Sustainable Carboxylation of Diamines with Hydrogen Carbonate

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Sustainable carboxylation of diamines with hydrogen carbonate Gianpiero Forte, Isabella Chiarotto, Frank Richter, Vinh Trieu, and Marta Feroci Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00229 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 2, 2018

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Organic Process Research & Development

Sustainable carboxylation of diamines with hydrogen carbonate Gianpiero Forte,a,* Isabella Chiarotto,a Frank Richter,b Vinh Trieu,b Marta Feroci a,* a

Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza Università di Roma, Via del Castro Laurenziano 7, 00161 Roma, Italy. *E-mail: [email protected]; [email protected]

b

Covestro Deutschland AG, Kaiser-Wilhelm-Allee 60, 51365 Leverkusen, Germany

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Organic Process Research & Development 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

TABLE OF CONTENT


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Abstract. A protocol for the carboxylation of diamines employing quaternary ammonium hydrogen carbonates as C1 source is presented. The approach is used to obtain industrially relevant bis-O-alkyl carbamates with diverse structural features in very high yield, even on gram scale. The quaternary ammonium salts – formally acting as “transporters” of the carboxylating agent – can be recovered after the reaction, and recycled with high efficiency. Regeneration of the hydrogen carbonates on ion exchange resin grants excellent atom economy in the process. Keywords: Carboxylation, carbamates, diamines, green chemistry, atom economy.

INTRODUCTION The environmental impact of production processes is a topic of increasing awareness in current research and development.1 This has fueled the interest towards under-explored approaches to chemical synthesis, building up the idea of green and sustainable chemistry.2 A substantial number of studies on the reactivity of carbon dioxide, along with the amount of funding consistently allocated to develop reactions to exploit CO2, well attests this situation.3 Carbon dioxide is indeed regarded as the primary anthropogenic contribution to the greenhouse effect. As of 2013, its concentration in the atmosphere has exceeded the record value of 400 ppm,4 raising major concerns about impacts on climate change. To prevent irreversible environmental issues, recent international agreements called for an urgent and dramatic cut in global emissions of CO2.5 Resources spent on strategies that limit the release of carbon dioxide into the environment have since considerably increased.6 On the other hand, CO2 represents a cheap and readily accessible carbon source,7 and the prospect of capturing the gas from industrial exhausts has driven the quest for viable methods to overcome its inert nature. These methods generally require harsh reaction conditions, such as high pressure or the presence of strong nucleophiles, and appear quite cumbersome to operate on a large scale. Most importantly, they often make use of metal catalysis, raising concerns on the environmental impact of waste management. As part of our ongoing interest in carbamate chemistry, we are committed to investigate scalable protocols for a sustainable transformation of diamines into alkyl carbamates – direct precursors of di-isocyanates, and key intermediates in the manufacturing process of polyurethanes, whose global market is expected to top USD 105 billion by 2025.8 Our goals are two-fold: (1) devise potential approaches to activate carbon dioxide for the synthesis of bis-O-alkyl carbamates; (2) optimize existing protocols in the perspective of industrial applications: taking into account the nature of reaction byproducts, and the possibility to recycle solvents and catalysts; avoiding health hazards; addressing issues of scalability and costs associated with the process.9 We recently described the synthesis of bis-O-butyl carbamates from diamines and carbon dioxide at atmospheric pressure, using butyl chloride as alternative to more harmful and 3 ACS Paragon Plus Environment


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expensive bromides or iodides.10 The protocol involves the pre-activation of CO2 on a copper cathode under very mild conditions. Collateral reactions, however, make the atom economy of such electrochemical activation highly questionable, while current nonelectrochemical approaches to obtain carbamates from CO2 need further optimization to be applicable on large scale.11 Here we report that tetraalkylammonium hydrogen carbonates, prepared from the corresponding chlorides, can efficiently carboxylate diamines. Notably, formed tetraalkylammonium chlorides can be recovered after the synthesis of carbamate esters, and used to regenerate the carboxylating agent with excellent atom economy in the overall process.12

RESULTS AND DISCUSSION Reduction of CO2 on copper electrodes in polar aprotic solvents, such as acetonitrile, mainly yields carbonates.13 Without additional bases, they react with amines to quantitatively form carbamate anions, whose nucleophilic behavior is particularly enhanced when tetraalkylammonium salts are used during the electrolysis.14 This activation method broadly benefits from the practical advantages of electrochemistry, including the use of mild conditions, and the absence of stoichiometric reductants.15 However, drawbacks like large initial amount of supporting electrolytes, and products of anodic reactions may hamper the effective sustainability of the protocol in industrial applications. Earlier studies from our group demonstrated that tetraethylammonium hydrogen carbonate (TEAHC) can carboxylate amines as efficiently as carbonates generated in the electrochemical reduction of CO2.16 TEAHC can be conveniently prepared from the corresponding chloride (TEACl) in a two-step process (Scheme 1, panel A),17 and is a promising alternative to overcome the aforementioned issues.

Scheme 1. Synthesis of TEAHC (A); synthesis of butyl carbamates using TEAHC for the carboxylation of amines (B).

Our investigation began with assessing the reactivity of TEAHC towards diamines – maintaining ratios optimized for monoamines (1.5 equiv. of TEAHC per amine group), but 4 ACS Paragon Plus Environment

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using 2.0 equiv. of butyl chloride instead of 5.0 equiv. of ethyl iodide per diamine – to rule out the unwanted formation of partially carboxylated, or N-alkylated, byproducts. Hexamethylenediamine (HMDA) was used to carry out the study because of the symmetric structure, which facilitates the NMR analysis of reaction products. Under these conditions (see experimental section), bis-O-butyl carbamate ester 1 is isolated in 92% yield as pure product (Scheme 2). The reaction can also be transferred to gram scale with a negligible performance loss: 1 is indeed obtained in 90% yield (19.8 g) when 70.0 millimoles of HMDA are carboxylated with TEAHC. It is interesting to notice that a stoichiometric amount of TEACl is formed during the alkylation step with butyl chloride (Scheme 1, panel B). Being the starting material for the synthesis of the hydrogen carbonate, TEACl can be recycled for subsequent reactions. The negligible solubility of both ammonium salts in diethyl ether largely facilitates their recovery, as solvent washings allow to separate the carbamate. In Table 1, column A, the yield of 1 for subsequent reactions are reported. After product extraction at the end of each run, the remaining solids were dried under high vacuum, then treated according to Scheme 1. A drop from 90% to 50% in the yield of 1 already occurs in the first recycling experiment. Slight improvements were observed by filtering off insoluble KCl after the residue was dissolved in methanol. Yet results remain quite far from being satisfactory, with the yield of 1 dropping to 65% between run four and five (Table 1, column B).

Scheme 2. Synthesis of dibutyl hexane-1,6-diyldicarbamate (1) using TEAHC as carboxylating agent. TEAHC: tetraalkylammonium hydrogen carbonate.

Excellent efficiency in recycling experiments was achieved when anion exchange resin Amberlite IRA-400 was utilized to convert TEACl to TEAHC (Table 1, column C). The resin is commercially available in chloride form, and stable to basic conditions. It is readily activated for our purposes by treatment with sodium hydrogen carbonate,18 which can be prepared by bubbling CO2 into aqueous solutions of sodium hydroxide. Besides leading to recycle TEACl up to four times without any appreciable loss in the the yield of 1, the use of these resins is generally regarded as a “green” asset for sustainable applications, and they are currently employed in a wide range of industrial processes.19 By the end of five subsequent reactions with such recycling approach, we calculated a 6.5% total loss of active TEAHC, which may be ascribed to Hofmann elimination on TEA cation at high temperatures.20



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Table 1. Regeneration of carboxylating agent. Yield of 1 for the reaction in Scheme 1. a

TEAHC: tetraethylammonium hydrogen carbonate; SpiroHC: 5-azoniaspiro[4.4]nonane hydrogen carbonate. a Synthesis of dibutyl hexane-1,6-diyldicarbamate from hexamethylenediamine (0.5 mmol), quaternary ammonium hydrogen carbonate (1.5 mmol) and butyl chloride (2.0 mmol) under experimental conditions b reported in Scheme 1. For each set of experiments, the carboxylating agent is reported in parentheses; c isolated product; after product extraction, the remaining solids were dried under vacuum and used to d regenerate TEAHC; prior to regenerate TEAHC, the remaining solids were dissolved in methanol, and the e impurities filtered off; the remaining solids were dissolved in water and passed on Amberlite IRA-400 resin, activated as described in the Experimental Section.

We reasoned that our process could benefit from the high thermal stability of spirocyclic ammonium salts recently reported.21 In a set of experiments TEAHC was replaced by 5azoniaspiro[4.4]nonane hydrogen carbonate (SpiroHC), whose corresponding chloride 2 can be conveniently prepared from pyrrolidine (Scheme 3). The data (Table 1, column D) indicate results comparable to the use of TEAHC, even in gram scale carboxylations: 88% yield (1.4 g) of 1 is indeed obtained from 5.0 millimoles of HMDA (see SI). However we calculated up to 27% total loss of active SpiroHC after five subsequent reactions.

Scheme 3. Synthesis of 5-azoniaspiro[4.4]nonane chloride (2).

Because diamines with diverse structural features are precursors of industrially important carbamates, we tested the range of applicability of carboxylation with TEAHC and SpiroHC on compounds reported in Table 2. Both salts efficiently react with aliphatic diamines, yielding results comparable to those obtained with electrochemically activated CO2 (entries 1-3). Benzyl diamines are slightly affected by inductive effect of the aromatic ring, resulting in decreased efficiency of the carboxylation process, and lower yield of the final carbamate (entry 4). Unexpected drop in performance of the reaction with SpiroHC is observed in this case. The spirocyclic cation appears to play a detrimental role that we find hard to 6 ACS Paragon Plus Environment

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rationalize at this stage. Aromatic diamines suffer from a poor nucleophilic character, and cannot be carboxylated either with TEAHC or SpiroHC under our mild experimental conditions. As observed already in the case of electrochemically activated CO2, formation of very low amounts of carbamate can only be observed when ethyl iodide is used in the alkylation step (entries 5-6).

Table 2. Carboxylation of different diamines using TEAHC or SpiroHC.a

TEAHC: tetraethylammonium hydrogen carbonate; SpiroHC: 5-azoniaspiro[4.4]nonane hydrogen carbonate. a Diamines (0.5 mmol) were treated with quaternary ammonium hydrogen carbonate (1.5 mmol), then with b butyl chloride (2.0 mmol) at 80 °C. isolated product. The yield for each first recycling experiment is reported c in parentheses; CO2 activated on a copper cathode prior to reaction with the diamine. For further details, d see ref. 10; ethyl iodide used in the alkylation step (room temperature, overnight).

CONCLUSIONS In conclusion, quaternary ammonium hydrogen carbonates carboxylate benzyl diamines with good yield, and aliphatic diamines with high to very high yield even on gram scale. We proved that anion exchange resins can be repeatedly used to obtain the carboxylating agent from the corresponding chloride, which is regenerated in the synthesis of bis-O-alkyl carbamates. Quaternary ammonium ions thus formally act as “transporters” of an activated 7 ACS Paragon Plus Environment


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form of CO2, and can successfully be recycled to achieve a sustainable process with excellent atom economy. TEAHC and SpiroHC are both effective carboxylating agents towards diamines, with the former giving generally better results when used in the synthesis of bis-O-alkyl carbamates. Active SpiroHC, supposedly more stable to temperature, experiences a significant loss during recycling experiments; by contrast, TEAHC is regenerated in virtually quantitative way.

EXPERIMENTAL SECTION Materials and methods. NMR spectra were recorded on a Bruker AC 200 spectrometer, using CDCl3 as internal standard. HPLC grade acetonitrile (≥ 99.9%, SigmaAldrich) was used in all the experiments. Tetraethylammonium chloride (≥ 99.9%, SigmaAldrich) was stored in a desiccator. All other reagents were used as received. Each described experiment was repeated at least three times, and the yields (reported as average values in the manuscript) are calculated on the amount of isolated pure product. Amberlite IRA-400 resin (chloride form, Sigma-Aldrich) was loaded with water into a glass column (height: 18 cm; internal diameter: 1 cm), and activated as follows: • washed with water until AgNO3 test was negative to chlorides (only when used for the first time); • slowly washed with 20 ml of NaHCO3 solution (1.0 mol/L in water); • washed with water until HCl test for bicarbonates was negative. Synthesis of 5-azoniaspiro[4.4]nonane chloride, 2. KOH (1.12 g, 20.0 mmol) was stirred in water (15 ml) at 100 °C. After 10 min, pyrrolidine (1.65 ml, 20.0 mmol) was slowly added, followed by slow addition of 1,4-dichlorobutane (2.2 ml, 20.0 mmol). The reaction mixture was kept under stirring at 100 °C for 18h, then overnight at room temperature. The solvent was evaporated under reduced pressure to obtain a solid residue, which was dissolved in CHCl3 and filtrated. CHCl3 was evaporated under vacuum to afford 2.85 g of pure chloride 2 (88% yield). Preparation of TEAHC and SpiroHC on Amberlite resin. Tetraethylammonium or 5azoniaspiro[4.4]nonane chloride (1.5 mmol) in 2.0 ml of water was slowly passed through a column containing the hydrogencarbonate form of the ion-exchange resin (see Materials and Methods section). The column was washed with water until 50 ml of solution were collected. The solvent was removed under reduced pressure to afford TEAHC or SpiroHC as a white solid, which was kept overnight under high vacuum. General protocol to the synthesis of bis-O-butylcarbamates using TEAHC or SpiroHC. In a 50 ml round bottom flask, TEAHC or SpiroHC (1.5 mmol) was dissolved in 12 ml of acetonitrile. The diamine (0.5 mmol, dissolved in 1 ml of acetonitrile) was added, and the reaction was allowed to stir at room temperature for 1h. Butyl chloride (2.0 mmol) was added, and the mixture stirred at 80 °C for 3h. The solvent was removed under reduced pressure, and the resulting residue was extracted with diethyl ether (3 × 40 ml). The collected organic fractions were concentrated to dryness to yield the pure butyl 8 ACS Paragon Plus Environment

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dicarbamate ester, while the diethyl ether used for the extractions was condensed and used in following experiments. Carboxylation scale-up with TEAHC. Synthesis of dibutyl hexane-1,6diyldicarbamate, 1. TEAHC (40.2 g, 210.0 mmol) was dissolved in 1.3 L of acetonitrile, and hexamethylenediamine (8.1 g, 70.0 mmol, dissolved in 70 ml of acetonitrile) was added. The solution was allowed to stir at room temperature for 2h. Butyl chloride (29 ml, 280.0 mmol) was added, and the reaction mixture heated at 80 °C for 3h. The solvent was removed under reduced pressure, and the resulting residue was extracted with ethyl acetate (3 × 300 ml). The collected organic fractions were concentrated to dryness to afford 19.8 g of the product (90% yield). Recovery and recycle of TEACl or chloride 2. The solid residue, after ethereal extractions, was dried in vacuo, dissolved in 2 ml of water, and treated with the resin to regenerate TEAHC or SpiroHC as described above.

ACKNOWLEDGEMENTS This work was financially supported by Covestro Deutschland AG, Sapienza Università di Roma and MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca).

ASSOCIATED CONTENT Supporting Information. Reported procedure for TEAHC preparation from TEACl without using Amberlite resin; carboxylation scale-up with SpiroHC; 1H, 13C NMR spectra and characterization of synthesized compounds (PDF).

The authors declare no competing financial interests.

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Sustainable Carboxylation of Diamines with Hydrogen Carbonate

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