Dehydrative condensation of β-aminoalcohols with CO2: An environmentally benign access to 2-oxazolidinone derivatives

Journal of CO₂ Utilization 25 (2018) 194–204

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Dehydrative condensation of β-aminoalcohols with CO2: An environmentally benign access to 2-oxazolidinone derivatives

T

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Sepideh Farshbaf a, Leila Zare Fekrib, , Mohammad Nikpassandc, Robab Mohammadib, Esmail Vessallyb a

Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA Department of Chemistry, Payame Noor University, P.O. Box, 19395-1697, Tehran, Iran c Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran b

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon dioxide 2-oxazolidinones β-amino alcohols Dehydrative condensation Metal catalysts

In this review, we try to provide a comprehensive and updated overview of recent developments in the synthesis of various 2-oxazolidinones through dehydrative condensation of easily available β-amino alcohols with carbon dioxide with special emphasis on the mechanistic aspects of the reactions. Literature has been surveyed until the end of 2017.

1. Introduction Needless to say, global warming and ocean acidification is primarily a problem of too much carbon dioxide (CO2) in the Earth’s atmospherewith around 300 billion tons [1]. This climate change could accelerate in years to come, as many scientists predict, causing a serious threat to all life on the planet [2], hence there is an urgent need to reduce the accumulation of this greenhouse gas in the atmosphere. Carbon dioxide capture, storage, and utilization (CCSU) technologies can be a probable and efficient solution for mitigating climate change and recently have attracted significant attention in the research community [3]. From the viewpoint of chemistry, CO2 is considered as a safe, abundant, inexpensive, nontoxic, nonflammable, and renewable C1 resource for the synthesis of a variety of value-added chemicals (e.g. alcohols, carboxylic acids, esters, aldehydes, amides, urethanes, ureas, and carbonates) [4–10]. However, this is a great challenge, owing to its inert nature and low reactivity [11,12]. Accordingly, development of new and highly efficient catalytic systems and synthetic protocols for chemical transformation of CO2 are among the most important topics in modern organic synthesis. Nitrogen-containing heterocyclic compounds are common structural units in marketed drugs [13] and in natural products [14]. These compounds are also very crucial in drug discovery programs. Therefore, the development of novel, practical, and convenient methodologies for the synthesis of this important class of organic compounds is highly desirable [15]. 2-Oxazolidinones are an important class of nitrogen-

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containing heterocycles, which has many applications in health (Fig. 1) [16–18] and in the production of herbicides [19]. Besides their biological importance, these useful heterocycles also have many applications in organic synthesis [20]. Moreover, they are very suitable solvents for lithium-ion batteries [21] and ink-jet printing [22]. There are many reported methods to synthesize 2-oxazolidinone derivatives such as carbonylation of β-amino alcohols with phosgene [23], carbonylation of β-amino alcohols with carbon monoxide [24], the addition of isocyanates to epoxides [25], and the reaction of isocyanates with propargylamines [26]. However, the cost of certain reagents (e.g., aziridines) and the higher toxicity of others (e.g., phosgene, carbon monoxide, and isocyanates) do not allow their preparation to be extended to large-scale production [27]. Hence, the development of safe methods for the efficient synthesis of titled compounds is highly desirable. Replacement of toxic carbonylating agents with CO2 is an ideal alternative. Recently, a series of CO2-based processes for the synthesis of functionalized 2-oxazolidinones have been developed [28], including threecomponent coupling of propargyl alcohols, amines, and CO2, carboxylative cyclization of propargyl amines and CO2, cycloaddition of aziridines with CO2, and dehydrative condensation of β-amino alcohols with CO2. Among these, carbonylation of easily available β-amino alcohols using CO2 is efficient and permits the construction of functionalized and chiral 2-oxazolidinones from simple starting materials with water as a by-product of the reaction (Scheme 1). The range of different synthesis methods have been developed to generate β-amino alcohols.

Corresponding author. E-mail address: [email protected] (L.Z. Fekri).

https://doi.org/10.1016/j.jcou.2018.03.020 Received 29 January 2018; Received in revised form 16 March 2018; Accepted 30 March 2018 Available online 10 April 2018 2212-9820/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. Selected examples of drugs containing 2-oxazolidinone core.

Scheme 1. Dehydrative condensation of β-aminoalcohols with CO2.

Hydrogenation of amino acids [29], reaction of epoxides with amines [30], three-component Mannich reactions of ketones, aldehydes, and amines [31], and catalytic reduction of 2-cyano-, 2-azido-, or 2-nitroacetophenones [32] are among the most general way to prepare βamino alcohols. Synthesis of functionalized 2-oxazolidinones through carbonylation of corresponding β-amino alcohols with CO2 was nicely highlighted by the groups of Pulla [28a] and Tomishige [28b] in their interesting reviews in 2013. In continuation of our reviews on the conversion of CO2 to value-added chemicals [11,28a,b,33], our aim in this review is to try to provide a comprehensive and updated overview of recent developments in the synthesis of 2-oxazolidinone derivatives via dehydrative condensation of β-amino alcohols with carbon dioxide with special emphasis on the mechanistic aspects of the reactions. In this review, we have classified these synthetic reactions based on the type of catalysts. 2. Metal catalyzed condensations In this section, we describe the current literature on metal catalyzed condensation reactions. The Tin-catalyzed reactions are discussed first. This is followed by cerium and silver-catalyzed reactions. Finally, the only reported example for alkali metals catalyzed reactions will be covered at the end of the section. Scheme 2. (a) Sn-catalyzed dehydrative condensation of β-aminoalcohols 4 with CO2 developed by Sasaki; (b) Mechanistic proposal for the formation of 2oxazolidinones 5.

2.1. Tin The possibility of tin catalyzed dehydrative condensation of βamino alcohols with CO2 into corresponding 2-oxazolidinones was first realized by Tominaga and Sasaki, who synthesized a range of highly substituted 2-oxazolidinone derivatives 5 from the reaction of both Nalkylated and N-unsubstituted β-amino alcohols 4 with CO2 (5 MPa) in the presence of 10 mol% of commercially available air-stable nBu2SnO. The reactions were carried out in NMP (N-methyl-2-pyrrolidone) at 180 °C and generally provided the desired products in moderate to excellent yields (Scheme 2a). The results demonstrated that substrates with secondary amines gave higher yields of products than those with primary amines. It is noted that the corresponding 2-oxazolidinones were also formed in the absence of the catalyst; albeit in fair yields. As shown in Scheme 2b, the reaction starts with the formation of a 1,3,2oxazastannolidine intermediate A by reaction between the tin oxide and β-amino alcohol 4. Next, the insertion of CO2 into intermediate A leads to 1,3,5,2-dioxazastannepan-4-one B. Finally, intramolecular nucleophilic attack of alkoxy group on a carbonyl carbon atom in intermediate

B affords the expected 2-oxazolidinones 5 with concomitant loss of the starting tin oxide [34]. In 2013, the group of Ghosh developed a series of 1,3-dichloro1,1,3,3-tetraalkyldistannoxanes catalysts 6 for the dehydrative condensation of β-amino alcohols 7 with CO2 (Fig. 2). The catalytic activity of these various chlorostannoxanes was found to be of the order 6a > 6b > 6c > 6d. Therefore, catalyst 6a was chosen for the synthesis of 2-oxazolidinones. Under optimized conditions [6a (0.4 mol %), CO2 (1.72 MPa), MeOH, 150 °C] a variety of β-amino alcohols react to give corresponding 2-oxazolidinones 8 in moderate yields with high turnover numbers (TON = 138). Notably, the chirality of the starting amino alcohols 7c–e is preserved (up to 99%) under the reaction conditions (Scheme 3) [35]. It should be mentioned that the same authors also successfully applied this catalytic system in the synthesis of fivemembered cyclic carbonates via coupling of epoxide with CO2 [36].

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Fig. 2. Structure of 1,3-dichloro-1,1,3,3-tetraalkyldistannoxanes 6.

Scheme 5. (a) Dehydrative condensation of β-amino alcohols 11 with CO2 using CeO2 as a heterogeneous catalyst; (b) CeO2-catalyzed synthesis of sixmembered-ring cyclic carbamates 14 through carbonylation of corresponding 1,3-amino alcohols 13 with CO2.

complete retention of configuration. Under optimized conditions 1,3amino alcohols 13 were also converted to the synthetically challenging six-membered-ring cyclic carbamates 14 in high yields (Scheme 5b). According to data obtained by kinetics and FTIR studies, the authors proposed that the mechanism for the synthesis of 2-oxazolidinones 12 from CO2 and β-amino alcohols 11 involves the following steps (Scheme 6): (i) formation of carbonate and carbamate adspecies of amino alcohol on CeO2 from starting amino alcohol 11 and CO2; (ii) decomposition of the carbonate moiety to the hydroxyl group and CO2; (iii) nucleophilic addition of the hydroxyl group to the carbamate adspecies on CeO2, providing 2-oxazolidinone-coordinated metal complex; and (iv) desorption of 2-oxazolidinone 12 and regeneration of CeO2 [40].

Scheme 3. Ghosh’s synthesis of 2-oxazolidinones 8.

2.2. Cerium Cerium oxide (CeO2) is one of the most employed catalysts in organic synthesis, owing to its availability, cost efficiency, and versatility [37]. This metal oxide has proven to be high efficient catalysts for a wide variety of carbon dioxide fixation reactions [38]. In 2010, Corma and García along with their co-workers used nanoparticulated ceria as a robust and highly efficient catalyst for the coupling of β-amino alcohols with CO2. Thus, a series of 2-oxazolidinones 10 were synthesized by treatment of aminoethanols 9 with CO2 (0.7 MPa) in refluxing ethanol (Scheme 4). Good to high conversions with good selectivity towards 2oxazolidinones were achieved for N-substituted amino alcohols. However, this protocol for carbonylation of N-unsubstituted amino alcohols was considerably less efficient. It is noteworthy that other metal oxides like MgO, ZrO2, TiO2, Y2O3, and γ-Al2O3 are not effective to afford the desired 2-oxazolidinones by this reaction [39]. In a closely related study, the group of Tomishige showed that a variety of substituted 2-oxazolidinones 12 were successfully formed from β-amino alcohols 11 and carbon dioxide through a CeO2-catalyzed dehydrative condensation reaction using acetonitrile as a solvent at 150 °C. According to the nature of the starting amino alcohols the corresponding cyclic carbamates were obtained in high to almost quantitative yields (Scheme 5a). The reaction is noteworthy in that both N-substituted and N-unsubstituted amino alcohols are well tolerated. It is noted that in the cases of the amino alcohols having a chiral center at the α-position of the hydroxyl group the reaction has occurred with

2.3. Silver Recently, the He laboratory described an interesting one-pot multicomponent approach for the synthesis of functionalized 2-oxazolidinones (Scheme 7). The Ag2CO3/Xantphos catalyzed three-component reaction of 2-aminoethanols 15, terminal propargylic alcohols 16 and carbon dioxide offered a new strategy for the straight forward access to N-substituted 2-oxazolidinone rings 17. The presence of Xantphos as an additive was critical for the success of this reaction, no reaction occurred in the absence of the additive. The reaction took place under relatively mild reaction conditions (1.0 MPa, 60 °C) and tolerated a variety of sensitive functional group, including nitro, hydroxyl, methoxy, and chloro groups, which provided the opportunity to further functionalize the 2-oxazolidinone products using other convenient reactions. These authors demonstrated significant scope of the 2-aminoethanol reagent, but limited scope of the propargylic alcohol substrate; however, the yields were moderate to excellent. The first step of this three-component reaction is the α-alkylidene cyclic carbonate A formation by the Ag(I)-catalyzed carboxylative cyclization of propargylic alcohol 16 with CO2 (Scheme 8). In the next step, a nucleophilic ring-opening reaction of intermediate A with the 2-aminoethanol 15 afforded the β-oxopropylcarbamate intermediate B. Finally the expected product 17 is obtained from the intermediate B through an intramolecular nucleophilic cyclization along with the concurrent formation of synthetically and biologically important α-hydroxyl ketone 18 as a by-product [41]. Very recently, the same authors improved the efficiency of this cascade reaction in the terms of yield and reaction time by performing the process in acetonitrile employing Ag2O as a commercially available

Scheme 4. Nano-CeO2 catalyzed synthesis of 2-oxazolidinones 10 reported by Corma-García. 196

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Scheme 6. Mechanistic proposal for the reaction in Scheme 5.

detect of carbamic acid A as the key intermediate in this reaction by 1H NMR (DMSO-d6). They proposed a mechanism for this transformation is shown in Scheme 10 and indicates that the hydroxyl group of amino alcohol acts as a nucleophile in the CeO bond formation and the hydroxyl group in the carbamic acid moiety is liberated during the cyclization process [43].

inexpensive catalyst and TMG (1,1,3,3-tetramethylguanidine) as a base. On the basis of the control experiments, NMR studies, kinetic curves, and DFT calculation, the authors stated that TMG simultaneously activate both hydroxyl group of propargyl alcohol and CO2. Thus, facilitates the formation of α-alkylidene cyclic carbonate A. then, ring opening of intermediate A through nucleophilic addition by starting 2aminoalcohol 15 gives the β-oxopropylcarbamate intermediate B which is in equilibrium with intermediate C. Finally, intramolecular nucleophilic cyclization of intermediate C yields the expected 2-oxazolidinone 17 (Scheme 9) [42].

3. Electrogenerated base-induced condensations Anionic species generated cathodically often have useful basic properties and have been widely used in synthetic organic chemistry [44]. Such bases commonly called electrogenerated bases (EGBs) and can be radical anions, anions or dianions [45]. Electrochemically promoted incorporation of CO2 into organic compounds is a well-known synthetic tool and has been the subject of a number studies in recent years [46]. In 2000, one of the earliest electrogenerated base-induced dehydrative condensation of β-amino alcohols with CO2 was published by Casadei and co-workers, who showed that the reaction of chiral βamino alcohols 21 with CO2 in the presence of 4 equiv. of 2-pyrrolidone electrogenerated base and 2 equiv. of tosyl chloride at room temperature in MeCN afforded chiral 2-oxazolidinones 22 in good to excellent yields (Scheme 11). It is noted that the reaction has occurred with complete retention of configuration. It should be mentioned that 2pyrrolidone electrogenerated base was prepared ex situ through

2.4. Alkali metals With the objective of designing a milder and greener procedure to 2oxazolidinones through dehydrative condensation of β-amino alcohols with CO2, in 2013, Foo and co-workers were able to demonstrate that a variety of chiral oxazolidinones 20 could be successfully obtained via the incorporation of CO2 under atmospheric pressure (0.1 MPa) into corresponding chiral β-amino alcohols 19 employing Cs2CO3 as an inexpensive and easily available catalyst in DMSO (Table 1). It is noted that other alkali metal carbonates, such as Li2CO3, Na2CO3, K2CO3, and Rb2CO3 were also found to promote the reaction; however, in lower yields. The results proved that the catalytic activity decreased in the order Cs+ ≈ Rb+ > K+ > Na+ > Li+. The authors were able to

Scheme 7. Ag(I)-catalyzed synthesis of 2-oxazolidinones 17 through three-component reaction of 2-aminoethanols 15, propargylic alcohols 16, and CO2. 197

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Scheme 8. Mechanism that accounts for the formation of 17 and 18.

Scheme 9. Cooperative catalytic mechanism by the silver cation and TMG.

4. Ionic liquid catalyzed condensations

Table 1 Cs2CO3-catalyzed synthesis of chiral oxazolidinones 20 reported by Saito.

Entry 1 2 3 4 5 6 7 8

Amino alcohol 19 1

2-Oxazolidinone 20 2

(S)-19a (R = Me, R = H) (S)-19b (R1 = Et, R2 = H) (S)-19c (R1 = iBu, R2 = H) (S)-19d (R1 = Bn, R2 = H) (R)-19e (R1 = iPr, R2 = H) (S)-19f (R1 = sBu, R2 = H) (R)-19 g (R1 = Ph, R2 = H) (R)-19i (R1 = H, R2 = Me)

1

Ionic liquids are low melting point salts that have emerged as an environmentally friendly alternative to the volatile organic solvents. Though primarily used as reaction media, they are now finding increasing application as catalysts for a great variety of chemical reactions [49]. In the past decades, numerous ionic liquids as catalysts/ reaction media have been applied in the synthesis of valuable compounds via fixation of CO2 by chemical ways [50]. In 2006, Fujita and co-workers demonstrated for the first time the usefulness of ionic liquids to catalyze the dehydration condensation of β-amino alcohols and CO2. Thus, in the presence of BMIM-Br (1-butyl-3-methylimidazolium bromide) as a catalyst and K2CO3 as an additive at 150 °C, the reaction of β-amino alcohols 23 with CO2 furnished corresponding 2-oxazolidinones 24 in moderate yields, along with small amounts of the substituted cyclic urea 25 side products (Scheme 12) [51]. Although this approach does not require dehydrating agents (such as PCl5 or POCl3) and harmful organic solvents, it is not attractive from an industrial perspective because of drastic conditions of temperature (150 °C) and pressure (10 MPa). Therefore, there is an urgent need to develop novel and effective catalytic systems based on ionic liquids for the synthesis of 2-oxazolidinone derivatives through this chemistry.

Yield (%) 2

(S)-20a (R = Me, R = H) (S)-20b (R1 = Et, R2 = H) (S)-20c (R1 = iBu, R2 = H) (S)-20d (R1 = Bn, R2 = H) (R)-20e (R1 = iPr, R2 = H) (S)-20f (R1 = sBu, R2 = H) (R)-20 g (R1 = Ph, R2 = H) (R)-20i (R1 = H, R2 = Me)

42 63 57 88 89 81 61 50

electrolyzation of 2-pyrrolidone in MeCN-TEAP (tetraethylammonium perchlorate) solution (MeCN as a solvent, TEAP as a supporting electrolyte), in a divided cell (platinum gauze anode and cathode) under galvanostatic control [47]. Shortly afterwards, the same research team reported a safe and high yielding electrochemically induced synthesis of a variety of chiral 2oxazolidinones by direct electrolysis of solutions of MeCN–TEAP containing the corresponding β-amino alcohols, with subsequent CO2 bubbling and addition of TsCl. The reaction is noteworthy in that both N-substituted and N-unsubstituted amino alcohols are well tolerated. The reaction was also compatible with a variety of amino alcohols bearing primary, secondary, and tertiary hydroxyl groups [48].

5. Phosphorus electrophiles mediated condensations The possibility of organophosphorus mediated carbonylation of βamino alcohols with CO2 to 2-oxazolidinones was first realized by Kodaka and co-workers, who synthesized a series of functionalized 2oxazolidinone derivatives 27 from corresponding β-amino alcohols 26 under Mitsunobu conditions. As shown in Scheme 13a, the reactions were carried out in the presence of Ph3P/DEADC/Et3N combination in acetonitrile and provided the desired products in good to high yields. The authors found that Ph3P can be advantageously replaced by nBu3P. 198

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Scheme 10. Proposed mechanism for formation of 20.

variety of chiral 2-oxazolidinones in good yields from β-amino alcohols and CO2 in the presence of tetramethylphenylguanidine (PhTMG) as a base and diphenyl chlorophosphate as a phosphorus electrophile under mild conditions [56].

This change even led to improved yields in some cases. The mild reaction conditions, short reaction time, and good yields were the advantages have been claimed for this synthetic protocol. However, the application of this method generate wasteful by-products. According to the mechanistic studies, this reaction proceeds through the formation of a carbamic acid salt A from the reaction of starting amino alcohol 26 with CO2 in the presence of Et3N as a base. Meanwhile, the reaction of triphenylphosphine with DEADC produces the intermediate B, which then undergoes reaction with A to form intermediate C. Subsequently the intramolecular cyclization of C gives the final 2-oxazolidinone 27 (Scheme 13b) [52]. Shortly afterwards, the same research team slightly improved the efficiency of this protocol by replacing DEADC with CCl4 [53]. In 2001, Ariza and co-workers reported an alternative strategy for the preparation of 2-oxazolidinones 29 from the reaction between βazido alcohols 28 and CO2 under basic conditions using 1.4 equiv. of trimethylphosphine as a mediator at −78 °C (Scheme 14a). The mechanistic course of this reaction is shown in Scheme 14b, and involves the initial formation of the iminophosphorane intermediate A from the reaction of starting β-azido alcohol 28 and Me3P. The treatment of this intermediate with CO2 gives isocyanate B, which undergoes intramolecular cyclization in basic medium to produce final 2-oxazolidinone 29 [54]. In 2004, Dinsmore and Mercer described that the reaction of chiral β-amino alcohols with CO2 in the presence of nBu3P/DBAD/DBU combination, produced corresponding optically pure 2-oxazolidinones in good to excellent yields (69–99%). This carboxylation-Mitsunobu cyclization method was also successfully applied in the synthesis of sixmembered cyclic carbamates from corresponding γ-amino alcohols [55]. In a related investigation, the group of Muñoz synthesized a

6. Catalyst-free condensations The reported examples for catalyst-free synthesis of 2-oxazolidinones through dehydrative condensation reactions of β-amino alcohols with carbon dioxide are scarce and to the best of our awareness there are only two examples on this chemistry. In 1959, Steele and coworkers discovered that synthesis of 5-methyl-2-oxazolidinone derivatives 31 by treatment of corresponding N-substituted β-amino alcohols 30 with CO2 (4 MPa) are possible even in the absence of additional catalyst and organic solvent. The reactions were carried out in water at elevated temperatures (120–175 °C), and generally provided the expected products in moderate to good yields (Scheme 15) [57]. It is noted that all of the reactions were performed on gram scale quantity. It can be expected that secondary amines playing a dual role in this transformation; the substrate and the catalyst [58]. Forty-four years later, the Arai laboratory reported a relatively milder process for the synthesis of 2-oxazolidinone derivatives 33 through dehydrative condensation of N-unsubstituted β-amino alcohols 32 with CO2 under catalyst-free conditions by employing methanol as a solvent. The reaction was carried out under 6 MPa of CO2 pressure at 150 °C and the expected N-H free 2-oxazolidinones were obtained in yields ranging from 51 to 87%. The authors also showed the application of this procedure for the high yielding syntheses of six-membered-ring cyclic carbamates by replacing β-amino alcohols with γ-amino alcohols. It should be noted that the aforementioned temperature is crucial for

Scheme 11. Synthesis of chiral 2-oxazolidinones 22 from the reaction of β-amino alcohols 21 with CO2 in the presence of 2-pyrrolidone electrogenerated base. 199

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Scheme 12. Synthesis of 2-oxazolidinones 24 catalyzed by ionic liquid.

Scheme 13. (a) Formation of 2-oxazolidinones 27 from β-amino alcohols 26 and CO2 using DEADC and Ph3P; (b) Mechanistic proposal for the formation of 27.

Scheme 14. (a) Synthesis of 2-oxazolidinones 29 from the reaction between β-azido alcohols 28 and CO2 developed by Vilarrasa; (b) proposed mechanism for formation of 29.

Scheme 15. Synthesis of 2-oxazolidinones 31 from β-amino alcohols 30 and CO2 in water and under catalyst-free conditions.

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Scheme 16. Synthesis of 2-oxazolidinones 33 and 2-imidazolidinones 34 from coupling of amino alcohols 32 with pressurized CO2 in the absence of catalysts.

Scheme 17. (a) Ph3SbO-catalyzed dehydrative 2-oxazolidinones 36 synthesis from CO2; (b) Proposed mechanism for formation of 36.

Scheme 18. (TBAT)-catalyzed dehydrative condensation of β-amino alcohols 37 with CO2 developed by Takada et al.

system for the dehydrative condensation of β-amino alcohols with pressurized CO2. A variety of N-alkylated β-amino alcohols 35 worked well under optimized conditions and resulted in corresponding 2-oxazolidinones 36 in fair to high yields (Scheme 17a). The best results were obtained for amino alcohols which had a tertiary carbinol carbon. Thus, the reactivity of amino alcohols was found to be of the order tertiary > secondary > primary. It is worth noting that the presence of molecular sieves 3 Å as an adsorptive dehydrating agent is crucial to the success of the reaction. In the absence of the dehydrating agent the expected products were obtained in poor yields. This fact can be

this transformation, because at higher temperatures than 150 °C the yield of cyclic carbamates 33 is decreased in favor of the cyclic ureas 34 through reaction of generated cyclic carbamates with another molecule of starting amino alcohols (Scheme 16) [59].

7. Miscellaneous reactions In an effort towards the development of an effective methodology for the synthesis of 2-oxazolidinones, the group of Nomura employed Ph3SbO/molecular sieves 3 Å/benzene combination as an efficient 201

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Scheme 19. Mukherjee's synthesis of palladium-bis (3-(pyridin-2-ylmethyl)oxazolidin-2-one) complex 41.

methylamino)ethanol 39 with atmospheric CO2 in the presence of 1.3 equiv. of thionyl chloride under solvent-free conditions. As shown in Scheme 19, In the presence of SOCl2, the hydroxyl oxygen of the amino alcohol is activated and becomes a good leaving group [63]. Previously, Paz and co-workers applied the similar procedure in the synthesis of a series of chiral 2-oxazolidinones. They have shown that β-amino alcohols 42 underwent dehydrative condensation with CO2 in the presence of SOCl2 as an activating agent in basic condition. The target 2-oxazolidinones 43 were obtained in moderate yields (Scheme 20) [64].

Scheme 20. Synthesis of 2-oxazolidinones 43 through dehydrative condensation β-amino alcohols 42 with CO2 employing SOCl2 as an activating agent.

8. Conclusion

explained by the strong sensitivity of the catalyst to hydrolysis. The authors found that other organic antimony compounds also promoted the reaction (e.g., Ph3Sb, Me3SbBr2, 4-Cl-C6H4SbO3H2); albeit in lower yields. The mechanism proposed to explain this cyclization starts with the formation of an antimony 2-aminoethoxide A from the β-amino alcohol 35 and Ph3SbO, which is followed by addition of CO2 to its amino ends to furnish 2-antimonoxyethylcarbamic acid B. Finally, intramolecular cyclization of intermediate B produces the final 2-oxazolidinones 36 (Scheme 17b) [60]. Subsequently, the same authors extended their methodology to the synthesis of 2-imidazolidinones (five-membered cyclic ureas), using 1,2-diamines instead of β-amino alcohols [61]. In 2014, Takada and co-workers prepared various optically pure 2oxazolidinones 38 in moderate to excellent yields through tetrabutylammonium difluorotriphenyl silicate (TBAT)-catalyzed dehydrative condensation of β-amino alcohols 37 derived from natural amino acids with CO2 (Scheme 18). However, the authors found some limitation in their methodology, since considerable difficulties were found when they attempted to react unsubstituted 2-aminoethanol and 2-aminocyclohexanol. In these cases, the expected products were obtained in very poor yields. In addition the reaction does not give good yields with sterically more demanding β-amino alcohols. It is noted that other silicon-fluorine-based compounds such as Bu4N[Ph3SiF2], Ph3SiF/ Me4NF, and Ph3SiF/KF were also found to promote the reaction but in lower yields. The optimized protocol tolerated a variety of sensitive functional groups, including hydroxyl and thioalkoxy groups. This made possible the further derivatization of the products. Mechanistic studies with C18O2 revealed that the alcohol OH in carbamic acid intermediate A acts as the nucleophile in CeO bond formation and the acid OH is liberated [62]. Recently, the group of Mukherjee developed a palladium-bis (3(pyridin-2-ylmethyl)oxazolidin-2-one) complex 41 as an efficient and robust catalyst for Suzuki–Miyaura cross-coupling between aryl halides and phenylboronic acid under microwave irradiation. The novel 3(pyridin-2-ylmethyl)oxazolidin-2-one ligand 40 was synthesized in gram-scale in 60% yield by the reaction of 2-((pyridin-2-yl)

2-Oxazolidinones are important targets in organic synthesis. They find wide application as chiral auxiliaries, as ligands, and as solvents. These important heterocycles also have been widely used in the preparation of pharmaceuticals, agrochemicals, and polymers. Their synthesis has been classically achieved by processes involving the use of toxic and gaseous carbonyl reagents such as phosgene or carbon monoxide. Several research groups have focused their efforts on alternative syntheses by replacement of toxic carbonylating agents with CO2. As illustrated, the one-pot synthesis of various 2-oxazolidinones through the dehydrative condensation of β-amino alcohols with CO2 has attracted a lot of attention in recent years. Beside inexpensive, nontoxic, and easily accessible starting materials, scale-ability, high yields, high atom and step economy were advantages of this interesting synthetic approach. Despite the notable achievements in this field over the past few years, many challenges still remain to be overcome: (a) almost all of the metal catalyzed these reactions are limited to the use of Sn, Ce, and Ag catalysts. Thus the exploration of cheaper and more easily available metal catalysts (such as Fe and Cu catalysts) are highly desirable in term of the cost and availability; (b) the number of reported examples in some reactions such as ionic liquid catalyzed and electrochemically promoted condensations are narrow and there is an urgent need to study the scope and limitations of these reactions; and (c) other catalysts such as bi-metallic, tri-metallic, and multi-metallic systems should be explored. We hope that this review will be beneficial in eliciting further research in this domain References [1] A. Samanta, A. Zhao, G.K. Shimizu, P. Sarkar, R. Gupta, Post-combustion CO2 capture using solid sorbents: a review, Ind. Eng. Chem. Res. 51 (2011) 1438–1463. [2] D.D. Chiras, Jones & bartlett publishers, Environmental Science, (2011). [3] (a) D.Y. Leung, G. Caramanna, M.M. Maroto-Valer, An overview of current status of carbon dioxide capture and storage technologies, Renew. Sust. Energ. Rev. 39 (2014) 426–443; (b) E.S. Rubin, J.E. Davison, H.J. Herzog, The cost of CO2 capture and storage, Int. J. Greenh. Gas Control 40 (2015) 378–400. [4] J. Rintjema, A.W. Kleij, Substrate-assisted carbon dioxide activation as a versatile approach for heterocyclic synthesis, Synthesis 48 (2016) 3863–3878.

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