Open Access Article
This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence

Activating harmful small molecules under mild conditions: theoretical insights into cinchonine-based valorization of CO2, CS2, and COS

Lucia Invernizzia, Caterina Damiano*a, Gabriele Manca*b and Emma Galloa
aDepartment of Chemistry, University of Milan, Via C. Golgi 19, 20133 Milan, Italy. E-mail: caterina.damiano@unimi.it
bIstituto di Chimica dei Composti Organo Metallici – CNR-ICCOM Sede Secondaria di Bari, c/o Dipartimento di Chimica, Università degli Studi di Bari, Via Orabona 4, 70126 Bari, Italy. E-mail: gmanca@iccom.cnr.it

Received 6th August 2025 , Accepted 11th September 2025

First published on 11th September 2025


Abstract

DFT calculations have been employed to deeply investigate the mechanism of CO2 cycloaddition to aziridines catalyzed by cinchonine hydrochloride salt, forming oxazolidin-2-ones under ambient conditions (room temperature, 0.1 MPa CO2). Computed energy barriers align with experimental observations and support a dual activation mechanism involving hydrogen bonding and nucleophilic attack at the aziridine carbon atom. The theoretical study also accounts for the observed regioselectivity, rationalizing the preference for nucleophilic attack at the more substituted aziridine carbon atom. Consistent with experimental findings, the calculations reveal that the reaction efficiency is influenced by the nature of the substituent at the aziridine nitrogen atom, explaining the lack of reactivity observed with N-aryl aziridines due to steric and electronic factors that hamper the reaction. Furthermore, the DFT study suggests that COS and CS2 can be activated for analogous cycloaddition reactions. Although these transformations involve higher energy barriers compared to CO2 cycloaddition, the formation of oxazolidin-2-thiones and thiazolidin-2-thiones is predicted to be feasible under slightly elevated temperatures (for CS2) or near-ambient conditions (for COS). These findings highlight the potential of cinchonine hydrochloride salt as an efficient, biocompatible and cost-effective catalyst for the sustainable valorization of small harmful molecules under mild conditions.


Introduction

The activation of small molecules has long been a central theme in chemical research, driven by the challenge of breaking strong bonds in simple, thermodynamically stable species. Historically, much of this work has focused on inert molecules such as N2, O2 and CH4, which are starting materials in processes like nitrogen fixation, oxidation reactions, and hydrocarbon functionalization.1,2 During the last decades, scientific interest has progressively moved toward carbon dioxide. Although CO2 is similarly unreactive, it presents a distinctive combination of scientific and environmental urgency. Its massive atmospheric abundance, mainly due to anthropogenic emissions, and its well-established role in global warming have made its activation a priority in the context of sustainable chemistry. As a result, CO2 is now viewed not only as a waste product but also as a potential carbon feedstock, whose efficient transformation could contribute to both climate mitigation and resource circularity.

Advances in catalytic3–5 and electrochemical methods6–8 have enabled the efficient conversion of CO2 into a variety of useful chemicals9 and fuels10 such as carbon monoxide, formic acid, methanol, methane, and a range of value-added compounds. In light of the development of a circular and sustainable economy, the establishment of new and efficient processes for CO2 valorization has become interesting for the synthesis of fine chemicals. One particularly intriguing reaction is the 100% atom-efficient cycloaddition of CO2 into organic substrates, such as epoxides11,12 and aziridines,13–20 to produce high-value chemicals like cyclic carbonates and oxazolidin-2-ones, respectively, which have several applications including as pharmaceutical agents.21–23 Classic CO2 activation typically requires the co-presence of an electrophilic and a nucleophilic promoter. In the synthesis of oxazolidin-2-ones, the electrophile, generally a metal center, interacts with the nitrogen aziridine atom to render one of the two carbon atoms more susceptible to nucleophilic attack. The consequent cleavage of the C–N bond transfers electron density to the nitrogen atom, which in turn becomes capable of activating the CO2 molecule (Scheme 1). The need for both electrophilic and nucleophilic functionalities led to the development of various binary and bifunctional metal-based catalysts for the CO2 cycloaddition to aziridines.4–10,13 More recently, metal-free systems,14,15,18,19,24 which integrate both functional components enabling efficient aziridine activation and ring-opening without the use of external metal catalysts, have also been extensively developed.


image file: d5nj03189c-s1.tif
Scheme 1 General reaction mechanism of CO2 cycloaddition to aziridines (left); CO2, COS and CS2 cycloaddition products to aziridines (right).

Although CO2 has received widespread attention as a platform molecule for sustainable synthesis, its thio-analogues, carbon disulfide (CS2) and carbonyl sulfide (COS), have remained largely overlooked.

Since the early 1970s, when the reactivity of CS2 toward aziridines was first investigated,25,26 research on COS and CS2 has primarily focused on their interactions with metal complexes.27,28 Only in recent years, the interest in these triatomic molecules has expanded, with growing attention to their cycloaddition reactions with epoxides and aziridines, particularly via metal–organic framework (MOF)-based catalytic systems in combination with nucleophilic co-catalysts.29–32 The limited experimental examples reported for the CS2 cycloaddition to aziridines rely on expensive binary systems involving amidato divalent lanthanide complexes, among which the most active is represented by the europium derivative used in combination with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).29 Other examples include a cerium-based framework,30 a porous 3D cobalt–organic framework assembled by [Co15] and [Co18] nanocages31 and a 3D material assembled by twisted cylindrical [Dy24] cages.32 In all cases, TBAB (tetrabutylammonium bromide) was the co-catalyst; either the metal centre or the positive channel of the MOF-based material acts as a Lewis acid to activate the aziridine, while the external nucleophile promotes the ring-opening reaction enabling CS2 insertion and product formation. To date, no examples of active bifunctional organocatalytic systems have been reported. The above-described behavior aligns with the structural and electronic similarities between COS, CS2, and CO2, highlighting their potential for analogous activation and transformation pathways.27,29,33 It is important to note that CS2 and COS not only cause environmental effects but can also contribute to various health disorders, particularly affecting the human reproductive and nervous systems.34,35 Therefore, rather than eliminating them through traditional combustion methods, treating and reusing these compounds as carbon and sulfur resources offers an attractive alternative to reduce the presence of toxic sulfur-containing pollutants in the atmosphere. In this view, CS2 and COS could serve as valuable C1-building blocks for the synthesis of sulfur-containing heterocycles,36–38 such as thiazolidin-2-thiones and oxazolidin-2-thiones (Scheme 1), and other functionalized materials,39 offering new opportunities in the field of small-molecule activation and sustainable sulfur chemistry.

It is important to underline that the activation of small molecules, such as CO2, CS2, and COS, should occur under mild reaction conditions to ensure a favorable balance between the energy required for the transformation and the amount of CO2 effectively utilized. If harsh conditions are employed, such as high temperatures, elevated pressures, or energy-intensive inputs, the environmental benefit of using these molecules as feedstocks may be offset by additional CO2 emissions generated during the process. For this reason, the development of catalytic systems capable of operating efficiently under ambient or near-ambient conditions is essential to maximize the net carbon benefit and make these transformations truly viable from both an energetic and environmental standpoint.

Given the growing demand for low-toxicity synthetic procedures, the use of eco-friendly bifunctional organocatalysts has become increasingly attractive. Recently, we reported a combined experimental and computational investigation of the catalytic properties of the metal-free bis-protonated porphyrin TPPH4Cl2 (TPP = dianion of tetraphenylporphyrin) in promoting the cycloaddition of CO2 into N-alkyl aziridines to produce N-alkyl oxazolidin-2-ones at 100 °C and 1.2 MPa of CO2 pressure.16 Computational analysis revealed that the reaction occurred, thanks to porphyrin/aziridine synergic CO2 activation in which the protonated core of porphyrin acts, by establishing hydrogen bonding with the CO2 oxygen atom, as an electrophilic center to facilitate the nucleophilic attack of the aziridine nitrogen atom to the CO2 carbon atom. The study paved the way for developing other bifunctional organocatalytic systems, bearing the same nucleophilic moiety (chloride anion) but different electrophilic NH+-containing species, to promote the synthesis of N-alkyl oxazolidin-2-ones under milder experimental conditions.40 In this context, some of us recently published a study on the catalytic activity of hydrochloride salts of DBU, quinine, and cinchonine,40 which were active at room temperature and 0.1 MPa of CO2 pressure. In view of the very good results achieved, we decided to investigate the electronic/energetic features of the mechanism of these reactions by using DFT calculations. In addition, the potential catalytic activity of cinchonine hydrochloride salt to promote the cycloaddition of other triatomic harmful molecules such as CS2 and COS has been investigated in silico. The obtained computational results pave the way for developing future efficient and eco-compatible catalytic processes for the synthesis of fine chemicals by recycling waste.

Results and discussion

Cycloaddition of CO2 to 1-butyl-2-phenylaziridine (1butyl) promoted by cinchonine hydrochloride (3)

In light of remarkable data published for the synthesis N-alkyl oxazolidin-2-ones under mild experimental conditions,40 detailed computational analysis was carried out on the model CO2 cycloaddition to 1-butyl-2-phenylaziridine (1butyl) yielding 3-butyl-5-phenyloxazolidin-2-ones (2Abutyl + 2Bbutyl) (Scheme 2), catalyzed by the naturally derived cinchonine hydrochloride salt (3). Although particular emphasis was placed on elucidating the reaction mechanism catalyzed by 3, the mechanisms involving hydrochloride salts of DBU and quinine, organocatalysts 4 and 5 respectively, were also investigated and the collected data are reported in the SI. The structures of 3, 4, and 5 organocatalysts were optimized at the B97D-DFT level of theory,41 and are shown in Fig. 1. The solvent effects have been taken into account by using the CPCM model42,43 for acetonitrile that is the solvent employed in experimental studies already published on the CO2 cycloaddition to aziridines.40 Additional methodological details are provided in the SI.
image file: d5nj03189c-s2.tif
Scheme 2 Synthesis of 3-butyl-5-phenyloxazolidin-2-ones 2Abutyl and 2Bbutyl by CO2 cycloaddition to 1-butyl-2-phenylaziridine (1butyl).

image file: d5nj03189c-f1.tif
Fig. 1 Optimized structure of hydrochloride salts 3, 4 and 5. The hydrogen atoms were hidden for the sake of clarity, except for those linked to heteroatoms. Selected distances are given in Å.

According to the published experimental results,40 1-butyl-2-phenylaziridine (1butyl) was chosen as the model substrate to investigate the catalytic mechanism of the reaction promoted by 3, and CH3CN was the modeled reaction solvent.

Drawing from prior data on the TPPH4Cl2-mediated reaction,16 the first step of the computational analysis focused on the possible formation of adduct 6butyl in which CO2 is positioned between hydrochloride salt 3 and aziridine 1butyl. As shown in Fig. 2, 6butyl was computationally identified with a free energy cost of +15.0 kcal mol−1 resulting from a balance between the favorable enthalpic contribution of −6.5 kcal mol−1 and a severe unfavorable entropic contribution. The local electrophilicity index (ωK+) of the two aziridine carbon atoms, namely C2 and C3, was estimated by using the method developed by Domingo et al.44


image file: d5nj03189c-f2.tif
Fig. 2 Optimized structure of adduct 6butyl. The hydrogen atoms were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

The calculations revealed that the ωK+ value for C2 is twofold higher than that of C3, suggesting a more favored nucleophilic attack at C2 rather than at the C3 carbon atom. This is in line with the experimentally observed reaction regioselectivity,40 as 3-butyl-5-phenyloxazolidin-2-one (compound 2Abutyl in Scheme 2) was always detected as the major isomer. Consequently, the mechanism yielding isomer 2Bbutyl, deriving from the nucleophilic attack on C3, was not further studied by DFT calculations.

In adduct 6butyl, aziridine nitrogen atom N2 acts as a nucleophile toward CO2, which loses its linearity with the O–C1–O angle reduced to 135°, as confirmed by the appearance of an IR-active vibration at 1759 cm−1, associated with the asymmetric stretching of the C–O bonds. The chloride anion remains distant from the C2 center, with a Cl⋯C2 distance of 3.76 Å whose reduction to 2.80 Å revealed the presence of a transition state, designated TS6-7butyl (Fig. 3).


image file: d5nj03189c-f3.tif
Fig. 3 Optimized structure of transition state TS6-7butyl and intermediate 7butyl. The hydrogen atoms were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

In TS6-7butyl, the approaching chloride anion weakens the N2–C2 bond, which is elongated by 0.15 Å compared to adduct 6butyl. This facilitates the initial shift of electron density toward the aziridine nitrogen N2, enhancing its nucleophilicity toward CO2. This effect is further supported by a 0.07 Å shortening of the N2⋯C1 distance and a more pronounced bending of the CO2 moiety, with the O–C–O angle reduced by 4° with respect to adduct 6butyl. From an energetic perspective, the transition from 6butyl to TS6-7butyl involves an estimated free energy barrier of +4.3 kcal mol−1. The transition-state nature of TS6-7butyl was confirmed by the presence of a single imaginary frequency at −170 cm−1, corresponding to the approach of the chloride ion to the C2 atom and the complete cleavage of the C2–N2 bond. Following TS6-7butyl, the system evolves toward the minimum-energy structure 7butyl (Fig. 3) that displays the complete formation of the C2–Cl bond (1.90 Å) and the corresponding cleavage of the N2⋯C2 bond (2.48 Å). The C1–N2 bond is also fully formed, with a length of 1.42 Å. The formation of intermediate 7butyl was estimated to be exergonic, with a free energy change of −12.6 kcal mol−1. At this point, the electron density originally localized on the N2 center has shifted toward the oxygen atoms of the original CO2 moiety, leading to a strengthening of the O⋯H hydrogen bond, as indicated by a 0.23 Å shortening compared to the corresponding distance in TS6-7butyl.

As confirmed by this O⋯H shortening, the oxygen atom of intermediate 7butyl becomes electron-rich and capable of performing a nucleophilic attack to the C2 center yielding oxazolidin-2-one 2Abutyl by a ring-closure step and the regeneration of catalyst 3. Intermediate 8butyl (Fig. 4) was achieved through the formation of the transition state TS7-8butyl, whose structure was computationally identified (Fig. 4).


image file: d5nj03189c-f4.tif
Fig. 4 Optimized structure of transition state TS7-8butyl, intermediate 8butyl and product 2Abutyl. The hydrogen atoms were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

In TS7-8butyl, the O–C2–Cl moiety adopts a quasi-linear arrangement with an angle of 165°, and the C2 atom approaches a quasi-planar geometry. The nucleophilic approach of oxygen atom to C2 initiates the displacement of the chloride ion with an associated free energy barrier of +7.4 kcal mol−1. The transition state nature of TS7-8butyl is supported by the presence of a single imaginary frequency at −216 cm−1, corresponding to the formation of the O–C2 bond and the cleavage of the C2–Cl bond. Subsequently, intermediate 8butyl is obtained with a free energy gain of −20.1 kcal mol−1 (Fig. 4).

The complete release of 3-butyl-5-phenyloxazolidin-2-one (2Abutyl) from 8butyl is exergonic by −5.1 kcal mol−1 accompanied by the restoration of the salt 3, able to perform the activation of a new aziridine moiety. The overall free energy gain, associated with the complete catalytic cycloaddition of CO2 to 1-butyl-2-phenyl aziridine 1butyl yielding 2Abutyl, was estimated to be exergonic by −11.2 kcal mol−1 (Fig. 5).


image file: d5nj03189c-f5.tif
Fig. 5 Free energy (kcal mol−1) pathway for the cycloaddition reaction of CO2 to 1-butyl-2-phenyl aziridine 1butyl yielding 3-butyl-5-phenyloxazolidin-2-one 2Abutyl.

The energy profile of the reaction (Fig. 5) reveals that the largest cost for the synthesis of 2Abutyl is +19.3 kcal mol−1, with the main disfavoring contribution associated with the formation of the initial adduct 6butyl. For comparison purposes, the cycloaddition of CO2 to 1butyl was also investigated in the presence of catalysts 4 and 5. The free energy pathways for the three catalytic processes are reported in the SI (Fig. S1). No substantial changes in free energy were identified, except for the free energy stabilization of compounds analogous to intermediate 7butyl, namely 7’butyl (involving catalyst 4) and 7”butyl (involving catalyst 5) within 6 kcal mol−1. The maximum free energy costs are within +20.8 kcal mol−1, corresponding to the formation of compounds 6’butyl and 6”butyl, analogous to the initial adduct 6butyl, and to the subsequent transition states TS6’-7’butyl and TS6”-7”butyl. The energy barriers for TS6’-7’butyl and TS6”-7”butyl fall within the range of +6.2 to +9.2 kcal mol−1. All the free energy values associated with the single step of the processes are listed in Fig. S2.

Effect of the N-aziridine substituent on the reaction efficiency

Already published experimental data40 highlighted that the steric and electronic nature of the substituent on the aziridine nitrogen center may drastically influence the reaction efficiency. No conversion was observed when 1-(3,5-bis(trifluoromethyl)phenyl)-2-phenyl aziridine (1aryl) was used in the CO2 cycloaddition catalyzed by 3. In contrast, when a cyclohexyl substituent was present at the nitrogen atom, the reaction proceeded with a low efficiency, affording the corresponding oxazolidin-2-one in 21% yield.40 To clarify the influence of the substituent at the aziridine nitrogen atom on the reaction efficiency, DFT computational analysis was carried out. The study was run as already described for the reaction involving 1-butyl-2-phenylaziridine 1butyl. First, two adducts with structures similar to that of 6butyl were optimized by reacting CO2 and 3 either with 1-(3,5-bis(trifluoromethyl)phenyl)-2-phenylaziridine (1aryl) or 1-cyclohexyl-2-phenylaziridine (1cyhexyl), obtaining 6aryl and 6cyhexyl respectively. As shown in Fig. 6, the optimized structure of 6aryl displays an N2⋯C1 distance as large as 3.02 Å, as well as a very weak hydrogen bond between H(NH+) and O(CO2), with an H⋯O distance of 2.64 Å.
image file: d5nj03189c-f6.tif
Fig. 6 Optimized structure of adducts 6aryl and 6cyhexyl, presenting 1-(3,5-bis(trifluoromethyl)phenyl)-2-phenylaziridine and 1-cyclohexyl-2-phenylaziridine, respectively. The hydrogen atoms were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

These features, together with the nearly unperturbed linear structure of the CO2 moiety, suggest that the occurrence of the process is unlikely when an aryl substituent is present at the N2 aziridine center, in line with the experimental results. For this reason, the CO2 cycloaddition to 1-(3,5-bis(trifluoromethyl)phenyl)-2-phenylaziridine was not investigated further. Conversely, when 1-cyclohexyl-2-phenylaziridine is the involved substrate, the adduct 6cyhexyl exhibits an activated CO2 characterized by a bent O–C–O structure with an angle of 135° and a computed IR-active stretching at 1746 cm−1. From the energy viewpoint, the formation of adduct 6cyhexyl requires a free energy cost of +19.2 kcal mol−1, higher than that needed for achieving adduct 6butyl from 1-butyl-2-phenylaziridine (+15 kcal mol−1). Even in this case, the obtained results are in line with experimental data, which highlighted a less efficient process when 1-cyclohexyl-2-phenylaziridine was employed as the starting material instead of 1-butyl-2-phenylaziridine.

The free energy barrier for conversion of 6cyhexyl to the transition state TS6-7cyhexyl, whose optimized structure is reported in Fig. S3, is estimated to be +6.5 kcal mol−1 while the overall energy barrier for transforming the starting reagents into TS6-7cyhexyl is +25.7 kcal mol−1, a quite high value for a reaction performed at room temperature. The whole free energy associated with the formation of oxazolidin-2-one 2Acyhexyl from 1-cyclohexyl-2-phenylaziridine (1cyhexyl) and CO2 is less exergonic than that of the same process involving 1-butyl-2-phenyl aziridine 1butyl (-7.5 kcal mol−1 versus −11.2 kcal mol−1). The complete energy profile of the CO2 cycloaddition to 1-cyclohexyl-2-phenyl aziridine is reported in Fig. S4.

Cycloaddition of CS2 to 1-butyl-2-phenylaziridine (1butyl) promoted by cinchonine hydrochloride (3)

A precedent computational analysis has predicted the feasibility, at least in silico, of the CS2 cycloaddition to N-alkyl aziridines to provide thiazolidin-2-thiones in the presence of bifunctional TPPH4Cl2.45 The study revealed a free energy contribution of −22.1 kcal mol−1 associated with the cycloaddition of CS2 to 1-butyl-2-phenylaziridine 1butyl to provide the corresponding thiazolidin-2-thione 9Abutyl.45 Based on these results, we studied the reaction between CS2 and 1-butyl-2-phenylaziridine 1butyl also in the presence of catalyst 3. In this regard, the adduct 10butyl (Fig. 7) was optimized.
image file: d5nj03189c-f7.tif
Fig. 7 Optimized structure of adduct 10butyl. The hydrogen atoms were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

Despite the favorable enthalpic contribution of −2.1 kcal mol−1, adduct 10butyl was optimized with a free energy cost of +21.5 kcal mol−1. It should be noted that the energy cost is 6.5 kcal mol−1 larger than that of the analogous process involving CO2 (formation of adduct 6butyl), possibly due to a weaker hydrogen bonding between the proton of 3 and the sulfur atom of CS2, as confirmed by the long S–H distance of 2.28 Å.

After the formation of adduct 10butyl, the activated aziridine substrate can be attacked by a chloride nucleophile through the transition state TS10-11butyl (Fig. 8) with a free energy barrier of +7.7 kcal mol−1. The overall barrier of +29.2 kcal mol−1 for the conversion of the reactants into TS10-11butyl was larger than that calculated for CO2 activation and it can be overcome by performing the reaction at higher experimental temperatures. Transition state TS10-11butyl features a quasi-linear Cl–C2–N2 arrangement with an angle of 151° and a weakened C2–N2 bond, whose length is stretched by 0.15 Å compared to that in 10butyl. In TS10-11butyl, the chloride approaches the C2 atom with a consistent shortening of Cl⋯C2 distance by 1.0 Å.


image file: d5nj03189c-f8.tif
Fig. 8 Optimized structure of transition state TS10-11butyl and of intermediate 11butyl. The hydrogen atoms were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

The complete formation of the C2–Cl bond in intermediate 11butyl (Fig. 8) was estimated to be exergonic by −25.9 kcal mol−1.

Similar to the CO2 activation, the sulfur atom in 11butyl may perform a nucleophilic attack on the C2 center to form the 5-membered ring and regenerate the catalyst 3. The transition state TS11-12butyl, shown in Fig. S5, was obtained with a free energy barrier of +13.6 kcal mol−1, while the intermediate 12butyl (Fig. 9) was obtained with the large free energy gain of −29.9 kcal mol−1.


image file: d5nj03189c-f9.tif
Fig. 9 Optimized structure of the intermediate 12butyl and thiazolidin-2-thione, 9Abutyl. The hydrogens were hidden for the sake of clarity, except for those linked to a heteroatom. Selected distances are given in Å and O–C1–O angle in degrees (°).

The complete release of thiazolidin-2-thione 9Abutyl, shown in Fig. 9, was achieved with a further free energy gain of −9.1 kcal mol−1. The overall formation of compound 9Abutyl by CS2 cycloaddition to aziridine 1butyl is depicted in Fig. 10.


image file: d5nj03189c-f10.tif
Fig. 10 Free energy (kcal mol−1) pathway for the cycloaddition reaction of CS2 to 1-butyl-2-phenyl aziridine 1butyl forming thiazolidin-2-thione 9Abutyl.

A comparison between the energy profiles of CS2 (Fig. 10) and CO2 (Fig. 5) cycloaddition to 1butyl reveals key differences. While a higher barrier is required for the formation of the initial adduct 10butyl and for reaching the transition state TS10-11butyl in the case of CS2 activation, more energy is released during CS2 activation rather than during CO2 activation. In summary, in silico analysis predicts that the energy barriers for the catalytic cycloaddition of CS2 to aziridine rings are not prohibitively high, although temperatures above room temperature might be necessary to promote the reaction.

Cycloaddition of COS to 1-butyl-2-phenylaziridine (1butyl) promoted by cinchonine hydrochloride (3)

Until now, only the activation of symmetric triatomic molecules has been investigated. This raises the question if the reaction can also proceed when a non-symmetric substrate, such as carbonyl sulfide COS, is used in the cycloaddition to aziridines.

Unlike previous cases, different structural isomers can be obtained starting from 1-butyl-2-phenylaziridine 1butyl and COS. Depending on which heteroatom is involved in the cyclization step, 3-butyl-5-phenylthiazolidin-2-one (13Abutyl) or 3-butyl-5-phenyloxazolidin-2-thione (14Abutyl) can be formed (Fig. 11).


image file: d5nj03189c-f11.tif
Fig. 11 Potential products of the cycloaddition of COS to 1-butyl-2-phenylaziridine (1butyl).

To begin the computational analysis of the reaction between COS and 1butyl, both compounds shown in Fig. 11 were optimized. Isomers 13Bbutyl and 14Bbutyl, derived from the nucleophilic attack to the less electrophilic carbon atom C3 of the aziridine ring, were not theoretically modelled in view of the unfavorable energy costs related to their formation (see below).

Preliminary DFT calculations revealed that product 13Abutyl is more stable than 14Abutyl by 10.4 kcal mol−1 in free energy. Although the energy difference between the two potential products is significant, the processes yielding both isomers were investigated. Given the non-symmetrical nature of COS, both adducts 15butyl (evolving in 13Abutyl) and 16butyl (evolving in 14Abutyl) (Fig. 12) were optimized, featuring the alternative involvement of either oxygen or sulfur in hydrogen bonding, respectively. Adduct 15butyl was estimated to be 1.3 kcal mol−1 more stable than 16butyl, mainly due to more efficient hydrogen bonding when oxygen, rather than sulfur, is involved.


image file: d5nj03189c-f12.tif
Fig. 12 Free energy (kcal mol−1) pathway for the cycloaddition reaction of CS2 to 1-butyl-2-phenyl aziridine 1butyl forming thiazolidin-2-thione 9Abutyl. Selected distances are given in Å and O–C1–O angle in degrees (°).

Starting from the reactants, adduct 15butyl is obtained with a free energy cost of +15.9 kcal mol−1, while the formation of 16butyl requires a slightly higher cost of +17.2 kcal mol−1. For adduct 15butyl, the N2–C2 bond cleavage via nucleophilic attack of chloride proceeds through transition state TS15-17butyl (Fig. S6) with an associated free energy barrier of +5.8 kcal mol−1. The system then evolves toward intermediate 17butyl (Fig. 13), with a free energy gain of −16.7 kcal mol−1. Accordingly, an overall energy barrier of +21.7 kcal mol−1 must be overcome to reach TS15-17butyl from the separate reactants.


image file: d5nj03189c-f13.tif
Fig. 13 Optimized structure of the intermediate 17butyl. The hydrogens were hidden for the sake of clarity, except for those linked to a heteroatom center. Selected distances are given in Å and O–C1–O angle in degrees (°).

Accordingly, an overall energy barrier of +21.7 kcal mol−1 must be overcome to reach TS15-17butyl from the separate reactants. The potential nucleophilic attack of chloride on the C3 center has been also investigated highlighting a free energy barrier of +17.8 kcal mol−1 to obtain TS15-17butylC3, shown in Fig. S6, from adduct 15butyl. Since the overall estimated free energy barrier for reaching TS15-17butylC3 is as high as +33.7 kcal mol−1, this mechanism was discarded.

In the case of adduct 16butyl, where the sulfur atom is involved in hydrogen bonding, the estimated free energy barrier for reaching TS16-18butyl is +11.5 kcal mol−1, resulting in an overall barrier of +28.7 kcal mol−1 from the isolated reactants to TS16-18butyl. The system then proceeds to intermediate 18butyl, with a free energy gain of −25.4 kcal mol−1. Thus, in view of the quite high calculated barrier compared to that of TS15-17butyl, the energy pathway for the formation of 14Abutyl was discarded.

Starting from 17butyl, the transition state TS17-19butyl was computed with a free energy barrier of +7.9 kcal mol−1, in which the sulfur center of COS can perform a nucleophilic attack to the C2 center yielding the intermediate 19butyl that evolves into the final product 3-butyl-5-phenylthiazolidin-2-one 13Abutyl. The complete release of 13Abutyl and restoration of catalyst 3 is accompanied by a free energy gain of −6.9 kcal mol−1. A complete free energy pathway for the production of 3-butyl-5-phenylthiazolidin-2-one 13Abutyl, starting from COS and aziridine 1butyl promoted by metal-free 3, is depicted in Fig. 14 and shows an overall free energy gain of −20.8 kcal mol−1.


image file: d5nj03189c-f14.tif
Fig. 14 Free energy (kcal mol−1) pathway for the cycloaddition reaction of COS to 1-butyl-2-phenyl aziridine 1butyl forming 13Abutyl.

To better summarize and compare the reactivity of the investigated triatomic molecules, the energy profiles of their cycloaddition to 1-butyl-2-phenyl aziridine 1butyl catalyzed by 3 were superimposed, as shown in Fig. 15.


image file: d5nj03189c-f15.tif
Fig. 15 Superimposed free energy (kcal mol−1) profiles of the cycloaddition reaction of CO2, COS and CS2 to 1-butyl-2-phenyl aziridine 1butyl catalyzed by 3.

A clear difference in overall energy gains can be observed between the formation of oxazolidine-2-one 2Abutyl and its sulfur-containing analogues (9Abutyl and 13Abutyl). The overall energy gains calculated for CS2 and COS cycloaddition to 1butyl were nearly twice as high as that for CO2 (−20.8 kcal mol−1 and −22.1 kcal mol−1 versus −11.2 kcal mol−1), suggesting a thermodynamic preference for the formation of oxazolidin-2-thiones and thiazolidin-2-thiones over oxazolidin-2-ones.

However, the activation barrier computed for the formation of the key transition state was higher for CS2 (+29.2 kcal mol−1) compared to CO2 (+19.3 kcal mol−1), indicating that more forcing reaction conditions may be required to achieve the product formation. In contrast, the energy barrier to reach the first transition state from COS is only slightly larger than that calculated for CO2 (+21.7 kcal mol−1 versus +19.3 kcal mol−1), suggesting that its cycloaddition could proceed efficiently under the similar mild conditions successfully employed for the 3-catalyzed oxazolidin-2-one synthesis. These findings support the feasibility of COS cycloaddition to aziridines promoted by catalyst 3 under experimental conditions only slightly more drastic than those validated for CO2 transformations.

Conclusions

In conclusion, the energetic/structural DFT analyses suggest a mechanism for the CO2 cycloaddition to the aziridine ring for the synthesis of oxazolidin-2-ones, efficiently promoted by cinchonine hydrochloride 3 under very mild conditions (RT and 0.1 MPa of CO2 pressure). The calculated energy barriers are compatible to the employed experimental conditions and the DFT study confirmed the double activation of CO2 through hydrogen bonding interactions and nucleophile attack of the aziridine nitrogen atom to the CO2 carbon atom. The theoretical analysis explains the dependence of the catalytic efficiency on the steric hindrance and/or electronic effects of the N-aziridine substituents, providing a rationale for the lack of reactivity of N-aryl aziridines, observed experimentally.

To extend the activation of triatomic molecules mediated by 3 from CO2 to CS2 and COS, the DFT study reported here will be fundamental for managing in near future experimental reactions involving CS2 and COS, which until now have only been theoretically predicted. The computational analysis revealed that catalyst 3 could be a potential candidate for efficiently mediating both CS2 and COS activation. Even if theoretical calculations underlined that both processes involve higher energy barriers than those optimized for the CO2 activation, acquired data support a future experimental study on the CS2 and COS cycloaddition to aziridine in the presence of 3. By comparing the calculated energy barriers, the cycloaddition of COS is predicted to be feasible at temperatures close to the ambient one, under conditions therefore similar to those observed for the 3-catalyzed valorization of CO2. In contrast, the use of CS2 would require slightly higher temperatures. In both cases, the synthesis of thiazolidin-2-thiones and oxazolidin-2-thiones is predicted to be feasible using an inexpensive and biocompatible catalyst, such as cinchonine hydrochloride salt (3), under mild reaction conditions.

Conflicts of interest

There are no conflicts to declare.

Data availability

The datasets supporting this article are included in the SI. See DOI: https://doi.org/10.1039/d5nj03189c.

Additional data are available from the corresponding author upon reasonable request.

Acknowledgements

CD and EG thank Università degli Studi di Milano for the PSR 2023 grants. GM acknowledges the CINECA award under the ISCRA initiative, for the availability of high-performance computing resources and support.

References

  1. X. B. Li, Z. K. Xin, S. G. Xia, X. Y. Gao, C. H. Tung and L. Z. Wu, Chem. Soc. Rev., 2020, 49, 9028–9056 RSC.
  2. L. Wang, W. Chen, D. Zhang, Y. Du, R. Amal, S. Qiao, J. Wu and Z. Yin, Chem. Soc. Rev., 2019, 48, 5310–5349 RSC.
  3. D. Intrieri, C. Damiano, P. Sonzini and E. Gallo, J. Porphyrins Phthalocyanines, 2019, 23, 305–328 CrossRef CAS.
  4. C. Damiano, M. Cavalleri, L. Invernizzi and E. Gallo, Eur. J. Org. Chem., 2024, e202400616 CrossRef CAS.
  5. T. Yan, H. Liu, Z. X. Zeng and W. G. Pan, J. CO2 Util., 2023, 68, 102355 CrossRef CAS.
  6. K. Wiranarongkorn, K. Eamsiri, Y. S. Chen and A. Arpornwichanop, J. CO2 Util., 2023, 71, 102477 CrossRef CAS.
  7. L. Li, X. Li, Y. Sun and Y. Xie, Chem. Soc. Rev., 2022, 51, 1234–1252 RSC.
  8. L. Rotundo, R. Gobetto and C. Nervi, Curr. Opin. Green Sustainable Chem., 2021, 31, 100509 CrossRef CAS.
  9. Q. Zhang and X. Jin, Chem. – Eur. J., 2025, 31, e202500933 CrossRef PubMed.
  10. M. Aresta, A. Dibenedetto and E. Quaranta, J. Catal., 2016, 343, 2–45 CrossRef.
  11. S. Kaewsai and V. D’ Elia, J. Organomet. Chem., 2025, 1039, 123799 CrossRef.
  12. W. Natongchai, D. Crespy and V. D’ Elia, Chem. Commun., 2025, 61, 419 RSC.
  13. S. Arayachukiat, P. Yingcharoen, S. V. C. Vummaleti, L. Cavallo, A. Poater and V. D’Elia, Molecular Catalysis, 2017, 443, 280–285 CrossRef.
  14. P. Yingcharoen, W. Natongchai, A. Poater and V. D’ Elia, Catal. Sci. Technol., 2020, 10, 5544–5558 RSC.
  15. C. Damiano, P. Sonzini, G. Manca and E. Gallo, Eur. J. Org. Chem., 2021, 2807–2814 CrossRef.
  16. M. Cavalleri, C. Damiano, G. Manca and E. Gallo, Chem. – Eur. J., 2023, 29, e202202729 CrossRef PubMed.
  17. C. Damiano, P. Sonzini, M. Cavalleri, G. Manca and E. Gallo, Inorg. Chim. Acta, 2022, 540, 121065 CrossRef.
  18. P. Sonzini, C. Damiano, D. Intrieri, G. Manca and E. Gallo, Adv. Synth. Catal., 2020, 362, 2961–2969 CrossRef.
  19. P. Sonzini, N. Berthet, C. Damiano, V. Dufaud and E. Gallo, J. Catal., 2022, 414, 143–154 CrossRef.
  20. C. Damiano, A. Fata, M. Cavalleri, G. Manca and E. Gallo, Catal. Sci. Technol., 2024, 14, 3996–4006 RSC.
  21. A. Z. Bialvaei, M. Rahbar, M. Yousefi, M. Asgharzadeh and H. S. Kafil, J. Antimicrob. Chemother., 2017, 72, 354–364 CrossRef CAS.
  22. D. McBride, T. Krekel, K. Hsueh and M. J. Durkin, Expert Opin. Drug Metab. Toxicol., 2017, 4, 491 Search PubMed.
  23. F. Moureau, J. Wouters, D. Vercauteren, S. Collin, G. Evrard, F. Durant, F. Ducrey, J. Koenig and F. Jarreau, Eur. J. Med. Chem., 1992, 27, 939–948 CrossRef CAS.
  24. G. Bresciani, M. Bortoluzzi, G. Pampaloni and F. Marchetti, Org. Biomol. Chem., 2021, 19, 4152–4161 RSC.
  25. C. S. Dewey and R. A. Bafford, J. Org. Chem., 1965, 30, 491–495 CrossRef CAS.
  26. T. A. Foglia, L. M. Gregory, G. Maerker and S. F. Osman, J. Org. Chem., 1971, 36, 1068–1072 CrossRef CAS.
  27. K. K. Pandey, Coord. Chem. Rev., 1995, 140, 37–114 CrossRef CAS.
  28. M. Guo, B. Dong, Y. Qu, Z. Sun, L. Yang, Y. Wang, I. L. Fedushkin and X. J. Yang, Chem. – Eur. J., 2025, 31, e202403652 CrossRef CAS PubMed.
  29. Y. Xie, C. Lu, B. Zhao, Q. Wang and Y. Yao, J. Org. Chem., 2019, 84, 1951–1958 CrossRef CAS PubMed.
  30. Y. Shi, D. Wen and S. Q. Zhao, Inorg. Chem., 2025, 64, 4387–4392 CrossRef CAS PubMed.
  31. W. Ding, X. Tang, S. Jin, Z. Li, D. Xu, X. Kang and Z. Liu, Green Chem., 2024, 27, 218–226 RSC.
  32. Y. Shi, B. Tang, X.-L. Jiang, Y.-E. Jiao, H. Xu and B. Zhao, J. Mater. Chem. A, 2022, 10, 4889–4894 RSC.
  33. A. J. Plajer and C. K. Williams, Angew. Chem., Int. Ed., 2022, 61, e2021044952022 CrossRef.
  34. K. Sieja, J. von Mach-Szczypiński and J. von Mach-Szczypiński, Med. Pr., 2018, 69, 317–323 CrossRef.
  35. A. W. Demartino, D. F. Zigler, J. M. Fukuto and P. C. Ford, Chem. Soc. Rev., 2017, 46, 21–39 RSC.
  36. R. Morales-Nava, M. Fernández-Zertuche and M. Ordóñez, Molecules, 2011, 16, 8803–8814 CrossRef CAS.
  37. A. Khalaj and M. Khalaj, J. Chem. Res., 2016, 40, 445–448 CrossRef CAS.
  38. A. Biswas and S. Hajra, Adv. Synth. Catal., 2022, 364, 3035–3042 CrossRef CAS.
  39. M. Sengoden, G. A. Bhat and D. J. Darensbourg, Green Chem., 2022, 24, 2535–2541 RSC.
  40. L. Invernizzi, C. Damiano and E. Gallo, Chem. – Eur. J., 2025, 31, e202500473 CrossRef CAS PubMed.
  41. S. Grimme, J. Comp. Chem., 2006, 27, 1787–17991 CrossRef CAS PubMed.
  42. V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–20013 CrossRef CAS.
  43. M. Cossi, N. Rega, G. Scalmani and V. Barone, J. Comp. Chem., 2003, 24, 669–681 CrossRef CAS.
  44. L. R. Domingo, M. Ríos-Gutiérrez and P. Pérez, Molecules, 2016, 21, 7482016 Search PubMed.
  45. C. Damiano, N. Panza, J. Nagy, E. Gallo and G. Manca, New J. Chem., 2023, 47, 4306–4312 RSC.

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2025
Click here to see how this site uses Cookies. View our privacy policy here.