DBU and DBU-Derived Ionic Liquid Synergistic Catalysisfor Conversion of CO2/CS2 To

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DBU and DBU-derived Ionic Liquid Synergistic Catalysisfor Conversion of CO2/CS2 to 3-Aryl-2-oxazolidinones/[1,3]Dithiolan-2-ylidene-phenylamine

Binshen Wang,Zhoujie Luo, Elnazeer H. M. Elageed, Shi Wu, Yongya Zhang, Xiaopei Wu, Fei Xia*, Guirong Zhang and Guohua Gao*[a]

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Abstract:An intermolecular synergistic catalytic combination of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and DBU-derived bromide ionic liquid has been developed for conversion of CO2, epoxides and amines under metal-free and solvent-free conditions.Various 3-aryl-2-oxazolidinones areproduced in moderate to excellent yields within a short reaction time. NMR spectroscopy and DFT calculations demonstrate that DBU as a hydrogen bond acceptor and the ionic liquid as a hydrogen bond donor activate the substrates cooperatively by means of inducing hydrogen bond to promote the reaction effectively. Based on these results, a possible reaction mechanism on synergistic catalysis of DBU and the ionic liquid is proposed. In addition, the reaction of CS2, ethylene oxide (EO) and aniline catalyzed by the combination of DBU and the DBU-derived ionic liquid also proceeds smoothly, which opens a hitherto unreported route to [1,3]dithiolan-2-ylidene-phenylamine in a straightforward way.

Introduction

Atmospheric carbon dioxide level has been rising since the start of the industrial revolution due to the generation of energy by the combustion of fossil fuels. CO2 is a main greenhouse gas that causes climate change.[1] It is also an abundant, nontoxic, nonflammable and renewable carbon source.[2] Owing toenvironmental and economic concerns, intensive researches have been conducted employing CO2 as a feedstock in recentyears.[3]

As an efficient method for conversion of inert CO2 to valuable chemicals, the synergistic catalysis that activatesCO2 and other substrates simultaneously has drawn much attentions.Particularly, compared to intramolecular synergistic catalysts,[4] intermolecular synergistic catalysts exhibit advantages of readily available, inexpensive, avoiding tedious preparation process. Various combined intermolecular synergistic catalysts have been developed for conversion of CO2, such as organic bases combining with cellulose,[5] N-bromosuccinimide (NBS)[6] and polyethylene glycol.[7] In addition, organometallic complexes combining with organic bases,[8] CO2 adducts of N-heterocyclic carbenes,[9] ammonium salts,[8a, 10] polyether-potassium salt complexes;[11] metal salts combining with formic acid,[12]biomass,[13] triphenylphosphine and phenol,[14] ionic liquids;[15] and ammonium saltscombining with alcoholic/phenolic compounds,[16]metal complexes[17] have also been proved to be prominentcombinations for catalytic chemical fixation of CO2. However, these intermolecular synergistic catalysts are mainly limited to produce cyclic carbonates.

3-Aryl-2-oxazolidinones are important heterocyclic compounds due to their bioactivity. They exhibit selective and reversible inhibition of monoamine oxidase A, and potent activity against Gram-positive bacterial pathogens including methicillin-resistant Staphylococcus aureas (MRSA) and vancomycin-resistant Enterococci (VRE).[18] The conventional conversions to 3-aryl-2-oxazolidinones are through the reaction of phosgene with 2-alkamine[19] and the reaction of isocyanate with epoxide,[20] which are considered nongreen as toxic starting materials are used. The alternative synthetic routes via the reaction of aryl halides with oxazolidin-2-ones,[18a, 21] the reaction of aryl iodides with amino alcohol carbamates,[22] and the reaction of CO2with amino alcohols are also reported.[23] However, most of these routes rely on metal catalysts that are toxic or expensive. He and others have reported the synthesis of 3-benzyl-2-oxazolidinones by the reaction of CO2with aziridines.[24] Additionally, ionic liquids catalyzed preparations of3-aryl-2-oxazolidinones by the reaction of aromatic amines with cyclic carbonates,[25] and the reaction of 2-(arylamino) alcohols with diethyl carbonate have been reported by us.[26]

Recently, we developed a novel approach for one-pot conversion of CO2, ethylene oxide (EO) and amines to 3-aryl-2-oxazolidinones in the presence of ionic liquids as catalysts.[27] Although the approach is regarded as green, it still suffers from drastic reaction conditions (extended reaction time 9 h), and the

Scheme 1. Conversion of CO2/CS2, epoxides and amines to 3-aryl-2-oxazolidinones/[1,3]dithiolan-2-ylidene-arylamine

reaction mechanism is still unclear. As a continuing work, herein,we present a study on the utilization of intermolecularsynergistic catalysis of readily available organic bases andorganic base-derived ionic liquids to effectively synthesizevarious 3-aryl-2-oxazolidinones by the reaction of CO2, epoxides and amines. NMR spectroscopy and DFT calculations were used to investigate the reaction mechanism. Moreover, ahitherto unreported reaction of CS2, EO and aniline also proceeded using the combination oforganic base and ionic liquid as catalysts, which produced [1,3]dithiolan-2-ylidene-phenylamine in a moderate yield (Scheme 1).

Results and Discussion

The synergistic catalysis of organic bases and organic base-derived ionic liquids for conversion of CO2 to 3-phenyl-2-oxazolidinone

To investigate the synergistic catalytic effects, several typical organic bases with different alkalinities and their derived ionic liquids were applied as catalysts for the reaction of CO2, EO and aniline to synthesize 3-phenyl-2-oxazolidinone(1a) (Table 1). Specially, bromide ionic liquids were elaborately used because bromide was found to play an important role in the reactions involving CO2 and epoxide owing to its good nucleophilicity and leaving ability.[10b-e] When using the single superbase diazabicyclo[5.4.0]-undec-7-ene (DBU, aqueouspKa11.5) alone, a moderate yield of 54% was obtained after 1 h at 140 oC. The single DBU-derived ionic liquid HDBUBr only afforded the desired product in a poor yield of 5%. Notably, the combination of DBU with HDBUBr gave an excellent yield of 93% within 1 h (Table 1, entries 1-3). Besides, single dimethylaminopyridine (DMAP, aqueouspKa9.2) or N-methylimidazole (MIm, aqueouspKa 7.1) provided the product in the yields of 53, 44%, respectively (Table 1, entries 4 and 7).[7, 28] And their derived ionic liquids, HDMAPBr and HMImBr, gave 1a in low yields of 8,

5%, respectively (Table 1, entries 5 and 8). However, both thecombination of DMAP with HDMAPBr, and the combination of MIm with HMImBr exhibited distinctly higher catalytic activities than single base or ionic liquid, which gave the product in the yields of 85, 77% respectively (Table 1, entries 6 and 9). These results showed the synergistic catalytic effects of the three organic bases with their corresponding derived ionic liquids for accelerating the reaction.

Effect of different organic bases and bromides

Differentorganicbases combining with HDBUBr were screened for the conversion of CO2 to 1a, and the results were summarized in Table 2. In addition to the bicyclic amidine DBU giving the excellent yield (Table 1, entry 3), cyclic DMAP and MIm also provided 1a in high yields of 89 and 84% (Table 2, entries 1 and 2). The tertiary amine triethylamine (TEA) afforded 1a in the yield of 82% (Table 2, entry 3). Diethylamine (DEA) and imidazole (Im), with secondary amino groups in their structures, produced 1a in moderate yields of 65 and 63%, respectively (Table 2, entries 4 and 5). Using pyridine (Py) as the organic base, the reaction afforded the product in the yields of 47% (Table 2, entry 6). 1,4-Diazabicyclo[2.2.2]octane (DABCO), whose catalytic activity might be affected by a high steric hindrance in part, provided 1a in the yield of 42% (Table 2, entry 7). Furthermore, triethanolamine (TEOA), diethanolamine (DEOA) and monoethanolamine (MEOA), all bearing hydroxylfunctional groups, gave 1a in low yields of 16-23% (Table 2, entries 8-10). The employed different organic bases showed quite strong effects on the reaction, probably resulted from their different alkalinity, steric hindrance as well as functional group.[5]

In the presence of the optimal organic base DBU, the effect of different bromides on the reaction was investigated (Table 3). Ionic liquid 1-butyl-3-methyl-imidazolium bromide (BmimBr) andtetrabutyl ammonium bromide (Bu4NBr) gave 1a in 90 and87%

yields respectively, which were slightly lower than that ofHDBUBr (93% yield) (Table 3, entries 1 and 2; Table 1, entry 3).It could be attributed to a better activation of HDBU+ toward substrates. When HBr (40 % aqueous solution) was applied as bromide, the reaction provided 1a in 78% yield. As a comparison, 0.5 g 4 Å molecular sieve was additionally added to minimize the possible prohibiting effect of water, and then the yield of 1a increased to 86% (Table 3, entries 3 and 4). The high yields afforded by HBr might be because of the in situ generating HDBUBr by the neutralization of HBr and DBU. Using 1-bromobutane, which might react with DBU to in situ generate ionic liquid Bu-DBUBr,[29] provided 1a in the yield of 74% (Table 3, entry 5). Moreover, NBS also gave a higher yield of 1a (70% yield) than not using any bromide (54% yield) (Table 3, entry 6; Table 1, entry 1). All applied bromides gave the desired product 1a in good to high yields, exhibiting moderate influences on the reaction.

Optimization of reaction conditions

To optimize other reaction parameters, the reaction of CO2, EO and aniline catalyzed by the combination of DBU and HDBUBrwas performed under different catalyst amounts,reaction temperatures, reaction times and CO2 pressures, as shown in Table 4. Decreasing the DBU or HDBUBr amount, the yields of 1a dropped from 93 to 86, 85%, respectively (Table 4, entries 1-3). When reaction temperature was reduced to 130 oC, the yield of 1a remained at 93%. However, further reducing reaction temperature to 120, 110 or 100oC gave 1a in lower yields of 82, 56 or 36%,respectively (Table 4, entries 4-7). The decrease in reaction time to 0.5 h led to reduced yield of 84% (Table 4, entry 8). When the CO2 pressure was decreased to 2.0 or 1.5 MPa, the yields of 1a were 77, 69%, respectively (Table 4, entries 9 and 10).In addition, under extended reaction time to 12 h along with lower catalyst amounts, reaction temperatures or CO2 pressures, the reaction only afforded 1a in moderate yields (Table 4, entries 11-14).

Generalities of the synergistic catalytic combination

Under optimized reaction parameters, the reactions of CO2, different epoxides and amineswere carried out to explore the generalities of the synergistic catalytic combination (Table 5). Using the combination of DBU and HDBUBr as catalysts at 130 oC within 1 h, aromatic amines with either electron-withdrawing substituents (such as halides) or electron-donatingsubstituents (such as alkyl or alkoxy groups) could be converted to corresponding 3-aryl-2-oxazolidinones 1b-1g in moderate to excellent yields (Table 5, entries 1-6). Naphthalen-1-ylamine provided desired product 1h in 85% yield (Table 5, entry 7). However, aliphatic amine that possess stronger electronegativity, such as cyclohexylamine, only gave the 1i in 27% yield (Table 5, entry 8). Furthermore, propylene oxide could also be employed as epoxide to synthesize 5-methyl-3-aryl-2-oxazolidinones 1j and 1k in high yields under slightly harsh reaction conditions (160 oC,9h) (Table 5, entries 9 and 10).

Reaction mechanism studies by NMR spectroscopy and DFT calculations

In our previous study,[27] 2-(phenylamino)ethanol has been revealed to be the key intermediate in the one-pot conversion of CO2, EO and aniline to 1a. To further rationalize the catalytic role of DBU and HDBUBr in the reaction, the interactions between catalysts and the substrates were studied by NMR spectroscopy and DFT calculations.

Firstly, the interactionswere investigated by 1H and 13C NMRspectroscopy. As shown in Figure 1a and 1b, upon addition of 1

eq EO, the N-H1 proton of HDBUBr shifted to downfield from 10.005 to 10.019 ppm, which indicated the formation of hydrogen bond between HDBUBr and EO. This suggested that HDBUBr could activate EO as a hydrogen bond donor.As shown in Figure 1c and 1d, an upfield shift of C1 of EO from 39.85 to 39.53 ppm was observed after adding 1 eq HDBUBr. In contrast, Jeromeet al. reported that carbon signal of EO underwent a downfield shift by adding neutral alcohols as hydrogen bond donors.[16a] The shifting difference implied that besides the hydrogen bond interaction between HDBU+ and EO, Br- also interacted with carbon of EO which increased the electron density around the carbon atom resulting in the upfieldshift of C1.The N-H2 proton of aniline in the mixture with 1 eq DBU underwent a downfield shift from 4.070 to 4.139 ppm and peak broadening, suggesting that aniline could be activated byDBU as a hydrogen bond acceptor (Figure 2a and2b).Furthermore, in the presence of 1 eq DBU, the O-H3 proton of 2-

Figure 1.Partial 1H NMR spectra of (a) only HDBUBr (0.1 mmol), (b) complex of HDBUBr (0.1 mmol) with EO (0.1 mmol), and partial 13C NMR spectra of (c) only EO (0.5 mmol), (d) complex of EO (0.5 mmol) with HDBUBr (0.5 mmol) in CD3CN (0.5 mL).

(phenylamino)ethanol dramatically shifted to downfield from2.796 to 3.923 ppm and became much broader, meanwhile the N-H4 proton of 2-(phenylamino)ethanol mildly shifted to downfield from 4.401 to 4.525 ppm (Figure 2c and 2d). This demonstrated that DBU could activate the hydroxyl group and NH group of 2-(phenylamino)ethanol via hydrogen bonds, especially on the hydroxyl group.

Subsequently, the interactions between catalysts and thesubstrates were investigated by DFT calculations at the M06-2x/6-31+G* level. As show in Figure 3, in the complex of HDBUBrwithEO, the bond length of N1-H1 of HDBUBr was elongated from 1.012 to 1.018Å, and the hydrogen bond between H1 of the hydrogen bond donor HDBUBr and O1 of EO was formed with a bond length of 1.910Å. Meanwhile, the bond length of the C1-O1of EO was elongated from 1.418 to 1.428Å, thus facilitatingthe ring opening of EO.As shown in Figure 4, in the complex of DBU with aniline, the hydrogen bond between N3 of the hydrogen bond acceptor DBU and H2 of aniline was formed with a bond length of 2.043Å. And the bond length of N2-H2 of aniline was elongated from 1.012 to 1.025Å, which made the nucleophilicity of N2 increase. In the complex of DBU with 2-(phenylamino)ethanol, two hydrogen bonds between N3 of DBUand 2-(phenylamino)ethanol were formed, in which the hydrogenbond between N3 and H3 of hydroxyl group (1.885 Å) was

Figure 2.Partial 1H NMR spectra of (a) only aniline (0.1 mmol), (b) complex of aniline (0.1 mmol) with DBU (0.1 mmol), (c) only 2-(phenylamino)ethanol (0.1 mmol), (d) complex of 2-(phenylamino)ethanol (0.1 mmol) with DBU (0.1 mmol) in CD3CN (0.5 mL).

Figure 3.DFT optimized structures of (a) HDBUBr, (b) EO and (c) the complex of HDBUBr with EO. The bond lengths are in the units of angstroms.

shorter than the one between N3 and H4 of NH group (2.104 Å). Accordingly, the bond length of O2-H3 of hydroxyl group was elongated from 0.967 to 0.984Å, and the bond length of N4-H4of NH group was elongated from 1.009 to 1.018Å, whichshowed the hydroxyl group accepted stronger activation from DBU than NH group.

Figure 4. DFT optimized structures of (a) aniline, (b) the complex of aniline with DBU, (c) 2-(phenylamino)ethanol and (d) the complex of 2-(phenylamino)ethanol with DBU. The bond lengths are in the units of angstroms.

The NMR spectroscopy and DFT calculation results consistently indicated DBU and HDBUBr could cooperatively activate the substrates through means of inducing hydrogen bonds, which catalyzed the reactionsynergistically. Based on these results and previous studies,a possible mechanism was proposed for the DBU and HDBUBr synergistically catalyzed the one-pot conversion of CO2,EO and aniline to 1a (Scheme 2). Additionally, a more detailed DFT calculation of reaction pathways and energy profiles was carried out for the one-pot reaction (Figure 5).In first Circle 1,EO is activated through a hydrogen bond interaction with the proton of HDBUBr and nucleophilic attacked by bromide anionvia the transition state Ts-C1-1 with a remarkable energy barrier of 38.0 kcal mol-1, resulting in forming ring-opened Int-C1-1.Then, the nucleophilic attack of Int-C1-1towards the carbon atom in CO2via Ts-C1-2 affordsan alkyl carbonate salt Int-C1-2. An intramolecular ring-closure ofInt-C1-2 viaTs-C1-3 gives the ethylene carbonate (Pro-C1). In the parallel Circle 2, aniline is activated by DBU through the hydrogen bond interaction. Thereafter,the activated aniline nucleophilic attacks on the carbon atom of 2-bromoethanolin Int-C1-1via the transition state Ts-C2-2, giving the 2-(phenylamino)ethanol (Pro-C2),with releasing anenergyof 21.4kcalmol-1. In the final Circle 3, the hydroxyl group of 2-(phenylamino)ethanol is activated by DBU, and then nucleophilic attacks on the carbonyl group of ethylene carbonate to produce a salt Int-C3-1, with an energy of 14.8 kcalmol-1. A subsequent intramolecular proton transfervia Ts-C3-2 leads to a carbonate Int-C3-2.[26]In the intermediate Int-C3-2, the NH group formsthe hydrogen bond interaction with DBU. By a following intramolecular nucleophilic attack of nitrogen atom to carbonyl groupvia Ts-C3-3, a desired product 1a and a byproduct ethylene glycol are produced.

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Scheme 2. The proposed reaction mechanism for the synthesis of 1a.

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DBU and HDBUBr catalyzed conversion of CS2 to [1,3]dithiolan-2-ylidene-phenylamine

To take full advantage of the catalytic effect, the combination of DBU and HDBUBr was then employed to catalyze the reaction of CS2, EO and aniline, which produced [1,3]dithiolan-2-ylidene-phenylamine (2a) as main product, rather than 3-phenyl-2-thiazolidinethione or 3-phenyl-2-oxazolidinethione. [1,3]Dithiolan-2-ylidene-arylamines, as bioactive cyclic dithiocarbonimidates, are effective anti-inflammatory analgesicswith low ulcerogenicity.[30]The traditionalmethods forthe synthesis of [1,3]dithiolan-2-ylidene-arylamines rely on the use of toxic isothiocyanates.[31] Another alternative method established by the reaction of aromatic amines, CS2 and dibromoethane has also been reported.[32] However, it suffers from a tedious and wasteful multi-step operational procedure. Recently, the preparation of [1,3]dithiolan-2-ylidene-arylamines by the reaction of ethylene trithiocarbonate and aromatic amines has also been developed.[25b] To the best of our knowledge, this is the first report on the direct preparation of 2a from CS2, EO and aniline in a one-pot reaction (Table 6).

In the presence of 0.4 mmol DBU and 0.1 mmol HDBUBr as catalysts, the reaction proceeded smoothly affording 2a in 56%

yield at 140 oC within 6 h (Table 6, entry 1). Increasing the reaction temperature or reaction time did not improve the yields.