Tetra-butylphosphonium arginine-based ionic liquid-promoted cyclization of 2-aminobenzonitrile with carbon dioxide

Abstract

An easily prepared amino acid ionic liquid (AAIL) i.e. [TBP][Arg] comprising a tetra-butylphosphonium cation and an arginine anion was found to be an efficient and recyclable catalyst for the synthesis of quinazoline-2,4(1H,3H)-diones from 2-aminobenzonitriles and CO₂ under solvent-free conditions. As a result, various 2-aminobenzonitriles bearing electron-withdrawing or electron-donating substituents worked well to afford quinazoline-2,4(1H,3H)-diones in excellent yields. Notably, this type of AAIL showed good stability, and could be easily recovered and reused five times without significant loss of its catalytic activity. This process represents an alternative approach for greener chemical fixation of CO₂ to afford valuable compounds.

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Introduction

The detrimental influence of increasing accumulation of CO₂ in the atmosphere has received more and more attention. Nevertheless, CO₂ can also be regarded as a sustainable feedstock with several characteristics such as low cost, nontoxicity, nonflammability, renewable and so on.¹ To date, numerous valuable chemicals such as formic acids,² methanol,³ dimethyl carbonate,⁴ cyclic carbonate,⁵ polycarbonates,⁶ ureas,⁷ urethanes,⁸ α,β-unsaturated carbonyl compounds,⁹ and carbon monoxide¹⁰ have been prepared from CO₂.

Quinazoline-2,4(1H,3H)-diones and their derivatives have found widespread applications in the pharmaceutical industry.¹¹ Traditionally, quinazoline-2,4(1H,3H)-diones are synthesized through the reaction of anthranilamides with phosgene or anthranilic acids with urea, potassium cyanate and chlorosulfonyl isocyanate, respectively.¹² However, most of those methods generally suffer from tedious workup procedures, use of toxic reactants and low efficiency. Therefore, development of more efficient, simple and green approaches using environmentally benign reagents would be highly desirable. Since a promising process for the synthesis of quinazoline-2,4(1H,3H)-diones from CO₂ and 2-aminobenonitriles was developed for the first time by Mizuno,¹³ a diverse library of catalysts such as 1,1,3,3-tetra-methylguanidine (TMG),¹⁴ Cs₂CO₃,¹⁵ poly(amidine),¹⁶ MgO/ZrO₂,¹⁷ monomeric tungstate,¹⁸ 1-butyl-3-methylimidazolium hydroxyl ([BMIm]OH),¹⁹ [BMIm]OAc,²⁰ amine-functionalized MCM-41,²¹ mesoporous smectites,²² TBD-functionalized Fe₃O₄,²³ and protic ionic liquids (ILs)²⁴ have been proved to be effective catalysts. In this context, development of more convenient and efficient catalytic systems with ease of recyclability remains a challenge.

As organic salts with melting point of lower than 100 °C, ILs have attracted much attention in recent years.^25–27 ILs containing cations such as imidazolium, pyridinium, ammonium and anions e.g. chloride, dicyanamide, bis(trifluoromethylsulfonyl)amide, tetra-fluoroborate, hexa-fluorophosphate are most common ILs. Notably, phosphonium-based AAILs have been explored with a number of advantages, for example, tetra-alkylphosphonium-based AAILs displayed lower viscosity and higher decomposition temperature than traditional ammonium-based AAILs.²⁸ Additionally, application of anion-functionalized ionic liquids including AAILs have been extended to CO₂ capture and fixation.²⁹ Zhang and co-workers found that the porous silica gel-supported tetra-butylphosphonium AAILs were used for fast and reversible CO₂ absorption.³⁰ Moreover, ILs, i.e. phosphonium-based ILs could also act as reaction media for the transesterification reaction and gave higher efficiency than traditional organic solvents.³¹ Recently, our group also showed that 1-butyl-3-methylimidazolium alanine-based AAILs i.e. [Bmim][Ala] could act as an effective catalyst for the cycloaddition of epoxides with CO₂ to produce cyclic carbonates in excellent yields under solvent-free conditions.³²

It was also worth mentioning that phosphonium-based AAILs are low-toxic, environmentally benign, and biodegradable. As known, ionic liquids with tetra-alkylphosphonium as cation can be more easily detected by ³¹P NMR technique. On the other hand, CO₂ can be fixed and activated by guanidines, which is a kinetically reversible process.³³ In this aspect, carboxylic group is able to form hydrogen bond with N–H, thus weakening the N–H bond and facilitating nucleophilicity of the NH₂ group.^20,34 These inspired us to design the reactive arginine-based ionic liquid with the guanidine motif denoted as [TBP][Arg] as a multifunctional base for CO₂ activation and carboxylic anion for N–H bond activation. In this work, we found that the carbonylation of 2-aminobenzonitriles with CO₂ performed smoothly to give a series of quinazoline-2,4(1H,3H)-diones in excellent yields by using [TBP][Arg] as a catalyst as depicted in Scheme 1. This phosphonium arginine-based IL featured numbers of advantages such as high catalytic activity, lower toxicity, environmentally benign characters, along with ease of detection by ³¹P NMR technique. Moreover, the catalyst could be easily recovered and reused for five times without appreciable loss of its catalytic activity.

Scheme 1 Cyclization of 2-aminobenzonitrile with CO₂ catalyzed by [TBP][Arg].

Results and discussion

At the start of this work, we prepared the bifunctional phosphonium arginine ([TBP][Arg]) ILs through the neutralization of tetra-butylphosphonium hydroxide ([TBP][OH]) aqueous solution with arginine at room temperature²⁸ (for detailed characterization of [TBP][Arg], see Experimental Section). It was worth mentioning that [TBP][Arg] is more stable than traditional ILs.²⁸ 2-Aminobenzonitrile (1a) was selected as a model substrate to evaluate catalytic activity of [TBP][Arg] as listed in Table 1. Gratifyingly, quinazoline-2,4(1H,3H)-dione 2a was obtained in 60% isolated yield by employing 1 equiv. [TBP][Arg] under 10 MPa CO₂ at 120 °C employing DMF as solvent (entry 1). Particularly, none of any byproduct was detected. As a result, the [TBP][Arg]-promoted cyclization of 2-aminobenzonitrile with CO₂ performed effectively with perfect chemoselectivity.

Table 1 ILs-promoted reaction of 2-aminobenzonitrile with CO₂^a

Entry	Catalyst	Amount (equiv.)	DMF (mL)	Yield^b (%)
a Reaction conditions: 1a (1 mmol, 0.118 g), CO2 (10 MPa), 120 °C, 24 h.b Isolated yield.c Tetra-butylphosphonium hydroxide (40% in water).d Tetra-butylammonium hydroxide (25% in water).
1	[TBP][Arg]	1	2	60
2	[TBN][Arg]	1	2	45
3	[Bmim][Arg]	1	2	50
4	[TBP][Ala]	1	2	0
5	Na[Arg]	1	2	0
6	[TBP][Arg]	1	—	85
7^c	[TBP][OH]	1	—	48
8^d	[TBN][OH]	1	—	52
9	[TBP][Arg]	0.5	—	84
10	[TBP][Arg]	0.3	—	96
11	[TBP][Arg]	0.1	—	95
12	[TBP][Arg]	0.05	—	88

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Encouraged by such results, a series of ILs including tetra-butylammonium arginine ([TBN][Arg]), tetra-butylphosphonium alanine ([TBP][Ala]), 1-butyl-3-methylimidazolium arginine [Bmim][Arg], Na[Arg], tetra-butylphosphonium hydroxide aqueous solution ([TBP][OH]), and tetra-butylammonium hydroxide aqueous solution ([TBN][OH]) were further examined for the reaction. As a result, [TBN][Arg] and [Bmim][Arg]-promoted the reaction with moderate yields (entries 2 and 3). While [TBP][Ala] and Na[Arg] showed inactivity at 120 °C with full recovery of the starting material under neat conditions (entries 4 and 5). This is probably because [TBP][Ala] with weak basicity is unable to activate 1a and CO₂.

On the other hand, poor solubility of Na[Arg] in DMF could account for activity. In addition, [TBP][OH] and [TBN][OH] were also evaluated for this reaction. As a consequence, the desired product 2a can also be attained in moderate yields (entries 7 and 8). Interestingly, the [TBP][Arg]-promoted reaction performed more efficiently without additional organic solvent compared with that in DMF (entry 6 vs. 1). Meanwhile, the influence of catalyst loading on the reaction was further investigated under identical reaction conditions (entries 9–12). Notably, [TBP][Arg] performed well even in the presence of 5 mol% catalyst relative to 1a, presumably due to synergistic effect of such dual activation of [TBP][Arg] consisting of two functional sites: carboxylic group and guanidine structure on the cyclization reaction. Carboxylic group is able to activate N–H bond through hydrogen bond interaction, thus improves the nucleophilicity of the NH₂ group. At the same time, CO₂ can be activated by guanidine structure as detected in ref. 33.

Subsequently, the influences of the reaction parameters including CO₂ pressure, reaction temperature and time on the reaction were investigated in detail. The results are summarized in Table 2. As easily seen, 2a yield was closely related to CO₂ pressure (entries 1–6). With CO₂ pressure decrease from 8.5 to 0.1 MPa, a sharp drop in 2a yield was observed from 95% to 16%, presumably because appropriate amount of CO₂ is beneficial for the reaction involving CO₂. Furthermore, temperature effect was also evaluated under 8.5 MPa CO₂. It was found that comparative yields were obtained at 120 °C and 100 °C (entries 2 and 7). However, 2a yield was reduced as further decrease in the reaction temperature to 60 °C (entries 8 and 9). Finally, we examined the effect of the reaction time under 8.5 MPa CO₂ at 100 °C. To be delighted, 70% 2a yield was obtained for 4 h. Further improved result (91%) was received after 12 h. With a longer time than 12 h, unchanged yield was acquired (entries 10–13).

Table 2 Effects of reaction parameters on chemical fixation of CO₂ with 1a^a

Entry	PCO2 (MPa)	T (°C)	Time (h)	Yield^b (%)
a All the reactions were carried out with 1a (1 mmol, 0.118 g) and [TBP][Arg] (0.1 mmol, 0.0433 g) unless otherwise specifically notified.b Isolated yield.c Without any catalyst.d KF instead of [TBP][Arg].e Cs2CO3 instead of [TBP][Arg].f Et3N instead of [TBP][Arg].g Melamine instead of [TBP][Arg].
1	10	120	24	95
2	8.5	120	24	93
3	7.5	120	24	62
4	7	120	24	32
5	4	120	24	25
6	0.1	120	24	16
7	8.5	100	24	91
8	8.5	80	24	79
9	8.5	60	24	65
10	8.5	100	16	91
11	8.5	100	12	91
12	8.5	100	8	83
13	8.5	100	4	70
14^c	8.5	100	12	0
15^d	8.5	100	12	0
16^e	8.5	100	12	0
17^f	8.5	100	12	7
18^g	8.5	100	12	16

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In addition, several typical inorganic and organic bases were also tested by employing 8.5 MPa CO₂ at 100 °C for 12 h. Obviously, no reaction occurred without any catalyst (entry 14). Inorganic bases such as Cs₂CO₃ and KF showed inactive under the given conditions (entries 15 and 16), presumably being ascribed to the poor insolubility. On the other hand, organic bases such as Et₃N and melamine, being difficult to activate both 1a and CO₂, only showed low activity (entries 17 and 18). Consequently, suitable basicity and good solubility are crucial to promoting the cyclization of 2-aminobenzonitrile with CO₂.

With the optimized reaction conditions in hand, the generality and the utility of this protocol were then examined using different 2-aminobenzonitriles bearing various substituents as listed in Table 3. As expected, 2-aminobenzonitrile derivatives with electron-withdrawing or electron-donating groups at benzene worked well to afford quinazoline-2,4(1H,3H)-diones 2 in excellent yields under the given reaction conditions. The target product 2b and 2g were obtained in 87% and 91% isolated yield from the dimethoxy substituted 1b and para-methyl substituted 2g, respectively (entries 2 and 7). Notably, both the meta and para-halogen substituted 2-aminobenzonitriles could almost quantitatively be converted into the corresponding quinazoline-2,4(1H,3H)-diones (entries 3–6).

Table 3 Synthesis of quinazoline-2,4(1H,3H)-diones 2 from various substrates 1 and CO₂^a

Entry	Substrate	Product	Yield^b (%)
a Reaction conditions: 1 (1 mmol), [TBP][Arg] (0.1 mmol, 0.0433 g), CO2 (8.5 MPa), 100 °C, 12 h.b Isolated yield.
1			91
2			87
3			96
4			94
5			96
6			95
7			91

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The reusability of the catalyst i.e. [TBP][Arg] was also studied by performing the reaction of 2-aminobenzonitrile and CO₂ under the optimum reaction conditions. After the reaction, the catalyst was recovered through adding water, filtration, and desiccation in vacuo for 24 h, which can be reused directly for the next run. As seen from Fig. 1, the catalyst could be reused for five times without appreciable loss of its catalytic activity.

Fig. 1 Catalyst reusability.

¹H NMR, ¹³C NMR and ³¹P NMR examination displayed that change of structure was not observed for the recovered catalyst in comparison with the fresh one (for the details, see ESI†).

A tentative mechanism for the present [TBP][Arg]-catalyzed chemical fixation of CO₂ with 2-aminobenzonitrile is illustrated in Scheme 2. Both amino group in 1a and CO₂ are initially activated by the bifunctional anion (carboxyl group and guanidine group, respectively) in [TBP][Arg], which facilitates the nucleophilic attack of 1a at CO₂ to generate the carbamate salt A. Subsequently, a cyclization occurs resulting in the formation of the intermediate B, followed by a ring opening to generate a crucial isocyanate intermediate C. C undergoes further ring-closing pathway to afford the species D. Finally, the final product 2a can be obtained through proton transfer of D with catalyst regeneration. Additionally, the formation of the isocyanate intermediate C assisted by the o-cyano group seems to be of great importance.³⁵

Scheme 2 Possible pathway for the reaction of 2-aminobenzonitrile with CO₂ catalyzed by [TBP][Arg].

To further investigate the effectiveness of the present simultaneous activation of N–H bond, we explored the [TBP][Arg]-catalyzed chemical transformations via different nucleophiles such as anthranilamide and urea as a CO₂ equivalent (Scheme 3). In general, the reaction of anthranilamide and urea^36,37 gave low yield of quinazoline-2,4(1H,3H)-dione even under harsh reaction conditions (e.g. 190 °C). Interestingly, the [TBP][Arg]-promoted reaction of anthranilamide with urea afforded the target product with a good yield even at milder conditions. Yet, in the absence of any catalyst, the reaction only gave 20% yield.

Scheme 3 The reaction of anthranilamide with CO₂ catalyzed by [TBP][Arg].

Conclusions

In conclusion, tetra-butylphosphonium arginine-based IL [TBP][Arg] able to activate both amino-group and CO₂ was proved to be a very efficient and recyclable catalyst for the synthesis of quinazoline-2,4(1H,3H)-diones from 2-aminobenzonitriles and CO₂ under solvent-free conditions. The catalyst [TBP][Arg] works well with a range of 2-aminobenzonitrile with electron-donating or electron withdrawing groups at the ring of benzene. Moreover, easy recovery and excellent reusability of the catalyst render this protocol to have much potential application for the catalytic conversion of CO₂ into valuable compounds and materials in industry.

Experimental

Materials and methods

Experiments using compressed gases CO₂ are potentially hazardous and must only be carried out by using appropriate equipment and under rigorous safety precautions. CO₂ with a purity of 99.999% was commercially available. All starting materials were obtained from TCI, Aladdin or Alfa Aesar Company and used as received. ¹H NMR spectra were recorded on Bruker 400 MHz spectrometer using CDCl₃ or DMSO-d₆ as solvent referenced to CHCl₃ (7.26 ppm) or DMSO-d₆ (2.50 ppm). ¹³C NMR spectra were recorded at 100.6 MHz in CDCl₃ (or DMSO-d₆) using CDCl₃ (77.0 ppm) (or DMSO-d₆, 39.5 ppm) as internal reference. ESI-MS were recorded on a Thermo Finnigan LCQ Advantage spectrometer in ESI mode with a spray voltage of 4.8 kV. High resolution mass spectrometry was conducted using a Varian 7.0 T FTICR-MS by ESI technique. Melting points were measured on an X4 apparatus and uncorrected. Infrared (IR) spectra were recorded on a Bruker Tensor 27 FT-IR spectrophotometer with KBr pellets. All the products were characterized by ¹H, ¹³C NMR, ESI-MS and IR analysis here and were identified by comparison of their characterized data with those reported in the literature.

Typical procedure for the synthesis of [TBP][Arg]

[TBP][Arg] was synthesized according to the reported procedure.²⁸ In a typical process, an [TBP][OH] 40% aqueous solution (20 mmol, 13.8 g) was added dropwise to a aqueous solution of slightly excess amino acid (22 mmol, 4.0 g). After stirring for 24 h at room temperature, water was removed by evaporation in vacuum. A mixture (30 mL) of acetonitrile and methanol at the percentage of 9 : 1 was added to the reaction mixture. Then the mixture was agitated intensely for 2 h to deposit the excess amino acid. Then excess amino acid was removed by filtration. The target product was obtained with a yield of 80% by evaporation and then dried in vacuo for 1 day at 70 °C. Other AAILs such as [TBN][Arg], [Bmim][Arg] and [TBP][Ala] was prepared similarly.

General procedure for the reaction of 2-aminobenzonitrile and CO₂

All of the reactions were conducted in a 50 mL stainless steel reactor with a magnetic bar. In a typical experiment, 2-aminobenzonitrile (1 mmol, 0.118 g) and [TBP][Arg] (0.1 mmol, 0.0433 g) were introduced into the inner glass tube successively. Afterwards, CO₂ with an appropriate pressure was charged. The reaction mixture was stirred for desired time at pre-set temperature. When the reaction finished, the reactor was cooled in ice-water and CO₂ was ejected slowly. After addition of 20 mL water, the residue was filtrated to yield the crude product with recovery of the catalyst. The crude product was washed with water and t-BuOMe, respectively and then dried in vacuo at 70 °C for 24 h. The product was further identified by NMR, IR and ESI-MS.

Characterization data of the products

Quinazoline-2,4(1H,3H)-dione (2a). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3253, 3054, 2846, 1702, 1670, 1618, 1443, 755; ¹H NMR (400 MHz, DMSO-d₆) δ 11.29 (s, 1H), 11.14 (s, 1H), 7.87 (dd, J = 8.3, 1.4 Hz, 1H), 7.63–7.59 (m, 1H), 7.15 (t, J = 7.4 Hz, 2H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ 163.10, 150.57, 141.11, 135.23, 127.21, 122.59, 115.57, 114.58 ppm; ESI-MS calcd for [C₈H₆N₂O₂]⁻ 161.13, found 161.17 (M − H).

6,7-Dimethoxyquinazoline-2,4(1H,3H)-dione (2b). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3471, 3379, 3294, 3175, 1704, 1627, 1427, 1436, 1269, 1103; ¹H NMR (400 MHz, DMSO-d₆) δ 11.11 (s, 1H), 10.93 (s, 1H), 7.25 (s, 1H), 6.67 (s, 1H), 3.82 (s, 3H), 3.78 (s, 3H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ 162.31, 154.89, 150.40, 145.02, 136.54, 107.13, 106.19, 97.75, 55.80, 55.70 ppm; ESI-MS calcd for [C₁₀H₉N₂O₄]⁻ 221.05, found 221.07 (M − H).

6-Chloroquinazoline-2,4(1H,3H)-dione (2c). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3210, 3060, 1728, 1668, 1482, 1431, 1284, 877; ¹H NMR (400 MHz, DMSO-d₆) δ 11.35 (s, 2H), 7.81 (s, 1H), 7.67 (s, 1H), 7.17 (d, J = 8.8 Hz, 1H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ 161.83, 150.05, 139.75, 134.76, 126.27, 125.84, 117.54, 115.71 ppm; ESI-MS calcd for [C₈H₄ClN₂O₂]⁻ 195.58, found 195.17 (M − H).

7-Chloroquinazoline-2,4(1H,3H)-dione (2d). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3306, 3171, 3050, 1744, 1687, 1617, 1430, 1286, 862; ¹H NMR (400 MHz, DMSO-d₆) δ 11.31 (s, 2H), 7.86 (d, J = 7.4 Hz, 1H), 7.20 (d, J = 8.0 Hz, 1H), 7.17 (s, 1H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ 162.05, 150.21, 142.03, 139.27, 129.04, 122.46, 114.71, 113.34 ppm; ESI-MS calcd for [C₈H₄ClN₂O₂]⁻ 195.58, found 195.21 (M − H).

6-Bromoquinazoline-2,4(1H,3H)-dione (2e). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3193, 3066, 1742, 1701, 1613, 1480, 1433, 1286, 836; ¹H NMR (400 MHz, DMSO-d₆) δ 11.33 (s, 2H), 7.93 (s, 1H), 7.80 (d, J = 8.5 Hz, 1H), 7.11 (d, J = 8.6 Hz, 1H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ 161.75, 150.09, 140.16, 137.48, 128.90, 117.82, 116.22, 113.78 ppm; HRMS calcd for C₈H₅BrN₂O₂ (M − H) 238.9462, found 238.9455.

6-Fluoroquinazoline-2,4(1H,3H)-dione (2f). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3236, 3054, 1718, 1678, 1500, 1439, 1378, 1287, 884; ¹H NMR (400 MHz, DMSO-d₆) δ 7.19 (m, 1H), 7.52–7.60 (m, 2H), 11.29 (s, 2H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ (111.7, 112.0), (115.2, 115.3), (117.4, 117.5), (122.7, 122.9), 137.4, 150.0, (156.0, 158.4), (162.05, 162.07) ppm; HRMS calcd for C₈H₅FN₂O₂ (M − H) 179.0262, found 179.0255.

7-Methylquinazoline-2,4(1H,3H)-dione (2g). Mp > 300 °C; IR (KBr) ν/cm⁻¹ 3193, 3066, 2950, 1742, 1701, 1613, 1480, 1433, 1286, 836; ¹H NMR (400 MHz, DMSO-d₆) δ 11.12 (s, 2H), 7.75 (d, 1H), 6.98 (t, 2H), 2.35 (s, 3H) ppm; ¹³C NMR (100.6 MHz, DMSO-d₆) δ 162.68, 150.44, 145.57, 140.93, 126.89, 123.63, 115.04, 112.04, 21.42 ppm; HRMS calcd for C₈H₅BrN₂O₂ (M − H) 175.0586, found 175.0583.

Acknowledgements

We are grateful to the National Natural Sciences Foundation of China, Specialized Research Fund for the Doctoral Program of Higher Education (20130031110013), MOE Innovation Team (IRT13022) of China for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Procedure of catalyst recovery; NMR spectra of fresh and recovered catalyst, and all products. See DOI: 10.1039/c4ra16057f

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