Introduction

Means of viable development, typically relying on more sensible resource management, is a conceit challenge in front of modern human society. Sustainability level in recent economic growth requires a massive improvement as it is far from an adequate level. According to the data released by Intergovernmental Panel on Climate Change (IPPC 2018), global surface temperature has mounted by approximately 1.5 °C from 1880 to 2018, which is a phenomenon caused by anthropogenic activities, predominantly greenhouse gases like CO2 emissions from fossil carbon to accomplish the escalating energy demand. Under this circumstances, melting of thousand years old glaciers, desertification of fertile land, rise in ocean water level and acidification of ocean water had caused enormous detriment to diverse ecological environment1. Scientific and technical advancements to curve atmospheric CO2 concentration via limiting industrial emission and use CO2 as an alternative fuel source in the renewable energy sector, had been a recurrent course of study for past few years2,3,4,5. The reduction of CO2 can be considered as a typical cohesive technology to rise artificial efficiency in producing various valuable hydrocarbons like formic acid, methanol, methane, and C2–C4 olefins6,7,8,9,10,11,12. Several fascinating integrated protocols have freshly been reported for hydrothermal and photochemical CO2 reduction, e.g., metal/metal oxide redox reaction based solar two-step water-splitting thermochemical cycle for CO2 reduction via hydrogen generation13,14,15,16,17,18,19,20,21,22. Alongside, chemical fixation of CO2 has gained substantial importance in synthetic chemistry because CO2 could be used as a benign, abundant, inexpensive, and renewable C1 reserve to yield a variety of value-added chemicals e.g. esters, amides, aldehydes, carboxylic acids, alcohols, organic carbonates and 2-oxazolidinones, etc23,24,25,26. In particular, synthesis of therapeutically cherished and synthetically convenient five-membered cyclic urethanes such as oxazolidinones via cycloaddition of CO2 with aziridines has become one of the most promising approaches in this area, because this process possess 100% atom efficiency, which exactly matches one of the most substantial criteria of green chemistry27,28,29,30,31. Despite being an admirable strategy to chemically capture and recycle CO2, most of these protocols suffer from high energy demand and utilize costly catalysts/ionic liquids to achieve ambient or near ambient condition for CO2 fixation, even from highly enriched CO2 source32,33,34,35,36,37,38,39,40. However, emissions from thermal power plants contain numerous gaseous components like SO2, NO2 along with CO241. In these context, post-combustion CO2 capture, release, and storage (CCS) had been the most abundantly used protocols for CO2 purification from industrial exhausts. Various strategies are being industrialized for capture, release and storage (CCS) of CO2 from gas streams, where gas–solid adsorption by metal–organic frameworks, gas–liquid chemi-absorption by amines and carbonation by quick/slacked lime are notable42,43,44,45,46. However, chemical fixation of CO2 from contaminated sources under mild conditions to produce industrially vibrant chemicals and products faces great defies because of two main reasons: (1) the high ionization potential (IP), and (2) the negative adiabatic electron affinity (EA) of carbon dioxide. Therefore, most of the reports use harsh reaction conditions to overcome the high thermodynamic stability and chemical inertness of carbon dioxide. Hence, the development of a cost-effective and robust protocol for CO2 capture, storage, and release in ambient conditions along with utility is highly desirable. Further, the chemical fixation is an irreversible process producing stable covalent compounds and thus, till now it could not be utilized for CO2 capture and release. It might be a potential CCS protocol as it would produce valuable chemicals, provided the chemical fixation and the defixation (release) done under ambient conditions, the latter is an unmet challenge. Herein, we report the first regenerable chemical fixation, where CO2 fixation by spiroaziridine oxindole under atmospheric pressure at rt (30 °C) without any catalyst producing stable spirooxazolidinone, a potential drug candidate47,48,49, further reversed back (defixation) to the spiroaziridine releasing CO2 under mild conditions. This fixation and defixation cycle can be repeated in a single loop for several times using a recyclable reagent.

Results and discussion

Uniqueness of spiroaziridine- and spirooxazolidinone oxindoles

CO2 is an overall non-polar molecule, but the presence of net partial charges [O−δ–C+2δ–Oδ] makes its susceptibility to nucleophilic as well as electrophilic attack at carbon and oxygen, respectively. As a consequence, substrate such as epoxide and aziridine with both reactivity centres are suitable for the fixation of CO220,36,37,38,39,40. However, all these require high pressure, -temperature and/or catalyst/additive. Designing substrate with tuned reactivity may lower the pressure and temperature for the chemical fixation of CO2 and may further facilitate the CO2 release. We envision that NH-free spiroaziridine oxindole 1 could be a suitable substrate with desired reactivity as the presence of oxindole unit may enhance the nucleophilicity of aziridine-nitrogen via an electron-donating resonance effect of nitrogen of oxindole unit and/or its anchimeric assistance (Fig. 1), simultaneously these may increase the electrophilicity of the C3 center of oxindole via resonance structure 1A and/or the formation of intermediate 1B under neutral or mild basic condition50,51,52. It is further envisioned that the presence of oxindole unit in spirooxazolidinone similarly will enable the release of CO2 under acidic conditions as shown in Fig. 1)53,54. More importantly, the spirooxazolidinoyl oxindole is a potential drug candidate47,48,49, so this CO2 fixation could be excellent and cheap method for its production.

Figure 1
figure 1

Proposed reactivity of spiroaziridine 1 and spirooxazolidinone 2 towards CO2 fixation and release.

Full size image

Optimization of auto-chemical fixation of CO2 by NH-free spiroaziridine 1a under ambient conditions

According to the presumption, we started our studies initially on synthesis of NH-free spiroaziridne oxindole 1a and its reactivity towards CO2 under different conditions. We have developed a new and efficient method for the synthesis of NH-free spiro aziridine 1a from easily available amino alcohol 3a on successive treatment with chlorosulfonic acid (ClSO3H) in dioxane and aqueous KOH. The exclusive formation of NH-free spiroaziridine 1a was detected by MS and NMR analysis. With great delight, when a slow stream of CO2 was passed through an aqueous dioxane solution of in situ synthesized spiroaziridine 1a at rt, within 30 min it produced exclusively CO2 adduct, spiro oxazolidione 2a in excellent yield (Table 1, entry 1) without any catalyst. This might be the first report of catalyst-free spontaneous chemical fixation of CO2 under ambient condition and also in aqueous medium. Instead of aqueous, solid KOH was also found to be suitable for the in situ synthesis of spiroaziridine 1a and subsequent fixation of CO2, but it took a bit more time than the aqueous-dioxane (entry 2). The dioxane was the optimized solvent for both in situ spiroaziridine formation and the chemical fixation of CO2. NaOH instead of KOH is also equally effective for the synthesis of spiroaziridine and subsequent CO2 fixation (entries 3 and 4). Further, when in situ generated spiroaziridine was taken in ethyl acetate and treated with slow stream of CO2 in absence of any base, it also gave the CO2-adduct within 1.5 h in 69% isolated yield (entry 5). Thus it can be concluded that the chemical fixation of CO2 by spiroaziridine does not require base as a catalyst/promoter. Ultimately with our great delight, the auto-chemical fixation of CO2 was successful with 12.5% CO2 in N2 as well as a stimulated coal flue gas (12.5% CO2, 7.5% O2 and 80% N2) without any appreciable loss in the yield of the adduct (entries 6–8). These took longer reaction time, may be due to low concentration and retention of CO2 in solution.

Table 1 Optimization of in situ synthesis of spiroaziridine 1a and fixation of CO2.
figure a
Full size table

Defixation of CO2 at near ambient conditions

We next sought to explore the possibility to regenerate the spiroaziridine via decarboxylation, which is an unmet challenge in CO2-chemical fixation. As per the presumption, the decarboxylation (CO2 release) was initiated with the reaction of spiroxazolidinone in the presence of different Brǿnsted acids and the subsequent treatment of base to regenerate the spiroaziridine and its regeneration was quantified with the further chemical fixation of CO2 leading to spirooxazolidione again. Both the CO2 defixation and the fixation were optimized in dioxane and briefly summarized in Table 2. The regeneration of spiroaziridine 1a was detected when a dioxane solution of spirooxazolidinone was heated with triflic acid at 100 °C. The extend of formation of spiroaziridine was confirmed by its chemical fixation of CO2 and it gave only 24% yield of the resynthesized spirooxazolidinone 2a (Table 2, entry 1). With our great delight, near quantitative formation of spiroaziridine 1a was achieved, when the compound 2a was heated only at 70 °C with HI followed by treatment with aqueous NaOH (Table 2, entry 4). This was revealed with the re-synthesis of spirooxazolidinone 2a with 94% of isolated yield. HBr was also found to act on at 70 °C, but it took longer time with incomplete conversion (entry 6). Further, to avoid the cumbersome procedure for the preparation of dioxane-HX, we developed an efficient and handy reagent, NaI-phosphoric acid for the regeneration of spiroaziridine 1a (entry 8) (“Supplementary material”).

Table 2 Optimization of reaction condition for defixation and subsequent fixation of CO2.
figure b
Full size table

Mechanism of CO2 defixation

In TfOH mediated decarboxylation of 2a (defixation of CO2), the formation of spiroaziridinium ion 1a′ was detected by MS analysis prior to the treatment with base. However, in case of HI or NaI-H3PO4, exclusive formation of intermediate compound 3-(aminomethyl)-3-iodooxindole 4a′ and no 1a′ was observed by MS analysis prior to the reaction with base. The intermediate iodo-amine 4a′ was isolated and identified as N-tosyl compound 5a by MS and NMR analysis (Fig. 2). The intermediate compound 4a′ on treatment with base regenerated the spiroaziridine 1a. Its in situ formation was confirmed by MS and NMR analysis and further isolated as N-tosyl spiroaziridine 6a.

Figure 2
figure 2

Intermediate compounds during defixation of CO2 from 2a.

Full size image

Chemical fixation and defixation cycle of CO2

The defixation of CO2 at 70 °C and the subsequent fixation of CO2 at rt (25 °C) was repeated for five times through the isolation of regenerated spirooxazolidinone 2a using solid NaOH. All the cycles required equal CO2-defixation and fixation time-scale and exhibited quantitative regeneration of 2a (≥ 95%; Fig. 3). Further the CO2 defixation (at 70 °C) and the fixation (at rt) were successfully continued for consecutive five cycles in one-pot by treating with NaI-H3PO4 and solid NaOH. Excitingly the overall yield of spirooxazolidinone after five cycles was found to be excellent (overall GC yield 95% and isolated yield 90%).

Figure 3
figure 3

The recycling of spiroaziridine 1a via re-synthesis of spirooxazolidinone 2a (The yield in each cycle referred to the GC yield of resynthesized spirooxazolidinone 2a; Standard deviation: cycle 1 and 2 = 0.58; cycle 3–5 = 1).

Full size image

Again, if we deeply look into the chemical reactions involved during the release and capture of CO2 of the process, NaI supposed to regenerate after the treatment of 4a′/1a′ with NaOH. So, in principle, NaI may be reused for the subsequent cycles. For the purpose, the first regeneration cycle with release of CO2 was carried out as usual with the combination of NaI-H3PO4 and NaOH and subsequent chemical fixation of CO2 produced the spirooxazolidione. The subsequent cycles for the regeneration of spiroaziridine (CO2-release/defixation) and CO2-fixation were performed without further addition of NaI, only varying with the equivalent of H3PO4 and NaOH (Fig. 4). Thrillingly these were smoothly continued for five cycles. It showed almost quantitative yield of spirooxazolidinone 2a in each cycle and finally the spirooxazolidinone 2a was isolated with excellent overall yield (90%).

Figure 4
figure 4

Fixation–defixation cycle of CO2 with recyclable NaI.

Full size image

In some of the developed technologies, the sorbent (liquid or solid) loaded with the captured CO2 is transported to a different vessel, where it releases the CO2 (regeneration) either after being heated or after a pressure decrease or after any other change of conditions around the sorbent. The sorbent resulting after the regeneration step is sent back to capture more CO2 in a cycle. This makes additional cost of the process. It will be desirable to conduct both CO2 capture and the release in a single vessel, this is possible when both are near similar conditions. In our case, 70 °C was found to be optimum temperature for the CO2 defixation. So, we further studied the temperature effect on CO2 fixation. Interestingly, it showed a near horizontal line for the fixation at 5 °C, 30 °C, 50 °C, 60 °C and 70 °C, respectively, with > 95% yield in each case (Fig. 5).

Figure 5
figure 5

Temperature effect in chemical fixation of CO2 by spiroaziridine 1a.

Full size image

Inspired by the above findings of temperature effect on CO2 fixation, we performed both CO2 defixation and fixation at 70 °C and continued for five cycles. With our great delight, it showed almost quantitative yield of spirooxazolidinone 2a in each cycle and an excellent overall yield after five cycles. This chemical fixation-defixation (five) cycles at 70 °C are repeated for three times with a standard deviation of 0.47–1.70 (Fig. 6).

Figure 6
figure 6

Chemical fixation-defixation cycles at 70 °C (The yield in each cycle referred to the GC yield of resynthesized spirooxazolidinone 2a; Standard deviation: cycle 1 and 2 = 0.82; cycle 3 = 1.25, cycle 4 = 1.70, cycle 5 = 0.47).

Full size image

The spirooxazolidonyl oxindoles are important bioactive compounds21,22. Thus further efforts are made to generalize the developed method for the synthesis of various spirooxazolidines by catalyst-free CO2 fixation of in situ generated spiroaziridines (Fig. 7). Irrespective of N-protection- and substitution of arene moiety of the oxindole unit, all underwent smooth auto-chemical fixation of CO2 providing the excellent isolated yields of the adducts 2, albeit N-benzyl and N-allyl substrates took longer time in comparison with others for the CO2 fixation. Further, alike 1a, the spiroaziridines derived from 3b, 3e, 3f and 3j also efficiently produced the corresponding CO2-adducts 2b, 2e, 2f and 2j with the flue gas in similar yields as with pure CO2. The regioselectivity of the fixation and the structure of the compound 2 was confirmed from the single crystal X-ray analysis of the compounds 2g (Fig. 7; CCDC 1898609). All the CO2-adducts 2 are solid compounds with melting point > 100 °C and bench stable for a couple of months under ambient conditions. Thus the developed regenerable chemical fixation protocol can be utilized for CO2 capture, storage and release, if and when it needed.

Figure 7
figure 7

Generalization of catalyst-free CO2 fixation in synthesis of various spirooxazolidinoyl oxindoles 2. The values in parenthesis refer to the GC yield using stimulated flue gas as a source of CO2.

Full size image

Conclusion

In summary, the first regenerable chemical fixation by spiroaziridine oxindole proved to be an excellent protocol for spontaneous and reversible CO2 fixation and defixation. We have demonstrated that the CCS [CO2 fixation and defixation cycle (regeneration)] can work well in one-pot (single vessel) for several cycles with excellent recovery using recyclable reagent under near ambient conditions. More importantly, the process regioselectively produced bioactive spirooxazolidinoyl oxindole in quantitative yields under extremely mild conditions (no extra reagent/catalyst, 1 atm., and rt). The CO2-adducts are stable compounds with high melting points, these can be stored for months under ambient conditions and can be reversed back to the sorbent as and when it requires. So, these findings in the ongoing research can open up a new avenue of the chemical fixation for the development of smart innovative technology towards the energy and cost effective practical CCS and would find abundant applications in CO2 fixation and -defixation chemistry towards chemical utilization of CO2 in industry.

Methods

Auto-chemical fixation of CO2 by in situ generated spiroaziridine 1a

Amino alcohol 3a (500 mg, 2.60 mmol) was dissolved in dry dioxane (8 ml) and cooled to 0 ºC. Chlorosulfonic acid (174 µl, 2.6 mmol) was added drop wise and the reaction mixture was stirred for 2 h at room temperature (rt). 14 ml of 1 M aqueous NaOH solution was added dropwise to quench the acid at 0 ºC and stirred at 70 °C for 16 h. The complete conversion to spiroaziridine was detected by MS analysis. Next, a slow stream of CO2 was passed through the solution at rt for 30 min. After complete consumption of 1a (monitored with TLC and also by MS analysis) the dioxane was removed under reduced pressure and the residue was extracted with EtOAc (3 × 10 ml), washed with brine solution and dried over anhydrous Na2SO4. Combined organic layer was concentrated and purified by silica gel flash chromatography using EtOAc/hexanes (1:1) to afford the desired CO2-adduct 2a (528 mg, 93%).

Note In case of stimulated flue gas (12.5% CO2, 80% N2 and 7.5% O2) or 12.5% CO2 in N2, the stream of gas was passed through the solution for 18 h.

Defixation of CO2 from spirooxazolidinone 2a and re-fixation of CO2

To a solution of spirooxazolidinone 2a (150 mg, 0.69 mmol) in dry dioxane (6 ml), sodium iodide (414 mg, 2.76 mmol) and o-phosphoric acid (144 μl, 2.76 mmol) were added. The mixture was stirred at 70 °C and the consumption of 2a was monitored by TLC and GC–MS. After 5 h, aqueous NaOH solution (0.7 M, 10 ml) was added and stirred for 30 min. The exclusive regeneration of spiroaziridine 1a was confirmed by MS analysis. No spirooxazolidinone 2a and iodoamine 4a were detected in MS analysis at this stage. The crude solution containing spiroaziridineoxindole 1a was further used for the chemical fixation of CO2. So, the slow stream of CO2 was passed through the solution for 30 min. The GC–MS analysis of the crude mixture with naphthalene as an internal standard showed quantitative formation of spirooxazolidinone 2a (98%). Usual work and flash column chromatographic purification as discussed in general procedure gave the compound 2a (143 mg, 95%).

CO2-defixation and fixation cycles through in situ regeneration of spiroaziridine 1a and the isolation of spirooxazolidinone 2a

To a stirred solution of spirooxazolidinone 2a (150 mg, 0.69 mmol) in dry dioxane (6 ml), sodium iodide (414 mg, 2.76 mmol) and o-phosphoric acid (144 μl, 2.76 mmol) were successively added at 70 °C and the reaction (consumption of 2a) was monitored by TLC. After complete consumption of 2a (5 h), it was brought to 0 °C and solid NaOH powder (390 mg, 9.75 mmol) was added to the reaction mixture. After attaining rt, it was stirred for additional 1 h. The stream of 100% CO2 was passed through to the suspended mixture for 1 h at rt. The solid mass was filtered off and washed with dioxane (2 × 5 ml). The combined organic solvent was evaporated to dryness under reduced pressure. The crude compound was dissolved in dioxane (6 ml) and 150 μl of the solution was taken out for the GC–MS analysis with naphthalene (5 mg) as an internal standard. The analysis showed 97% yield of the spirooxazolidinone 2a. So the calculated amount of resynthesized 2a was found to be 148.5 mg and 150 μl of the solution contained 3.7 mg of 2a. The resynthesized compound 2a (148.5 − 3.7 = 144.8 mg) was used for the second cycle for the regeneration of spiroaziridine and the fixation of CO2 using the same procedure as mentioned above i.e. the use of NaI-H3PO4, solid NaOH and the stream of CO2. The GC-yield of the second cycle was observed to be 96%. Similarly, another three cycles were carried and the GC-yields were found to be 99%, 97% and 95%, respectively.

One-pot CO2-defixation and fixation cycles without isolation of re-synthesized spirooxazolidinone

The one-pot CO2-defixation and fixation cycles were carried out following the similar procedure as above without separating out the solid by-products and isolation of re-synthesized spirooxazolidinone in the intermediate cycles.

To a stirred solution of spirooxazolidinone 2a (150 mg, 0.69 mmol) in dry dioxane (6 ml), sodium iodide (414 mg, 2.76 mmol) and o-phosphoric acid (144 μl, 2.76 mmol) were successively added at 70 °C and the reaction (consumption of 2a) was monitored by TLC. After complete consumption of 2a (5 h), it was brought to rt and solid NaOH powder (390 mg, 9.75 mmol) was added to the reaction mixture at 0 °C. After attaining to rt, it was stirred for additional 1 h. The stream of CO2 was passed through to the suspended mixture for 1 h at rt. The complete consumption of in situ regenerated spiroaziridine and the formation of spirooxazolidinone 2a were monitored by TLC and MS analysis. Without separating out the solid mass and the isolation of spirooxazolidinone, another consecutive four cycles were repeated by adding the same amount of sodium iodide and o-phosphoric acid followed by solid NaOH and the stream of CO2 for each cycle in the same pot. The consumption of the intermediate substrate and regeneration of the product were monitored during each cycles by TLC and MS analysis. At the end of 5th cycles, the solid mass was filtered off and washed with dioxane (3 × 10 ml). The combined organic solvent was evaporated to dryness under reduced pressure. The crude compound was dissolved in dioxane (6 ml) and the GC–MS analysis with naphthalene as an internal standard showed 95% overall yield of the spirooxazolidinone 2a for the five cycles. The silica gel flash column chromatographic purification of the crude with hexanes-EtOAc (1:1) gave the spirooxazolidinone 2a (134.9 mg, 90% overall yield) as a white solid.

Recycling of spiroaziridine and NaI for the fixation- and defixation of CO2

To a stirred solution of spirooxazolidinone 2a (150 mg, 0.69 mmol) in dry dioxane (6 ml), sodium iodide (414 mg, 2.76 mmol) and o-phosphoric acid (144 μl, 2.76 mmol) were successively added at rt and the reaction (consumption of 2a) was monitored by TLC. After complete consumption of 2a (5 h), solid NaOH powder (342 mg, 8.6 mmol) was added to the reaction mixture at 0 °C. After attaining to rt, it was stirred for additional 1 h. The stream of CO2 was passed through to the suspended mixture for 1 h at rt. The complete consumption of in situ regenerated spiroaziridine and the formation of spirooxazolidinone 2a (97% GC yield) were monitored by TLC and MS analysis. For the next cycle, the reaction mixture was acidified with o-phosphoric acid (292 μl, 5.6 mmol) and stirred at 70 °C without further addition of sodium iodide. After complete consumption of the spirooxazolidinone (monitored with TLC), solid NaOH powder was added (694 mg, 17.36 mmol) and the stream of CO2 was passed through for 1 h to reproduce the spirooxazolidinone 2a (98%, GC yield). This process was repeated for five consecutive cycles. GC–MS analysis showed almost quantitative yield of spirooxazolidinone in each stage and finally the spirooxazolidinone 2a (135.0 mg, 90%) was isolated after fifth cycle by flash chromatography using hexanes-EtOAc (1:1).

Note (a) GC yield is determined by using naphthalene as internal standard; (b) the release of CO2 from spiroxazolidinone and its subsequent regeneration using CO2 fixation is considered as one complete cycle. (c) At constant temperature (70 °C) five consecutive cycles of CO2 fixation and defixation was accomplished using above method (Supplementary material; General procedure 2). GC yield in resynthesis of spirooxazolidione 2a was monitored at each stage (Fig. 3).