Tandem Synthesis of β-Amino Alcohols from Aniline, Dialkyl

Mar 26, 2008 - 1501 Wakarusa Dr. A, Suite 110, Lawrence, Kansas 66047. An efficient tandem route for selective synthesis of β-amino alcohols from ani...
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Ind. Eng. Chem. Res. 2008, 47, 2484-2494

Tandem Synthesis of β-Amino Alcohols from Aniline, Dialkyl Carbonate, and Ethylene Glycol Anandkumar B. Shivarkar,† Sunil P. Gupte,*,† and Raghunath V. Chaudhari*,‡ Homogeneous Catalysis DiVision, National Chemical Laboratory, Pune-411008, India, and Department of Chemical & Petroleum Engineering, Center for EnVironmentally Beneficial catalysis, The UniVersity of Kansas, 1501 Wakarusa Dr. A, Suite 110, Lawrence, Kansas 66047

An efficient tandem route for selective synthesis of β-amino alcohols from anilines, dialkyl carbonate and ethylene glycol in the presence of recyclable Na-Y zeolite has been demonstrated. Transesterification of dialkyl carbonate by ethylene glycol produce ethylene carbonate which further reacts with aniline to give β-amino alcohols in a single step. This reaction system was studied under high-pressure as well as pot reaction condition. Various process parametric effects were investigated for the reaction of aniline, dialkyl carbonate, and ethylene glycol. It was observed that a maximum 51% yield of mono-β-amino alcohol, i.e., N-phenylethanolamine (NPEA). is obtained under pressure conditions. The yield of NPEA was improved drastically (>91%) by carrying out the reaction under pot conditions using diethyl carbonate as transesterification agent. Finally activity and selectivity of solid catalyst was explained on the basis of nature of active sites and pore structure of the catalyst. 1. Introduction The focus of chemical industry to make specialty chemicals using simple, atom efficient, and clean synthetic routes is gaining importance due to stringent environmental regulations and awareness of minimizing pollution as much as need to improve economics. The tandem reactions, wherein two or more reactions are possible to carry out in a single pot with compatible catalysts and reagents is one of the approaches used in developing clean processes, which would allow minimization of waste and thus the burden on the eco-system. Specialty chemicals such as β-amino alcohols are an important class of organic compounds due to their bifunctional nature having alcohol and amine functional groups in the same compound, which allow them to react in a wide variety of ways.1 These versatile compounds are extensively used in medicinal chemistry in the preparation of biologically active natural and synthetic products, artificial amino acids, and chiral auxiliaries for asymmetric synthesis.2-3 They are also useful as intermediates in the synthesis of perfumes,4 dyes,5 photo developers,5 and oxazolidones.6 β-Amino alcohols are usually produced from alkylene oxide and amine5,7 using a variety of activators and promoters such as metal triflates,8,9 metal halides,10-12 metal amides,13,14 and alkali metal salts such as perchlorates,15 clays,16 silica,17 and ionic liquids.18 However, these routes suffer from the drawback of either handling of hazardous alkylene oxides or the use of stoichiometric amount of reagents, longer reaction time, high pressure and temperature conditions. β-amino alcohols were also synthesized from non-hazardous alkylene carbonate19 using metal chloride20 metal amide,21 metal oxides,22 metal carbonate,22 and zeolite as catalysts.23 As a part of our ongoing program on expansion of environment-friendly methodologies for making fine chemicals, we developed a new methodology for the synthesis of β-amino * Corresponding authors. (S.P.G.) E-mail: [email protected]. Fax: +91-20-2590-2621. Tel: +91-20-2590-2169. (R.V.C.) E-mail: [email protected]. Tel : +1-785-864 1634. Fax : +1-785-864 6051. † Homogeneous Catalysis Division, National Chemical Laboratory. ‡ Department of Chemical & Petroleum Engineering, Center for Environmentally Beneficial catalysis, The University of Kansas.

alcohols from anilines, dialkyl carbonate, and ethylene glycol using a base catalyst via a tandem route. In this route (see Scheme 1), the transesterification and alkylation steps are carried out in a single step. This route has advantages over the earlier reported routes wherein amino alcohols are synthesized using aniline and alkylene carbonates,19,23 e.g., in situ generation of ethylene carbonate (EC) by transesterification reaction of dialkyl carbonate and ethylene glycol, get rid of the isolation and purification steps required for production of alkylene carbonates, and avoid the use of alkylene carbonates, which are conveniently produced from hazardous epoxides.24 The use of solid acid or base catalysts has received considerable attention in fine chemical synthesis due to their environmental compatibility, recyclability, greater selectivity, noncorrosiveness, lower cost, and longer shelf life. In this context, zeolites are promising due to their micro porous, aluminosilicate structure with highly ordered crystalline nature. The combination of acid and base properties and shape selectivity in the zeolite catalysts is an important factor for the synthesis of fine chemicals; particularly zeolite-Y is often employed as a preferred catalyst for various types of the reactions like alkylation, hydroamination, isomerization, polymerization, cyclization, nitrile hydrolysis, photoreduction, nitration of aromatic compounds, etc.25-27 Herein, we report for the first time one-pot synthesis of β-amino alcohols from aromatic amine, dialkyl carbonate, and ethylene glycol using highly efficient and recyclable Na-Y zeolite as a catalyst. The effect of operating this reaction system under pressure as well as pot reaction conditions has been shown to give different amino alcohols and N-alkylanilines. 2. Experimental Section Aniline and substituted aromatic amines, ethylene glycol (EG), dimethyl carbonate (DMC), diethyl carbonate (DEC), PbCO3, and ZnO were purchased from M/S S. D. Fine-chem Ltd., India. n-Dibutyltin oxide and tetraethylammonium bromide (TEAB) were purchased from Aldrich Chemicals. For the faujasites, Na-Y was provided by Su¨d-Chemie, India, and Na-X was obtained from Laporte-inorganic, U.K. Authenticity

10.1021/ie070617q CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008

Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 2485 Scheme 1

of Na-Y and Na-X was confirmed by XRD analysis. Dibutyl carbonate (DBC) was synthesized according to the literature procedure.28 Amines and diols were freshly distilled or recrystallized prior to use. Na-Y and Na-X zeolite were activated in air at 773 K for 6 h before use. 1H and 13C NMR spectra were recorded on Bruker AC 200 and Ac 400 instrument by using TMS as an internal standard (CDCl3 or acetone-d6 solution). IR spectra were recorded on Shimadzu 8000 series FTIR spectrometer. Mass spectrometry was performed on GCMS (M/s. Agilent, model 6890 GC with a 5973 N mass selective detector) and liquid-phase components were analyzed by GC (M/s. Agilent, model 6890) equipped with an auto-sampler instrument. Flash chromatography was performed using CombiFlash Companion instrument, supplied by Teledyne ISCO. 2.1. General Experimental Procedure for High-Pressure Reaction. High-pressure reactions were carried out in a parr autoclave (50 mL, SS-316). In a typical experiment, aniline (10.7 mmol), dimethyl carbonate (33 mmol), ethylene glycol (286 mmol), and the solid catalyst (0.25 g) were charged into the reactor. The reactor was flushed with nitrogen and then pressurized with nitrogen up to 3.4 MPa. Then the contents were heated up to 443 K under well mixed conditions and the progress of the reaction was monitored by withdrawing the intermediate samples which were quantitatively analyzed by GC for reactants and products. The reaction was continued for specified time, the contents cooled to room temperature and gas vented off. Liquid phase analysis was carried out by GC using HP-1 capillary column (30 m length × 0.32 mm i.d. × 0.25 µm film thickness). 2.2. General Experimental Procedure for Pot Reaction. In a typical experimental procedure, known quantities of aniline (10.7 mmol), diethyl carbonate (33 mmol), ethylene glycol (260 mmol), and a solid catalyst (0.25 g) were charged to a dried three-necked 100 mL round-bottom flask equipped with temperature controller, stirrer and fractional distillation assembly. The contents were flushed with nitrogen and heated at 399 K until the reflux of DEC commences, thereafter, the temperature was slowly raised to 418 K in 3 h. During this period, transesterification of ethylene glycol to ethylene carbonate occurs due to the effective removal of ethanol (for DMC as substrate, transesterification was carried out in the range of 363 to 398 K for 6 h). Excess of DEC was removed by increasing the temperature steadily from 418 to 433 K in 0.5 h. After the complete removal of DEC, the reaction was continued for 4 h at 433 K. A similar reaction procedure was followed for DBC as a substrate in which DBC being a high boiler (bp ) 480 K) could not be distilled out. Progress of reaction was monitored by time sampling and liquid phase was quantitatively analyzed using a gas chromatography as described in earlier section. 2.3. Isolation of Products. After completion of reaction, reaction mixture was cooled to room temperature and filtered to separate the catalyst. Water was added to the filtrate and it was extracted with diethyl ether (3 × 15 mL). The ether layer was dried over anhydrous Na2SO4 and concentrated. The

products were separated from concentrated organic phase by flash chromatography on a 4 g normal phase silica RediSep column employing n-hexane-ethyl acetate as an eluent with gradient programming. The products, N-phenylethanolamine, N-methyl-N-phenylethanolamine, and N-phenyldiethanolamine were thus isolated in pure form. [Caution! Due care should be exercised while handling β-amino alcohols as they can cause methemoglobinemia.] The products were confirmed by their GC-MS, IR, and 1H and 13C NMR spectra, and whenever possible, spectral data were matched with that reported in the literature.23 2.4. Product Characterization Data. 2-[(4-Methylphenyl)amino]ethanol (5b). IR (film): νmax ) 3382 (OH, NH), 2918 (CH), 1616 (CdC, Ar), 1519 (CdC, Ar), 1319 (C-N, Ar), 1062, 810 cm-1. 1H NMR (200 MHz, CDCl3): δ ) 2.24 (s, 3H; -CH3-Ar); 2.59 (brs, 2H; NH and OH); 3.27 (t, J ) 5.4 Hz, 2H; -NCH2-); 3.80 (t, J ) 5.4 Hz, 2H; -OCH2-); 6.57 (d, J ) 8.5 Hz, 2H; -CH, Ar); 6.98 (d, J ) 8.5 Hz, 2H; -CH, Ar). 13C NMR (50 MHz, CDCl3): δ ) 20.32 (-CH3, Ar); 46.57 (-NCH2); 61.10 (-OCH2); 113.58 (-CH, Ar); 127.32 (-C, Ar); 129.73 (-CH, Ar); 145.63 (-C, Ar). GC-MS (70 eV, EI) m/z (%): 151 (24) [M]+, 120 (100) [p-CH3-C6H4NH)CH2]+, 106 (3), 91 (17), 77 (6), 65 (7), 51 (2). 2-[(4-Methoxylphenyl)amino]ethanol (5c). IR (film): νmax ) 3421 (OH, NH), 2945 (CH), 1624 (CdC, Ar), 1514 (CdC, Ar), 1305 (C-N, Ar), 1076, 833 cm-1. 1H NMR (200 MHz, CDCl3): δ ) 3.04 (brs, 2H; NH and OH); 3.23 (t, J ) 5.3 Hz, 2H; -NCH2-); 3.73 (s, 3H; -OCH3-Ar); 3.80 (t, J ) 5.3 Hz, 2H; -OCH2-); 6.64 (d, J ) 9.1 Hz, 2H; -CH, Ar); 6.76 (d, J ) 9.1 Hz, 2H; -CH, Ar). 13C NMR (50 MHz, CDCl3): δ ) 47.84 (-NCH2); 55.64 (-OCH3, Ar); 60.53 (-OCH2); 114.81 (-CH, Ar); 115.61 (-CH, Ar); 140.83 (-C, Ar); 153.06 (-C, Ar). GC-MS (70 eV, EI) m/z (%): 167 (23) [M]+, 136 (100) [p-OCH3-C6H4NHdCH2]+, 121 (11), 108 (11), 93 (4), 77 (4), 65 (3). 2-[(4-Aminophenyl)amino]ethanol (5d). IR (film): νmax ) 3363 (OH, NH), 2945 (CH), 1614 (CdC, Ar), 1519 (CdC, Ar), 1325 (C-N, Ar), 1058, 827 cm-1. 1H NMR (200 MHz, acetoned6): δ ) 2.95 (brs, 4H; 3NH and OH); 3.19 (t, J ) 5.6 Hz, 2H; -NCH2-); 3.72 (t, J ) 5.6 Hz, 2H; -OCH2-); 6.47 (d, J ) 8.7 Hz, 2H; -CH, Ar); 6.60 (d, J ) 8.7 Hz, 2H; -CH, Ar). 13C NMR (100 MHz, acetone-d ): δ ) 47.18 (-NCH ); 61.04 6 2 (-OCH2); 113.72 (-CH, Ar); 121.64 (-CH, Ar); 142.07 (-C, Ar); 146.09 (-C, Ar). GC-MS (70 eV, EI) m/z (%): 152 (36) [M]+, 121 (100) [p-NH2-C6H4NHdCH2]+, 107 (5), 93 (23), 77 (6), 65 (11), 52 (3). 2-[(4-Chlorophenyl)amino]ethanol (5e). IR (KBr): νmax ) 3305 (OH), 3190 (NH), 2947 (CH), 1600 (CdC, Ar), 1499 (Cd C, Ar), 1312 (C-N, Ar), 1063, 813 cm-1. 1H NMR (200 MHz, CDCl3): δ ) 2.56 (brs, 2H; NH and OH); 3.27 (t, J ) 5.3 Hz, 2H; -NCH2-); 3.83 (t, J ) 5.3 Hz, 2H; -OCH2-); 6.56 (d, J ) 8.9 Hz, 2H; -CH, Ar); 7.1 (d, J ) 8.9 Hz, 2H; -CH, Ar). 13C NMR (50 MHz, CDCl ): δ ) 46.10 (-NCH ); 60.96 3 2 (-OCH2); 114.27 (-CH, Ar); 122.41 (-C, Ar); 129.04 (-CH, Ar); 146.55 (-C, Ar). GC-MS (70 eV, EI) m/z (%): 171 (26) [M]+, 140 (100) [p-Cl-C6H4NHdCH2]+, 111 (8), 105 (9), 91 (2), 77 (10), 65 (2). 2-[(4-Nitrophenyl)amino]ethanol (5f). IR (KBr): νmax ) 3442 (OH), 3274 (NH), 2968 (CH), 1599 (CdC, Ar), 1503 (Cd C, Ar), 1327 (C-N, Ar), 1040, 753 cm-1. 1H NMR (200 MHz, acetone-d6): δ ) 2.83 (brs, 2H; NH and OH); 3.38 (t, J ) 5.4 Hz, 2H; -NCH2-); 3.77 (t, J ) 5.4 Hz, 2H; -OCH2-); 6.73 (d, J ) 9.2 Hz, 2H; -CH, Ar); 8.01 (d, J ) 9.2 Hz, 2H; -CH, Ar). 13C NMR (50 MHz, acetone-d6): δ ) 46.23 (-NCH2);

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Table 1. Screening of Catalysts under Pressure Condition Using Aniline, DMC, and EGd

a Yields were determined by GC analysis and based on aniline conversion. b Products were confirmed by GC-MS. c In the absence of DMC. d Pressure reaction conditions: aniline, 10.7 mmol; DMC, 33 mmol; ethylene glycol, 286 mmol; catalyst, 2.3 mmol (entry 2-5), 0.25 g (entry 6-8); T, 443 K; N2 pressure, 34 bar; agitation speed, 13 Hz; time, 2 h; reaction volume, 20 mL.

60.83 (-OCH2); 111.76 (-CH, Ar); 126.79 (-CH, Ar); 137.82 (-C, Ar); 155.48 (-C, Ar). GC-MS (70 eV, EI) m/z (%): 182 (23) [M]+, 151 (100) [p-NO2-C6H4NHdCH2]+, 135 (5), 105 (58), 93 (2), 76 (7), 65 (4), 50 (4). 2-[(3-Hydroxyphenyl)amino]ethanol (5g). IR (film): νmax ) 3332 (OH, NH), 2947 (CH), 1606 (CdC, Ar), 1496 (CdC, Ar), 1338 (C-N, Ar), 1055, 767 cm-1. 1H NMR (200 MHz, acetone-d6): δ ) 2.96 (brs, 2H; NH and OH); 3.19 (t, J ) 5.6 Hz, 2H; -NCH2-); 3.72 (t, J ) 5.6 Hz, 2H; -OCH2-); 4.77 (brs, 1H; OH,Ar); 6.08-6.17 (m, 3H; -CH, Ar); 6.90 (dd, J ) 7.8 and 8.5 Hz, 1H; -CH, Ar) 13C NMR (100 MHz, acetoned6): δ ) 46.75 (-NCH2); 61.20 (-OCH2); 100.33 (-CH, Ar); 104.64 (-CH, Ar); 105.37 (-CH, Ar); 130.43 (-CH, Ar); 151.33 (-C, Ar); 159.17 (-C, Ar). GC-MS (70 eV, EI) m/z (%): 153 (34) [M]+, 122 (100) [m-OH-C6H4NHdCH2]+, 109 (3), 94 (13), 77 (5), 65 (9), 53 (2). 2.5. Catalyst Recycle Experiments. Na-Y zeolite catalyst was separated from the reaction mixture by filtration through Sartorius 393-grade filter paper. Separated catalyst was washed several times with acetonitrile to remove adhered organic impurities. The catalyst was dried at 373 K and then subjected to calcination at 773 K for 6 h in air and reused. 2.6. NH3, CO2 TPD, and BET Surface Area Measurements. The acidity and basicity of the solid catalyst was estimated by temperature programmed desorption (TPD) of NH3 on Quantachrome CHEMBET 3000 and CO2 on a Micromeritics AutoChem 2910 instrument by the following procedure. A weighed amount of the catalyst sample was placed in a quartz reactor and dehydrated at 773 K under the flow of He for 60 min. Then, the sample was cooled down to 298 K under the flow of helium and then treated with 10% CO2 in He under the flow of 30 mL/min for 40 min. The physisorbed CO2 was removed by heating the sample under the flow of He at 323 K for 10 min. CO2 was desorbed in a He flow by increasing the temperature to 923 K with a heating rate of 10 K/min measuring CO2 desorbed by TCD. For NH3 TPD measurement, similar procedure was followed except that samples were exposed to 2000 µL of NH3 at room temperature until saturation, and then physisorbed NH3 was desorbed at 423 K. The BET surface area was measured by means of nitrogen adsorption at 77 K preformed on a Quantachrome CHEMBET 3000 instrument.

3. Results and Discussions The objective of this work was to report one pot synthesis of β-amino alcohols using dimethyl carbonate, which has now been recognized as an effective reagent that can replace phosgene, methyl halides and dimethyl sulfate in conventional synthesis involving carbonylation and alkylation reactions.29-31 The route comprises of tandem synthesis of β-amino alcohol involving in situ transesterification reaction of dialkyl carbonate and ethylene glycol to ethylene carbonate followed by N-alkylation of aniline by ethylene carbonate (see Scheme 1). This reaction system was investigated under high-pressure as well as pot reaction conditions, and the wider applicability of the route is shown for different substrates and catalysts. 3.1. Tandem Synthesis of β-Amino Alcohol under HighPressure Reaction Conditions. The experimental results on catalyst screening, screening of dialkyl carbonates as substrates, and effects of catalyst loading, dialkyl carbonate concentration, and temperature were studied to understand the conversion of amines and selectivity pattern of amino alcohols. 3.1.1. Preliminary Experiments for Catalyst Screening. Transesterification of carbonates32-35 and N-alkylation of anilines by carbonates36-38 are well-known reactions in the literature. Generally, both acid and base catalysts are effective for transesterification; however base catalysts are often employed for N-alkylation of amines as well as for transesterification.39 For the purpose of preliminary catalyst screening, a few basic catalysts were tested for the reactions of aniline, dimethyl carbonate (DMC) and ethylene glycol (EG) and the results are shown in Table 1. Since, ethylene glycol was used in far excess compared to either aniline or DMC, it was a solvent as well as one of the reactants while, aniline was considered as the limiting reactant. Therefore, the results on parametric effects are presented on the basis of aniline conversion. In the absence of a catalyst, there was no reaction (Table 1, entry 1). From the catalyst screening, it was observed that basic catalysts like dibutyltin oxide (DBTO), tetraethylammonium bromide (TEAB), PbCO3, and ZnO showed good activity for the tandem reaction, giving rise to NPEA up to 20-50% yield (entry 2-5). Zeolites, such as Na-Y and Na-X, were also tested for this reaction and it was found that both the catalysts gave good yields of NPEA (∼50%, entry 6, 7) while Na-Y gave better conversion than Na-X (∼92%). Soluble catalyst, TEAB was more selective for methylated amino alcohol (N-methyl-N-phenylethanolamine)

Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 2487 Table 2. Characterization of Solid Catalyst

catalyst PbCO3 ZnO Na-X Na-Y a

NH3 desorbed

Si/Al ratio

BET surface area, (m2/g)

µmol/g

µmol/m2

1.2 2.4

2.67 4.20 522 584

0.74 0.62 5.35 6.89

0.28 0.15 0.01 0.01

CO2 desorbed Tmax,a

K

612; 629; 655 565 433; 451; 611 425; 425; 611

µmol/g

µmol/m2

Tmax,a K

acidity to basicity ratio

1.68 2.63 69.6 42.7

0.63 0.63 0.13 0.07

666; 697 571 547 532

0.44 0.24 0.08 0.16

Temperature at which maximum desorption is recorded

formation as well as bis-amino alcohol (N-phenyldiethanolamine), while DBTO showed preference for methylated anilines and amino alcohol (entries 2 and 3 respectively). Catalysts PbCO3 and ZnO showed good selectivity for NPEA (∼ 70%, entries 4 and 5), but the activity of these catalysts was lower (total yield of amino alcohol, ∼70%) compared to Na-Y zeolite (total yield of amino alcohol, 86%). In order to check the possibility of formation of β-amino alcohol (NPEA) from the condensation reaction of aniline and ethylene glycol, an experiment was carried out in the presence of Na-Y catalyst without DMC, wherein β-amino alcohol was not formed (entry 8). To explore the catalytic sites responsible for tandem synthesis of β-amino alcohol, the solid base catalysts were subjected to BET surface area as well as CO2 and NH3 TPD analysis, and these results are presented in Table 2. The metal oxide catalysts show poor surface area but have high density of Lewis acidicbasic sites per unit surface area. Further, the shape of NH3 and CO2 desorption curves shoved that, for Na-X and Na-Y catalyst a marked tailing is observed, which is indicative of diffusion controlled desorption characteristics of a channel architecture that exist in zeolite X and Y. While for ZnO and PbCO3 desorption peaks were sharper as expected for catalyst having low surface area, wherein active sites are distributed mainly on external surface of the catalyst. The NH3 desorption peaks for X and Y were similar in shape with three maxima of signal intensities (see Table 2). In both zeolites a broad hump was also observed at 611 K. The overall peak area was larger for Y zeolite compared to X indicating that Y zeolite has higher concentrations of acidic sites compared to Na-X. The NH3 TPD of ZnO showed sharp maxima at 565, while PbCO3 showed an intense NH3 desorption at 612, 629, and 655 K. The overall nature of NH3 TPD of X and Y zeolites showed weaker acidic sites compared to ZnO and PbCO3. The nature of Lewis and Brønsted acid sites of Na-Y zeolite was also qualitatively determined by pyridine sorption using DRIFT-IR technique at 373, 443, and 673 K. The detection of absorption bands at 1593 and 1442 cm-1 in the spectrum revealed the presence of Lewis acidity while absence of band at 1540 cm-1 indicated the absence of Brønsted acidity in the Na-Y zeolite of our catalyst. The CO2 TPD profile of the oxides showed that ZnO is having weak basic sites (Tmax at 571 K) compared to lead carbonate (Tmax at 666 and 697 K respectively). On the other hand zeolites X and Y showed weaker basic sites (a broad CO2 desorption peak in the range of 510-546 K; Tmax ) 532 K for Y and in the range of 510-571 K with Tmax ) 547 K for X) compared to oxide catalysts. Na-X zeolite shows higher density of basic sites as well as slightly stronger nature of these sites compared to Na-Y. Desorption of CO2 at lower temperatures in zeolites relative to lead carbonate corresponds to a weaker basic sites in Na-Y and this probably makes it (Na-Y) a better catalyst compared to metal oxide and in turn offer a better resistance toward deactivation by carbon dioxide (product), the same is also true with Na-X. A marked difference in the nature of acidic and basic active sites of X and Y zeolite revealed by NH3 and CO2 TPD is the smaller surface density of acidic sites of these zeolites compared to their basic sites (See Table 2, entries 3

and 4, for the acid to base ratio) indicating that basicity due to framework oxygen atoms seems to be playing a main role in substrate activation step, while on the other hand Lewis acidity is essential but seems to be less important in this case. Nevertheless combination of Lewis acid-base properties is preferred for an efficient catalysis. The high activity of zeolite catalyst is due to the high surface area associated with threedimensional structure of zeolite which accommodates active sites in their pores. It may be noted that the catalytic nature of active sites in zeolite looks similar to that exist in soluble catalyst TEAB (high conversion; see Table 1) indicating that distribution of active sites in zeolite X and Y is uniform. On the other hand due to the pore and cage size restrictions that exist in zeolite, a better selectivity for NPEA is realized with zeolite as catalyst compared to TEAB (note that yield of bis amino alcohol viz. NPDEA is ∼ 3-5 times lower for zeolite catalyst than that with TEAB; see Table 1, entries 3, 6, and 7). The metal oxide catalyst shows remarkably good control on formation of methylated derivatives and good yield of NPEA is realized despite of their poor surface area (entries 4 and 5). Thus ZnO, which shows the nature of the catalytic sites similar to those that exist in Na- X and Y with respect to their acidity and basicity, seems to be a good catalyst next to zeolite. A typical concentration -time profile for synthesis of β-amino alcohol using Na-Y zeolite catalyst is shown in Figure 1. Almost complete conversion of aniline was achieved at the end of 4 h and correspondingly DMC and EG (not shown in Figure 1) was also consumed with concurrent formation of amino alcohols and N-methylated anilines. Material balance was in complete agreement with the products formed as per the stoichiometry in Scheme 2. This also shows that the reaction is thermodynamically favorable under the experimental conditions. The Figure 1 shows that NPEA is rapidly formed (61%; Scheme

Figure 1. Progress profile of the reaction with time for high-pressure reaction. Pressure reaction conditions: aniline, 10.7 mmol; DMC, 33 mmol; ethylene glycol, 286 mmol; Na-Y zeolite catalyst, 0.25 g; T, 443 K; N2 pressure, 34 bar; agitation speed, 13.3 Hz; time, 4 h; reaction volume, 20 mL.

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Scheme 2

2, path 2) in the initial period from reaction of aniline and generated ethylene carbonate (generated in situ by transesterification reaction of DMC and ethylene glycol; Scheme 2, path 1). Thereafter, NPEA yield decreases due to further reaction of NPEA with DMC and EC to give N-methyl-N-phenylethanolamine (NMPEA; path 3) and N-phenyldiethanolamine (NPDEA; path 4), respectively (see Scheme 2). Usually reaction of aniline with dimethyl carbonate is known to give N-alkylated products (mono- and dimethylanilines; paths 5 and 6) at higher reaction temperatures.36-38 However, in the presence of ethylene glycol, reaction pattern of aniline and dimethyl carbonate is altered and dimethyl carbonate reacts more readily with ethylene glycol instead of aniline forming ethylene carbonate. Hence, lower yields of N-methylaniline (NMA) and N,N-dimethylaniline are realized compared to amino alcohols (discussed also in section 3.1.4). 3.1.2. Effect of Catalyst Loading. Effect of catalyst loading on initial rate of tandem reaction of aniline, dimethyl carbonate and ethylene glycol for β-amino alcohol synthesis was investigated in the range of catalyst loading 0.0063-0.018 g/cm3 at 433 K, and the results are presented in Figure 2. Initial rate of reaction showed linear dependence on catalyst loading, indicating the absence of any mass transfer effects under the conditions of present work. It was observed that both conversion of aniline and yield of β-amino alcohols were increased with increased in catalyst loading. 3.1.3. Effect of Temperature. The temperature effect was investigated in the range of 423-453 K and the results are

Figure 2. Effect of catalyst loading. Pressure reaction conditions: aniline, 10.7 mmol; DMC, 33 mmol; ethylene glycol, 286 mmol temperature, 443 K; N2 pressure, 34 bar; agitation speed, 13.3 Hz; reaction volume, 20 mL.

shown in Figure 3. The results show that with increase in temperature increased conversion of aniline and yields of NMPEA and NPDEA are obtained. While, yield of NPEA initially increase with temperature up to 443 K, but with a further increase in temperature, NPEA yield was found to decrease. Thus, higher temperature favors formation of NMPEA and NPDEA. It is interesting to note that with increase in temperature, yields of methylated anilines are not affected and remain below ∼5%. 3.1.4. Effect of Dimethyl Carbonate Concentration. Dimethyl carbonate plays a key role in the synthesis of amino alcohol derivatives, since DMC is involved in various reactions that take place under experimental conditions investigated in this study (see Scheme 2). The effect of DMC concentration on the yield of β-amino alcohols was investigated in the range 0.8 × 10-3 to 3.24 × 10-3 mol/cm3 (see Figure 4). Results show that yield of NPEA does not improve much on increase in DMC concentration from 0.81 × 10-3 to 1.62 × 10-3 mol/cm3. But as the DMC concentration is further increased from 1.62 × 10-3 to 3.24 × 10-3 mol/cm3, a decrease in the NPEA yield was observed, while, the yield of N-methyl-N-phenylethanolamine increased. The possibility of NMPEA formation can be realized via two pathways (see Scheme 2): in the first path NPEA is further methylated by DMC and in second case N-methylaniline

Figure 3. Effect of temperature. Pressure reaction conditions: aniline, 10.7 mmol; DMC, 33 mmol; ethylene glycol, 286 mmol; Na-Y zeolite catalyst, 0.25 g; N2 pressure, 34 bar; agitation speed, 13.3 Hz; time, 2 h; reaction volume, 20 mL.

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Figure 4. Effect of DMC concentration. Pressure reaction conditions: aniline, 10.7 mmol; DMC; ethylene glycol, 286 mmol; Na-Y zeolite catalyst, 0.25 g; T, 443 K; N2 pressure, 34 bar; agitation speed, 13.3 Hz; time, 2 h; reaction volume, 20 mL.

undergoes N-alkylation by EC giving rise to NMPEA. However, from Figure 4, it is clearly seen that major contribution of NMPEA formation is via N-methylation of NPEA as the yield profile of NPEA shows sharp decline with simultaneous increase in NMPEA yield. Whereas NMA yield profile is not much changed during this period. Therefore, high DMC concentration leads to more selective formation of NMPEA. 3.1.5. Effect of Dialkyl Carbonates. Various dialkyl carbonates, viz. dimethyl carbonate, diethyl carbonate, and dibutyl carbonate, were investigated for the tandem synthesis of corresponding β-amino alcohol derivatives, to examine their transesterification efficiency to form ethylene carbonate as well as their reactivity toward N-alkylation of anilines (see Figure

Figure 5. Effect of dialkyl carbonates. Pressure reaction conditions: aniline, 10.7 mmol; dialkyl carbonate, 33 mmol; ethylene glycol, 286 mmol; Na-Y zeolite catalyst, 0.25 g; T, 443 K; N2 pressure, 34 bar; agitation speed, 13.3 Hz; time, 2 h; reaction volume, 20 mL.

5). The results show that DMC is the most reactive substrate among the screened dialkyl carbonates but least selective for giving NPEA (because of its better methylating ability). Thus, screening results of carbonates indicate that highest yield of NPEA was obtained when DEC was used. DBC also showed promising results although it is the least reactive among the carbonates tested. 3.1.6. Effect of Amines. Various amines substrates were screened for tandem synthesis of β-amino alcohols from DMC and EG using Na-Y zeolite catalyst, and the results are presented in Table 3. It was observed that most of the amines show higher conversions (>92%) except p-chloroaniline and p-nitroaniline (see entries 4 and 5). The results are generally in agreement with the reactivity pattern expected for substituted aromatic amines.23 Electron-donating substituents such as -CH3 and -OCH3 enhance the nucleophilicity of aniline thus increas-

Table 3. Synthesis of β-Amino Alcohols from Anilines Using DMC and EG under Pressure Conditionsd

a Conversion and yields were determined by GC analysis. b Products were confirmed by GC-Ms. c Yield of 2-hydroxyethyl benzyl carbamate (12.3%) & 3-benzyloxazolidin-2-one (36.7%). d Pressure reaction conditions: amine, 10.7 mmol; DMC, 33 mmol; EG, 286 mmol; Na-Y catalyst, 0.25 g; T, 443 K; N2 pressure, 34 bar; agitation speed, 13 Hz; time, 2 h; reaction volume, 20 mL.

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Figure 6. (a) Catalyst screening under pot reaction conditions for DMC as TE agent. Pot reaction conditions: aniline, 10.7 mmol; dimethyl carbonate, 33 mmol; ethylene glycol, 286 mmol; catalyst, 2.3 mmol (ZnO, PbCO3, TEAB, DBTO) or 0.25 g (Na-Y, Na-X zeolite); T, 363-433 K; time, 9.5 h; reaction volume, 20 mL. (b) Catalyst screening under pot reaction conditions for DEC as TE agent. Pot reaction conditions: aniline, 10.7 mmol; diethyl carbonate, 33 mmol; ethylene glycol, 260 mmol; catalyst, 2.3 mmol (ZnO, PbCO3, TEAB, DBTO) or 0.25 g (Na-Y, Na-X zeolite); T, 399-433 K; time, 7.5 h; reaction volume, 20 mL.

ing the reactivity as well as total yield of amino alcohols (total yield of amino alcohol includes yield of mono-β-amino alcohol, methylated amino alcohol and bis-amino alcohol; see Table 3, entries 2 and 3). Electron-withdrawing substituents such as -Cl and -NO2 on the aniline ring decrease the reactivity as well as the yield of amino alcohols (see entries 4 and 5). In the case of m-aminophenol, both N-alkylation as well as O-alkylation by ethylene carbonate was observed (see entry 6). While in case of benzylamine having aliphatic nature, both alkylation as well as carboxylation takes place leading to formation of oxazolidone and carbamate in substantial amounts along with amino alcohols (entry 7). Similar observations were earlier made for benzylamine and dimethyl carbonate system giving rise to methylamine and carbamate.40 Since benzylamine is a stronger nucleophile41 than anilines, benzylamine can effectively attack both the electrophilic carbons of ethylene carbonate, viz. carbonyl carbon (which is hard) and methylene carbon (which is soft). Thus, benzylamine, which is neither too soft nor too hard in nature, is able to react with both the reactive centers of ethylene carbonate in accordance with HSAB theory. The reaction of benzylamine with soft electrophilic carbon gives rise to β-amino alcohols while reaction with hard electrophilic carbon produces 2-hydroxyethylbenzyl carbamate and 3-benzyloxazolidin-2-one (see Table 3, entry 7, 12.3% and 36.7% respectively). It may however be noted that, the explanation

Figure 7. (a) Concentration-time profile for pot reaction. Pot reaction conditions: aniline, 10.7 mmol; diethyl carbonate, 33 mmol; ethylene glycol, 260 mmol; Na-Y zeolite catalyst, 0.25 g; temperature, 399-433 K; time, 7.5 h; reaction volume, 20 mL. (b) Synthesis of N-phenyl diethanolamine. Pot reaction conditions: aniline, 10.7 mmol; diethyl carbonate, 33 mmol; ethylene glycol, 260 mmol; Na-Y zeolite catalyst, 0.25 g; After 7.5 h, 0.016 mol of DEC was added, and reaction was continued further for 12.5 h.

given here is qualitative in nature as applicability of nucleophilicity is much more complicated than described here.42 From the investigations on the tandem synthesis of β-amino alcohol from aniline, dimethyl carbonate and ethylene glycol under high-pressure reaction conditions, it can be concluded that all three amino alcohols, viz. N-phenylethanolamine, N-methyl-N-phenylethanolamine, and N-phenyldiethanolamine, are simultaneously formed. 3.2. Tandem Synthesis of β-Amino Alcohol under Pot Reaction Conditions. The tandem synthesis of β-amino alcohols was investigated under pot reaction conditions to investigate the possibility of maximizing the yield of one particular amino alcohol by controlling the reaction parameters. 3.2.1. Catalyst Screening. The screening of catalysts under pot condition was performed using both DMC and DEC as the

Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 2491 Table 4. Effect of Organic Carbonate on the Synthesis β-Amino Alcoholsc

a Conversion and yields were determined by GC analysis. b T, 363-433 K; time, 9.5 h. c Pot reaction conditions: aniline, 10.7 mmol; dialkyl carbonate, 33 mmol; ethylene glycol, 286 mmol; Na-Y catalyst, 0.25 g; T, 399-433 K; time, 7.5 h;

transesterification (TE) agents. This was thought necessary as DMC is more efficient toward transesterification and Nmethylation of aniline than DEC, while DEC is more selective for amino alcohols formation (see section 3.1.5). Parts a and b of Figure 6 show the formation of amino alcohols and N-alkylated anilines when DMC and DEC were used as TE agents, respectively. From the catalyst screening, it was observed that basic catalysts like ZnO, PbCO3, tetraethylammonium bromide, and dibutyltin oxide showed good activity for the tandem reaction, giving rise to NPEA up to 30-60% yield (Figure 6, parts a and b). Zeolites, such as Na-Y and Na-X were also tested for this reaction and it was found that Na-X gave moderate yield of NPEA (Figure 6a and b 55-70%) while Na-Y gave excellent yield of NPEA in presence of DEC (Figure 6b, yield ∼ 91%). It was also noticed that solid base zeolite catalysts were more selective toward NPEA formation (See Figure 6a and 6b) compared to TEAB and PbCO3. Formation of N-methylaniline was at its maximum with TEAB as the catalyst and DMC as the TE agent (See Figure 6a, ∼19% yield of N-methylaniline). On the other end, DEC as TE agent was more selective for NPEA synthesis and negligible Nethylaniline was seen with Na-Y as the catalyst. When DMC was used as the TE agent, PbCO3 and DBTO gave rise to 1214% yield of NPDEA; however, when DEC was used as the TE agent, PbCO3 and TEAB were found to give rise to a 7-9% yield of NPDEA (Figure 6b). These results suggest that DEC is the best TE reagent for tandem synthesis of NPEA from ethylene glycol and aniline, and hence, further reactions were carried out with DEC as the TE agent. A concentration-time profile (see Figure 7a; for the sake of convenience, the concentration profile for DEC and EG is not shown in this figure) of this reaction shows that ethylene carbonate is generated from transesterification of ethylene glycol and diethyl carbonate. During the transesterification step, the temperature of pot increases (399 to 418 K in 3 h) and EC reacts with aniline to form NPEA. At this stage, excess of DEC was distilled off within 0.5 h, meanwhile the reaction temperature is further raised from 418 to 433 K (for details see the Experimental Section, section 2.2), where upon concentration of EC and aniline decreases and NPEA formation increases with time. It was observed that as the concentration of NPEA increased, it further reacts with EC to give N-phenyldiethanolamine and simultaneously N-ethylaniline reacts with EC to give little amount of N-ethyl-N-phenylethanolamine (NEPEA). In order to see, if the yields of bis-amino alcohol (NPDEA) can be further enhanced. A fresh lot of DEC (0.016 mol) was injected slowly at 7.5 h to reaction mixture via dosing pump and reaction continued further up to 12.5 h. A maximum yield of 92% NPDEA was realized (see Figure 7b). 3.2.2. Effect of Dialkyl Carbonates. Various organic carbonates such as dimethyl carbonate and dibutyl carbonate were

also examined for tandem reaction under pot conditions (see Table 4). It was observed that, in the case of dimethyl carbonate, 100% conversion of aniline was achieved with 79.5% yield of NPEA (entry 1). However, during transesterification step N-alkylation of aniline by DMC gives rise to N-methylaniline in appreciable amounts which further undergoes N-alkylation by EC to give N-methyl-N-phenylethanolamine (entry 1, 12.6% yield). N-Alkylation of NPEA by EC to form NPDEA was also observed (entry 1, 6.4% yield). Both these reactions decrease the yield of NPEA (entry 1, 79.5% yield). When diethyl carbonate was used as a substrate, an appreciably smaller amount of N-ethylaniline was formed during transesterification step because diethyl carbonate is not a good N-alkylating agent as compared to dimethyl carbonate43 and NPEA was formed in high yields (91%, entry 2), while dibutyl carbonate as TE agent gives a lower yield of NPEA (56.5%, entry 3) as DBC is less reactive toward transesterification than both dimethyl and diethyl carbonate. The control on product selectivity under pot condition is better. 3.2.3. Effect of Aromatic Amines. In order to exemplify the wider range of applicability of the route, different types of amines were examined for β-amino alcohols synthesis from EG and DEC using Na-Y zeolite catalyst, and results are presented in Table 5. The different reactivities of aromatic amines were dependent on substituents of the benzene ring. p-toluidine, p-anisidine, and p-phenylene diamine, having electron donating groups present on the benzene ring, facilitated the rate of reaction and β-amino alcohols were obtained in excellent yields (entry 2-4). It was also observed in these anilines that continuing the reaction for longer contact time, further Nalkylation of amino alcohol by ethylene carbonate was taking place to give bis-amino alcohol (entry 1-3) derivatives. While p-chloroaniline and p-nitroaniline having electron withdrawing groups present on benzene ring decreases rate of reaction and poor yield of β-amino alcohols were obtained (entries 5 and 6). In most of the screened amines, N-ethylated amino alcohol and N-ethylated aniline were also formed in low yields and their combined yield in the range of 1.5 to 3.7% (see Table 5, last column, entry 1-4). It was also observed that in the case of p-phenylene diamine and m-aminophenol, the selectivity of mono-amino alcohol decreased due to further N- or O-alkylation of -NH2 and -OH groups present on the aromatic amines (entry 4 and 7). A different selectivity pattern was observed when m-aminophenol was used as a substrate. In this case, there is a possibility of N- as well as O-alkylation by ethylene carbonate and it was observed that O-alkylated product viz. 2-(3aminophenoxy)ethanol prevailed (10; 56.8%, Table 5, entry 7) over N-alkylated product, i.e., 3-(2-hydroxyethylamino)phenol (5g; 5.8%), while 2-{[3-(2-hydroxyethoxy)phenyl]amino}ethanol is formed as a result of both N- and O-alkylation (9; 27%).

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Table 5. Synthesis of β-Amino Alcohols from Anilines Using DEC and EG under Pot Conditionsc

a Conversions and yields were determined by GC analysis. b Side-products were confirmed by GC-MS, where side products corresponding to entry 1-4 are N-ethyl anilines and N-ethylated β-amino alcohols. c Pot reaction conditions: amine, 10.7 mmol; diethyl carbonate, 33 mmol; ethylene glycol, 260 mmol; Na-Y catalyst, 0.25 g; T, 399-433 K.

0.75 µm), and typically about 3-5% catalyst was lost during each recycle and these losses were accounted for in activity calculations. In this study, the catalyst recycle experiments were also carried out till complete conversion of aniline (contact time of 7.5 h), and it was observed that at the end of the fifth recycle ∼98% activity of catalyst was retained (for the sake of convenience, these data are not shown in the figure). These two sets of experiments show that catalyst is deactivated to some extent during reaction; however, the loss in catalyst activity can be compensated by increasing contact time of reaction. 4. Conclusion

Figure 8. Catalyst recycles study. Pot reaction conditions: aniline, 10.7 mmol; diethyl carbonate, 33 mmol; ethylene glycol, 260 mmol; Na-Y zeolite catalyst, 0.25 g; T, 363-433 K; time, 3.5 h; reaction volume, 20 mL

3.2.4. Catalyst Recycle Study. From the industrial point of view, one of the most important aspects is the reusability of the catalyst. The results of the catalyst reusability studies are presented in Figure 8. This figure depicts the results on catalyst recycle experiments at 3.5 h of contact time. The figure shows that catalyst can be recycled five times and it was observed that there was minor loss in its catalytic activity for each recycle and overall loss in catalyst activity was ∼7%. There were losses during handling of catalyst since the particles were fine (0.65-

Selective one-pot synthesis of β-amino alcohols from anilines, dialkyl carbonate and ethylene glycol has been demonstrated for the first time. In this synthesis, ethylene carbonate was generated in situ by transesterification reaction of dialkyl carbonate and ethylene glycol which further undergoes Nalkylation of aniline to give β-amino alcohol. Under highpressure reaction condition, the effect of reaction conditions was investigated for the reaction of aniline, dimethyl carbonate and ethylene glycol and it was observed that selectivity of monoβ-amino alcohol, i.e., NPEA was very poor (55%). The yield of NPEA decreased due to the formation of N-methylated amino alcohol by N-methylation of NPEA in the presence of DMC under high-pressure conditions. The selectivity of mono β-amino alcohol was improved significantly (>91%) by carrying out the reaction under pot reaction conditions using diethyl carbonate in which ethanol and excess DEC was removed during the reaction by reactive distillation. It was observed that the Na-Y catalyst was highly effective in converting various anilines to

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β-amino alcohols in high yields with excellent catalyst recyclability. Characterization of Lewis acidic and basic sites of solid catalyst indicated that, the basic sites are dominating the catalysis, while acidic sites are essential but seem to be playing secondary role in this case. Reactivity of various aromatic anilines and organic carbonates was studied and it was observed that electron-donating substituents enhance the nucleophilicity of aniline thus increasing the reactivity as well as yield of β-amino alcohols, while electron-withdrawing substituents on the aniline ring decrease the reactivity as well as the yield of β-amino alcohols. Diethyl carbonate was found to be the most suitable substrate for tandem synthesis of β-amino alcohol as it is not good N-alkylating agent, thus selectively β-amino alcohols are formed. Dimethyl carbonate is highly reactive which gave more N-methylated products that decrease the selectivity of NPEA, whereas dibutyl carbonate is less reactive for transesterification reaction that resulted in poor yield of β-amino alcohol. Several parameters play a major role in optimizing one particular product (due to complex nature of reaction network). For example, DMC in combination with TEAB as the catalyst gives rise to methylated amino alcohol and bis-amino alcohol under pressure conditions. Na-Y catalyst selectively gives NPEA when either DEC or DBC is used as the transesterification reagent for ethylene carbonate formation. It has been shown that in the presence of higher concentrations of ethylene carbonate, mono amino alcohol (NPEA) can be efficiently converted into bis-amino alcohol (NPDEA). Acknowledgment A.B.S. gratefully acknowledges the fellowship award for his research work by CSIR, Government of India. We also thank CSIR, New Delhi for financial support on network project P23CMM0005-J. We are very grateful to Su¨d-Chemie, India for supplying the Na-Y zeolite. The analytical help on FTIR and GC-MS by Mrs. S. K. Shingote is greatly appreciated. Nomenclature Abbreviations DMC ) dimethyl carbonate DEC ) diethyl carbonate DBC ) dibutyl carbonate EG ) ethylene glycol DBTO ) dibutyl tin oxide TEAB ) tetraethylammonium bromide NMA ) N-methylaniline NNDMA ) N,N-dimethylaniline NEA ) N-ethylaniline NPEA ) N-phenylethanolamine NMPEA ) N-methyl-N-phenylethanolamine NEPEA ) N-ethyl-N-phenylethanolamine NPDEA ) N-phenyldiethanolamine TE ) transesterification Literature Cited (1) Kirk-othmer; Encyclopedia of Chemical Technology, 4th ed.; HoweGrant, M., Ed.; Wiley-Interscience Publishers: New York, 1992; Vol. 2, p 1. (2) Ager, D. J.; Prakash, I.; Schaad, D. R. 1,2-Amino Alcohols and Their Heterocyclic Derivatives as Chiral Auxiliaries in Asymmetric Synthesis. Chem. ReV. 1996, 96, 835. (3) Chakraborti, A. K.; Kondaskar, A. ZrCl4 as A New and Efficient Catalyst for the Opening of Epoxide Rings by Amines. Tetrahedron Lett. 2003, 44, 8315 and references therein.

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ReceiVed for reView May 1, 2007 ReVised manuscript receiVed February 9, 2008 Accepted February 12, 2008 IE070617Q