 via intramolecular cyclization and rearrangement promoted by acetate. IM3 is nucleophilically attacked by amine 1a obtaining α-amino alcohol IM4, which is converted into another important intermediate iminium IM5 with the assistance of IM1'. Furthermore, IM5 undergoes nucleophilic addition by HCOO‾ to afford the key intermediate aminomethyl formate IM6, from which the selectivity between aminal and methylamine originates. If the amount of CO2 is deficient, the net reaction would terminate at the C0 level to generate kinetically stable aminal 1b as the final product. On the other hand, as the transformation of IM6 into aminal is reversible and excess CO2 can render the reversible reaction to proceed backward reverting to IM6, which will be further reduced to thermodynamically stable methylamine 1c.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Present Addresses †

State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China § Applied Chemistry and Engineering Research Institute, College of Chemistry, Nankai University Tianjin 300071, China. ‖ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China

Author Contributions ‡

These authors contributed equally.

Notes

CONCLUSIONS

The authors declare no competing financial interest.

To conclude, we have described the metal-free IL catalytic protocol for the synthesis of the challenging C0 species aminals starting from CO2, amine and hydrosilane. [nBu4N]OAc can even enable silicon waste i.e. PMHS as an effective reducing agent. Notably, with comprehensive

ACKNOWLEDGMENT This work was financially supported by National Key Research and Development Program (2016YFA0602900), National Natural Science Foundation of China (21472103,


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ACS Sustainable Chemistry & Engineering 21672119, 21421062, 21577071), the Natural Science Foundation of Tianjin (16JCZDJC39900)

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ACS Sustainable Chemistry & Engineering

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A brief (~ 20 word) synopsis： Recycling both carbon and silicon wastes to valuable chemicals has successfully been realized via efficient IL catalysis in combination of experimental with theoretical study.


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Waste Recycling: Ionic Liquid-Catalyzed 4-Electron Reduction of CO2

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