Reaction of CO₂ and CS₂ with aniline catalyzed by AlCl₃ under mild condition

Yuan Xinhua, Xu Hongxing, Ni Zhonghai^#, Zhang Lifang^#
(College of Material Engineering, Jiangsu University, Zhenjiang, 212013, China; ^#College of Chemical Engineering, China University of Mining and Technology, Xuzhou, 221008, China)

Received Mar.23, 2003; Supported by the National Natural Science Foundation of China (No. 20207003)

Abstract The reactions of aniline with CO₂ and CS₂ were investigated in the presence of sublimed AlCl₃ under the mild conditions (room temperature and ambient atmosphere). Reasonable mechanism, electrophilic substitution of carbonium and nitronium, was also provided according to GC/MS analysis of the reaction products. The resulting N,N'-diphenylurea(DPU, high selectivity of 92.5mol%) and N,N'-diphenyl thiourea (DPTU, high yield and selectivity of 80.1mol% and 99.7mol%, respectively) were purified by recrystallizing from ethanol and identified by melting point, element analysis, GC/MS, FTIR, UV and ¹H NMR data.
Keywords carbon dioxide, carbon disulfide, aniline, Lewis acid

1 INTRODUCTION    
Reducing CO₂ emissions for addressing climate change concerns is becoming increasingly important as the CO₂ concentration in the atmosphere has increased rapidly since the industrial revolution. Many mitigation methods, including CO₂ sequestration and novel CO₂ utilization, are currently under investigation. The current commercial processes for capturing CO₂ from flue gas use a chemical absorbing method, with monoethanol amine (MEA) as an absorbent. However, the method is expensive and energy intensive. There are also many reports about the syntheses of preparing carbonate, CO₂ polymers, methanol, ethanol, synthetic gas and formic acid or its derivatives using CO₂ as a reactant ^[1-8]. Utilization of CO₂ in the selective synthesis of carbonate esters from amines and alkylating agents has been reported ^[9]. CO₂ may be also used as a catalyst in trans-esterification reaction for the carbamation of amines ^[10]. Primary alkylamines and hindered arylamines give high yields of isocyanates when reacted with CO₂ and the Mitsunobu zwitterions generated from dialkyl azodicarboxylates and Bu₃P in dichloromethane at  -78^oC ^[11]. CO₂ is known to be a very stable compound, so there are few organic reactions in which CO₂ was actually used as C₁ resource. The condition of the reactions was usually under high temperature and high pressure, the selectivity and yield of target product were generally low.
    In this paper, we described a mild method for the preparation of N,N'-diphenylurea from aniline and CO₂ with the catalyst of AlCl₃ under the mild conditions of room temperature and ambient atmosphere. N,N'-diphenyl thiourea was also conveniently synthesized through this similar method with high yield and selectivity of 80.1mol% and 99.7mol%, respectively. The component of reaction products was identified by GC/MS analysis, while the structures of DPU and DPTU were confirmed by various analyses.

2 EXPERIMENTAL 
Ethanol and ether were dewatered by MgSO₄ and then distilled in vacuum or under nitrogen atmosphere. The catalyst of Lewis acid AlCl₃ was sublimed before use. All other chemicals used in this study were commercially available and used without further purification. The reaction process was carried out in a chest under free-water, nitrogen atmosphere. Melting point was determined in capillaries on a domestic melting point apparatus and was uncorrected. GC/MS analysis was carried out on HP 6860/5973 GC/MS. ¹H NMR spectrum was recorded on a JEOL Lambda 400 spectrometer (400 MHz) with TMS as the internal standard in C₄D₈O. Chemical shifts are expressed in parts per million (d, ppm). FTIR spectrum was obtained on a Nicolet Magna IR-560 spectrometer. UV spectrum was acquired on Hitachi U-2000 UV spectrometer. LC/MS/MS analysis was performed on a Bruker Esquire system (ESI source, positive ion detection, 113.4 V). Elemental analyses for C, H, N and O were carried out on a Leco CHN-2000 Elemental analyzer.
    The reactions of aniline with CO₂ and CS₂ were performed in a glove box because AlCl₃ is prone to lose activity for absorbing water in the atmosphere. Charged nitrogen into the glove box and vacuumized with vacuum pump for 3-4 times, and then put aniline, sublimed AlCl₃, a stirring seed and CS₂ or CO₂ into a 200mL flask. Put the flask on a magnetic force stirrer for several hours until the reaction was accomplished. Took out reaction products, and added mixture solvent (concentrated hydrochloric acid : water, V/V =1:10), and washed with water for several times till the pH value was about 7.0. Pure DPU and DPTU crystal were obtained by recrystallizing the reaction mixture from ethanol, and then identified by melting point, element analysis, GC/MS, FTIR, UV and ¹H NMR data.

3 RESULTS AND DISCUSSIONS
3.1 Reaction of aniline with CO₂    
3.1.1 GC/MS Analysis. According to GC/MS analysis, the reaction of aniline with a small quantity of carbon dioxide with the catalyst of AlCl₃, even at the mild condition of room temperature and ambient atmosphere, could afford phenyl isocyanate(119(M⁺, 100), 91(46.5), 77(1.5), 71(4.8), 64(25.2), 51(5.4)), azobenzene(182(M⁺, 45.8), 152(12.5), 105(20.6), 77(100), 51(20.4)), N,N'-diphenylcarbodiimides(194(M⁺,100), 118(2.5), 103(4.6), 91(15.0), 77(24.8), 64(7.1), 51(13.2)), 2-anilinoaniline(184(M⁺, 100), 168(9.2), 154 (3.8), 128(3.1), 107(8.2), 91(10.2), 77(2.5)) and N,N',N"-triphenylguanidine(287(M⁺, 59.8), 210(4.0), 194(100), 117(2.9), 93(55.0), 77(26.8), 64(12.9), 51(10.0)).
    DPU has recently been reported as a convenient intermediate in the phosgene-free synthesis of methylene diphenylene diisocyanate. And DPU can also be used to increase the cold resistance, and accelerate development and higher yields of cucumber and other plants ^[12]. In the case of DPU synthesis from aniline, nitrobenzene and carbon monoxide, the reaction has been presumed to occur according to the following equation (1) or (2) ^[13].
PhNO₂ + PhNH₂ + 3CO  --> PhNHCONHPh + 2CO₂ (1)
PhNO₂ + 5PhNH₂ + 3CO --> 3PhNHCONHPh + 2H₂O (2)
    In the study, DPU was synthesized as the following stoichiometry (3) when abundant CO₂ was used.
2PhNH₂ + CO₂ PhNHCO --> NHPh + H₂O (3)
    In most of current adsorption or chemical reaction processes, CO₂ has been used as a weak acid. However, CO₂ is also a weak electrophilic reagent and AlCl₃ has strong affinity for electron. The electrophilic power of CO₂ was reinforced when the complexation between AlCl₃ and CO₂ was formed. The electrophilic reagent of CO₂ complexation cannot attack benzene ring since the electronic density of "C" atom in benzene ring is low. However, the "N" atom in aniline is ready to be attacked for the existence of an isolated pair of electrons. Therefore, according to electrophilic substitution reaction pathway of carbonium and nitronium, the components of the reaction product of aniline and CO₂ could be reasonably explained ^[14].
3.1.2 Purification and spectra analyses of DPU. Colorless, needleshaped, pure DPU crystal was obtained by recrystallizing the reaction mixture from aniline and abundant CO₂. The molecular structure of DPU was identified through melting point (238-239^oC, literature value 239-240^oC^[15]), analysis calculated for C₁₃H₁₂N₂O: C, 73.56; H, 5.70; N, 13.20; O, 7.54, found: C, 73.59; H, 5.72; N, 13.18, O, 7.51, FTIR (KBr film, n=3326.9, 1648.6, 1596.9, 1555.4, 1494.4, 1443.2, 1312.6, 1233.8, 893.9, 754.7, 698.8 cm^-1), UV (l_max=205, 257 nm) and ¹H NMR(d_H: 7.504-7.523(4H, d), 7.233-7.261 (4H, t), 6.937-6.966 (2H, t)). According to the mass data, the crystal should be phenyl isocyanate, but in the FTIR spectra of the crystal, there is no characteristic peak value (about 2275-2250 cm^-1) of link double bond "X=Y=Z". The HPLC/MS/MS spectra indicated that the molecular weight of the crystal was about 212.1. Therefore, the crystal should be DPU, and the abnormal mass data may be caused by the loss of neutral molecule of aniline in the cleavage of mass spectra^[16]. Therefore, the selectivity may be up to 92.5mol%.
3.2 Reaction of aniline with CS₂
3.2.1 GC/MS analysis. The component of reaction product of aniline with CS₂ catalyzed by AlCl₃ should be similar to the reaction product of aniline with CO₂ since the structure of CS₂ is similar to CO₂. According to GC/MS analysis, the reaction mixture of aniline with CS₂, even at the mild condition of room temperature and ambient atmosphere, afford phenyl iso-thiocyanate (135(M⁺, 100), 91(5.1), 77(54.2), 51(16.4)), azobenzene, N,N'-diphenylcarbodiimides, hydride of diphenyl thiourea (234(M⁺, 19.7), 174(100), 144(10.0), 130(20.6), 118(5.1), 91(2.5), 77(2.5)) and N,N',N"-triphenylguanidine.
    The reaction mechanism of aniline and CS₂ has been reported by Ballabeni^[17] and the others ^[18]. The dithiocarbamic acid (Ph-NHSSH) initially formed by nucleophilic attack of aniline to CS₂ can be converted into the more electrophilic phenyl iso-thiocyanate. But Ballabeni himself did not isolate any dithiocarbamate-amine salts, and this mechanism can not explain the products in the reaction of aniline with CS₂ except for phenyl iso-thiocyanate and DPTU.
    The electrophilic power of CS₂ was also reinforced when the complexation between AlCl₃ and CS₂ was formed. The electrophilic reagent of CS₂ complexation can attack "N" atom in aniline more easily than CO₂ complexation. Therefore, according to electrophilic substitution reaction pathway of carbonium and nitronium, the components of the reaction product of aniline and CS₂ were reasonably explained.
3.2.2 Purification and spectra analyses of DPTU. DPTU has been widely used for the past decades as a medicine for leprosy, as an analytical reagent for some metals such as ruthenium and osmium, as a curing accelerator for caoutchouc and synthetic rubber, especially for natural latex and neoprene and as a thermal stabilizer for PVC; it also serves as an intermediate and a raw material for the preparation of diphenylguanidine, dyestuffs, curing bladder, rubber bag, tire cement, wires, cables, detergent for cleaning airport runways and many other commercial products ^[19]. The synthesis of DPTU from carbon disulfide and aniline has been widely studied in the past few decades, but the reaction condition was usually severe, with high temperature over the boiling point of CS₂, high pressure, expensive solvent or long reaction time. Herein, under mild conditions of room temperature and ambient atmosphere, aniline reacted with CS₂ for 2h with the catalyst of sublimed AlCl₃. Recrystallizing the reaction mixture from ethanol gave white, flakelike pure DPTU crystal. The yield of DPTU is about 80.1mol%. The molecular structure of DPTU was identified through melting point(153.5-154^oC, literature value 154-155 ^oC^[18])), analysis calculated for C₁₃H₁₂N₂S: C, 68.39; H, 5.30; N, 12.27; S, 14.04, found: C, 68.42; H, 5.32; N, 12.25; S, 14.01, FTIR(KBr film, n=3208.1, 3035.4, 1598.1, 1551.4, 1492.9, 1450.2, 1342.1, 1240.1, 1070.2, 760.6, 696.9 cm^-1), UV(l_max=205, 280.5nm) and ¹H NMR (d_H：7.06-7.15 (2H, t), 7.25-7.27 (4H, t), 7.41-7.51(4H, d), 9.01(2H, s)). According to the mass data, the crystal should be phenyl iso-thiocyanate, but in the FTIR spectra of the crystal, there is no characteristic peak value of link double bond "X=Y=Z". The HPLC/MS/MS spectra indicated that the molecular weight of the crystal was about 228.1. Therefore, the crystal should be DPTU, and the abnormal mass data may also be caused by the loss of neutral molecule of aniline in the cleavage of mass spectra. It is similar to DPU. Then, the selectivity of DPTU is over 99.7mol%.

4 CONCLUSION
DPU (high selectivity of 92.5mol%) and DPTU (high yield and selectivity of 80.1mol% and 99.7mol%, respectively) can be synthesized from aniline with CO₂ and CS₂ in the presence of sublimed AlCl₃ even under the mild conditions of room temperature and ambient atmosphere. The reaction mechanism, electrophilic substitution reaction pathway of carbonium and nitronium, was provided according to the components obtained from GC/MS data and confirmed by the reaction of aniline and CS₂ under the same condition. Pure DPU and DPTU were obtained by recrystallizing reaction product from ethanol, and the molecular structures of DPU and DPTU were identified through melting point, GC/MS, FTIR, UV and ¹H NMR data. Neutral loss of aniline may be occurred in the cleavage of mass spectra of DPU and DPTU. The research may provide a new way for us to mitigate the greenhouse effect of CO₂ and to prepare important organic intermediate of DPTU.

REFERENCES       
[1] Takasho Y, Hodeo M, Takahiroqh K. J. Chem. Soc. Chem. Commun., 1997: 1129.
[2] Sakakura T, Saito Y, Okano M. J. Org. Chem., 1998, 63: 7095.
[3] Cheng M, Lovbkofsky E B. J. Am. Chem. Soc., 1998, 120: 11018.
[4] Daremsbourg K J, Holtcamp M, Struck G E. J. Am. Chem. Soc., 1999, 121: 107.
[5] Aschcrofe A T, Kcheetham A, Green M L H et al. Nature, 1991, 225: 352.
[6] Mark M F, Maier W F. J. Catal., 1996, 164: 122.
[7] Krocher O, Koppel R A. J. Chem. Soc. Chem. Commun., 1997: 453.
[8] Cadadei M A, Moracci F M, Zappia G. J. Org. Chem., 1997, 62: 6754.
[9] Aresta M, Quaranta E. J. Org. Chem., 1988, 53: 4253.
[10] Dibenedetto A, Aresta M, Quaranta E. Prepr.-Pap. Am. Chem. Soc., Div. Fuel Chem., 2000, 45 (4): 681.
[11] Saylik D, Horvath M J, Elmes P S. J. Org. Chem., 1999, 64: 3940.
[12] Zauralov O A. Agrokhimiya, 1998, 12: 45;
[13] Lee C W, Lee J S, Lee S M et al. Journal of Molecular Catalysis, 1993, 81 (1): 17.
[14] Yuan X H, Ni Z H, Wei X Y et al. Chemistry (Huaxue Tongbao), 2002, 65 (9): 627.
[15] Baker J K. J. Chem. Soc., 1975: 4652.
[16] Yuan X H, Zhang L F, Ni Z H et al. Journal of Jiangsu University (Jiangsu Daxue Xuebao), 2003, 24 (2): 54-57.
[17] Ballabeni M, Ballini R, Bigi F et al. J. Org. Chem., 1999, 64: 1029.
[18] Bazavova I. M., Dubenko R G, Sbevehenko L I. Ukr. Khim. Zh., 1980, 46: 186.
[19] Ozaki S. Chem. Rev., 1972, 72: 457.

[ Back ] [ Home ] [ Up ] [ Next ]Mirror Site in USA  Europe  China  GBNet