Abstract

A novel application of titanosilicate molecular sieves in the synthesis of β-amino alcohols via aminolysis of epoxides, at mild and solvent-free conditions, is reported, for the first time. A range of β-amino alcohols with nearly 100% regio- and stereoselectivity was produced over these reusable solid catalysts in high yields.

Graphical abstract

A novel application of titanosilicate molecular sieves in the synthesis of β-amino alcohols via aminolysis of epoxides, at mild and solvent-free conditions, is reported, for the first time. A range of β-amino alcohols with nearly 100% regio- and stereoselectivity was produced over these reusable solid catalysts in high yields.

Keywords: Aminolysis of epoxides; Synthesis of β-amino alcohols; Titanosilicate molecular sieves; Ti-MCM-41

Article Outline

1. Introduction

2. Experimental

2.1. Catalyst preparation

2.2. Physicochemical properties

2.3. Reaction procedure

3. Results and discussion

3.1. Catalyst characterization and acidity measurements

3.2. Catalytic activity

4. Conclusions

Acknowledgements

References

1. Introduction

β-Amino alcohols are an important class of organic compounds used in the synthesis of biologically active natural and synthetic products, pharmaceuticals (β-blockers, for example), synthetic amino acids and pesticides [1], [2], [3], [4] and [5]. Their synthesis via ring-opening of epoxides with large excess of amines is a convenient route in organic synthesis. But this non-catalytic route requires high temperatures and hazardous solvents. Thermally sensitive epoxides cannot be selectively transformed due to formation of side products at high temperatures [6], [7], [8] and [9]. Several catalysts including sulfamic acid [10], Amberlyst-15 [11], metal triflates [12] and [13], metal alkoxides [14], metal halides [15], [16], [17], [18] and [19], transition metal salts [20], [21] and [22], heteropolymolybdate or tungstate [23] and [24], mono-dispersed silica nanoparticles [25], zeolites [26], montmorillonite clay [27] and ionic liquids [28] and [29] have been reported for this synthesis at mild conditions using stoichiometric amount of the amine reagent. However, the use of hazardous organic solvents, low catalytic efficiency (turnover frequency) and product selectivity, and non-reusability of the catalyst system are some of the issues with the existing methods. Less basic amines fail to open the epoxide ring at ambient conditions and require higher temperatures. Hence, there is a need for a more active and selective heterogeneous catalyst for this reaction.

Titanosilicate molecular sieves have been known for their remarkable selective oxidation activity of organic molecules at mild conditions using H₂O₂ as oxidant [30] and [31]. Recently, we reported their application as Lewis acid catalysts (1) for the synthesis of polycarbonate and polyurethane precursors utilizing CO₂ instead of toxic phosgene [32], [33], [34] and [35], and (2) for transesterification reactions of carbonates and carboxylic acid esters [33] and [36]. We, now, report their use as efficient, reusable, solid catalysts for the synthesis of a range of β-amino alcohols at room temperature and solvent-free conditions. The selectivity (both regio and stereo) of the products over these solids is much higher than that on the earlier catalysts for this reaction.

2. Experimental

2.1. Catalyst preparation

TS-1 (Si/Ti = 33; S_BET = 485 m²/g) was synthesized according to the reported procedure [37]. In a typical synthesis of Ti-MCM-41 (input ratio of Si/Ti = 30), 2.67 g of sodium hydroxide was dissolved in 147 g of distilled water. To that, 5.94 g of cetyltrimethylammonium bromide (CTMABr, S.D. Fine Chem. Ltd., India) was added. Then, 4 g of fumed silica (Aldrich) was added slowly while stirring over a period of 45 min. The stirring was continued for another 1 h; pH of the gel was maintained at 9–10 using dilute H₂SO₄ solution. To it, 0.657 g of titanium isopropoxide (Aldrich) in 5–10 ml of iso-propanol was added over a period of 15 min. The resultant gel was stirred for further 5 h at 298 K. It was then transferred into a teflon-lined stainless steel autoclave and heated to 373 K for 48 h. The solid product was filtered, washed with distilled water, dried at 353 K and finally calcined at 813 K for 6 h. Ti-MCM-41 with Si/Ti input ratio of 10, 20 and 40 were prepared in a similar manner taking appropriate amounts of the silicon and titanium sources. The output Si/Ti ratios were found to be 35, 20, 25 and 50, respectively.

2.2. Physicochemical properties

The catalysts were characterized as reported earlier [33] and [36]. IR spectra were recorded on a Shimadzu SSU 8000 DRIFT-IR spectrometer equipped with a liquid nitrogen-cooled MCT detector. Samples were activated at 698 K for 2 h under nitrogen flow. Then, they were cooled to 323 K and pyridine (30 μl) was adsorbed. The sample temperature was raised to a desired value and held at that temperature for 30 min and then, the spectrum was recorded. Temperature-programmed desorption (TPD) measurements were performed on a Micromeritics Autochem 2910 instrument. The sample (500 mg) was initially activated at 773 K for 2 h in He-flow (20 ml/min). It was then cooled to 353 K and 10% NH₃ in He was adsorbed for 30 min. The sample was flushed with He (30 ml/min) for 1 h at 373 K and the desorption was monitored by raising the temperature from 373 to 723 K at a ramp rate of 10 K/min.

2.3. Reaction procedure

In a typical reaction, equimolar amounts (5–30 mmol) of epoxide and amine were taken in a glass round-bottom flask (50 ml) placed in a temperature-controlled oil bath and fitted with a water-cooled condenser. To it, a known quantity of titanosilicate catalyst was added. The reaction was conducted at a specified temperature and for a desired period of time. The progress of the reaction was monitored by taking out aliquots of the sample, diluting it with a known quantity of dichloromethane, separating the catalyst by centrifugation and subjecting the diluted liquid to gas chromatographic analysis (Varian 3400; CP-SIL8CB column; 30 m-long and 0.53 mm-i.d.). The products were identified using GC–MS (Varian CP-3800; 30 m-long, 0.25 mm-i.d., and 0.25 μm-thick CP-Sil8CB capillary column). They were also isolated by column chromatography (eluent: petroleum ether–ethyl acetate mixture) and characterized by ¹H NMR studies. In some cases, for comparative studies, the experiments were conducted in the presence of solvent (5 ml). The characteristics of different isolated β-amino alcohol products are as follows:

2-Phenylamino-2-phenyl ethanol. GC–MS (m/e): 214.8, 213.8, 182.0. ¹H NMR: 3.7 (1H, dd), 3.9 (1H, dd), 4.4 (1H, dd), 6.5 (2H, d), 6.7 (1H, t), 7.1 (2H, t), 7.29 (5H, m).

2-(4-Chlorophenylamino)-2-phenyl ethanol. GC–MS (m/e): 249.1, 248.2, 246.3. ¹H NMR: 3.7 (1H, dd), 3.9 (1H, dd), 4.4 (1H, dd), 6.5 (2H, d), 7.05 (2H, d), 7.4 (5H, m).

2-(3-Methylphenylamino)-2-phenyl ethanol. GC–MS (m/e): 228.0, 227.4, 197.3, 196.4, 118.1 ¹H NMR: 2.3 (3H, s), 3.7 (1H, dd), 3.9 (1H, dd), 4.5 (1H, dd), 6.5 (3H, m), 7.0 (1H, t), 7.3 (5H, m).

2-(2-Methylphenylamino)-2-phenyl ethanol. ¹H NMR: 2.3 (3H, s), 3.8 (1H, dd), 3.9 (1H, dd), 4.5 (1H, dd), 6.37 (1H, d), 6.63 (1H, t), 6.95 (1H, t), 7.07 (1H, d), 7.35 (5H, m).

2-(4-Methoxyphenylamino)-2-phenyl ethanol. GC–MS (m/e): 244.9, 244.0, 136.2.

2-Butylamino-2-phenyl ethanol. GC–MS (m/e): 195.0, 194.2.

2-Phenylamino-cyclohexanol. GC–MS (m/e): 193.0, 192.1, 191.2, 132.2.

1-Chloro-3-phenylamino propan-2-ol. GC–MS (m/e): 188, 187, 186.2, 183.5, 106.2.

3. Results and discussion

3.1. Catalyst characterization and acidity measurements

The crystallinity and phase purity of TS-1 [33] and Ti-MCM-41 (Fig. 1(i)) were confirmed from their XRD patterns. TS-1 showed a characteristic oxygen-to-metal charge transfer band in the UV-visible spectrum with a maximum at around 210 nm [30]. Ti-MCM-41 samples (Fig. 1(ii)) showed a broad, asymmetric band in the region 200–300 nm indicating the presence of more than one type of Ti species in their structure. In a well-prepared calcined titanosilicate sample, at least two types of Ti species – tetrapodal Ti(OSi)₄ (showing characteristic UV band at 210 nm) and tripodal Ti(OH)(OSi)₃ (showing characteristic band at 220 nm) had been identified [30]. These species, on contact with air/moisture, form penta/hexa-coordinated Ti structures which show a band at around 280–290 nm. In fact, all these Ti species were present in our air-exposed Ti-MCM-41 samples (Fig. 1(ii)). The penta/hexa-coordinated Ti could be converted back to tetracoordinated, tri and tetrapodal Ti upon removal of water by thermo-evacuation. Absence of anatase-like TiO₂ phase (band at 330 nm) cannot be completely ruled out especially in Ti-MCM-41 samples containing higher amounts of Ti-content.

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Fig. 1. (i) X-ray diffractograms of calcined Ti-MCM-41. Si/Ti (output) molar ratio = 25 (a), 35 (b) and 50 (c). (ii) Diffuse reflectance UV-visible spectra of Ti-MCM-41. Si/Ti (output) molar ratio = 20 (a), 25 (b), 35 (c) and 50 (d).

The acidic properties of the samples were investigated by DRIFT spectroscopy of adsorbed pyridine and NH₃-TPD measurements. The samples showed peaks at around 1595 and 1445 cm⁻¹ due to H-bonded pyridine and 1580 and 1485 cm⁻¹ due to pyridine-coordinated to weak Lewis acid sites (Fig. 2(a and b)). Strong Lewis acid sites (peaks at 1623 and 1455 cm⁻¹) and Brönsted sites (peaks at 1639 and 1546 cm⁻¹) were absent. The IR bands were relatively more intense in Ti-MCM-41 than in TS-1 consistent with the easy accessibility of the Ti sites in the former than in the latter to the adsorbate molecules. The IR peaks due to pyridine disappeared completely above 398 K on TS-1 and 523 K on Ti-MCM-41 suggesting that the strength of the Lewis acid sites on Ti-MCM-41 is higher than on TS-1. Easy accessibility of Ti sites in Ti-MCM-41 with open tetrahedral Ti(OH)(OSi)₃ structure probably lead to the larger concentration of Ti-pyridine complexes than the Ti in TS-1 possessing closed tetrahedral Ti(OSi)₄ structure. NH₃-TPD studies showed a desorption peak at 448 K (Fig. 2(c)). This desorption in Ti-MCM-41 is more intense and asymmetric indicating, in agreement with the DRIFT studies, the differences in the strength of the Lewis acid sites. This difference in accessibility and acid strength of Ti sites in TS-1 and Ti-MCM-41 influence their catalytic activity (vide infra).

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Fig. 2. DRIFT spectra of adsorbed pyridine on (a) TS-1 and (b) Ti-MCM-41 (Si/Ti = 35). (c) NH₃-TPD of TS-1 and Ti-MCM-41 samples.

3.2. Catalytic activity

The ring-opening reaction of styrene oxide with aniline (20 mmol each) was carried out in solvent-free conditions at 308 K over TS-1 and Ti-MCM-41 catalysts (50 mg) (Table 1). Two types of regio-isomers A and B were obtained; the selectivity of the latter was significantly higher than that of the former (Scheme 1). Styrene oxide conversion of 80 mol% with B-isomer selectivity of 93.8 mol% was obtained over Ti-MCM-41 (entry 3). At similar reaction conditions, TS-1 showed only 11.5 mol% conversion and 90.8% of B-isomer product selectivity (Table 1, entry 2). The reaction occurs even in the absence of a catalyst but with very low styrene oxide conversion (6.6 mol%; selectivity of B-isomer = 89.7%). In other words, due to diffusion limitations, the reaction over TS-1 occurs mainly at the outer surface of particles. Most of the Ti sites in Ti-MCM-41 are possibly accessible to the reactants.

Table 1.

Influence of Ti structure: reaction of styrene oxide with aniline over different catalysts^a

Catalyst	Styrene oxide conv. (%)	Product selectivity (%)		TOF (h−1)
		A	B
Nil	6.6	10.3	89.7	–
TS-1	11.5	9.2	90.8	24
Ti-MCM-41	80.5	6.2	93.8	169
Ti-MCM-41 – 1st recycle	77.4	4.9	95.1	162
Ti-MCM-41 – 2nd recycle	77.0	5.8	94.2	162
TiO2 (2 mg)	9.1	14.0	86.0	18
TiO2 (50 mg)	48.0	13.7	86.3	4

^a Reaction conditions: catalyst (TS-1/Ti-MCM-41 (Si/Ti = 35)), 50 mg; styrene oxide, 20 mmol; styrene oxide:aniline (molar ratio), 1:1; solvent, nil; reaction time, 4 h; reaction temperature, 308 K.

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Scheme 1. Regioselective aminolysis of styrene oxide with aniline.

We have also carried out experiments with anatase-TiO₂. The Ti-content (in the sample) was kept the same as that used in the experiments with Ti-MCM-41. The conversion over anatase-TiO₂ was much lower (9.1 mol%) than that obtained over Ti-MCM-41 containing the same amount of Ti (80.5%) (Table 1, compare entries 6 and 3). This experiment, thus, indicates that dispersion, and surface accessibility of the Ti⁴⁺ ions are important in determining the catalytic activity of titanosilicate materials. The recovered catalyst was washed first, with dichloromethane and then, with methanol. It was dried at 373 K for 2 h and subjected to reuse (Table 1, entries 4 and 5).

The catalytic activity of Ti-MCM-41 decreased with increasing Ti-content (Table 2). Larger amounts of penta/hexa-coordinated Ti and anatase-like TiO₂ surface structure are present in high Ti-containing samples (Fig. 1(ii)). These anatase-like Ti structures were inactive leading to lower catalytic activity at higher Ti-content (Table 2, entries 3 and 4). Ti-MCM-41 with Si/Ti output ratio of 35 showed maximum aminolysis activity.

Table 2.

Influence of Si/Ti ratio of Ti-MCM-41 on the reaction of styrene oxide with aniline^a

Si/Ti mol. ratio	Epoxide conv. (%)	Product selectivity (%)		TOF (h−1)
		A	B
50	55.3	4.4	95.6	663
35	56.1	5.0	95.0	471
25	19.2	1.4	98.6	115
20	18.2	4.5	95.5	87

^a Reaction conditions: Ti-MCM-41, 50 mg; styrene oxide, 20 mmol; styrene oxide:aniline (molar ratio), 1:1; no solvent; reaction temperature, 308 K; reaction time, 1 h.

A few experiments were conducted also in the presence of solvents (Table 3). Styrene oxide conversion was lower in polar (acetone, methanol and acetonitrile) than in non-polar (toluene, dichloromethane and carbon tetrachloride) solvents. Interestingly, catalytic activity was higher when the reaction was conducted without any solvent (Table 2). Solvent molecules compete with those of the substrate for adsorption on the surface. Further experiments were conducted without any solvent.

Table 3.

Influence of solvent: reaction of styrene oxide with aniline^a

Solvent	Epoxide conv. (%)	Product selectivity (%)
		A	B
No solventb	99.5	2.4	97.6
Toluene	81.0	1.2	98.8
Dichloromethane	77.6	0.8	99.2
Carbon tetrachloride	71.2	1.2	98.8
Acetonec	31.2	1.5	24.7 (73.8)
Methanol	29.2	12.2	87.8
Acetonitrile	25.0	1.4	98.6

^a Reaction conditions: Ti-MCM-41 (Si/Ti = 35), 50 mg; styrene oxide, 1 mmol, styrene oxide:aniline (molar ratio), 1:1, solvent, 5 ml; reaction time, 4 h; reaction temperature, 303 K.
^b In reactions without solvent styrene oxide and aniline were taken 5 mmol each.
^c Selectivity in parenthesis corresponds to other product formed by condensation of acetone and aniline.

Fig. 3 shows the influence of reaction time and temperature on styrene oxide conversion and product selectivity. The conversion increased with reaction time but the product selectivity was unaffected. Although the reaction proceeded much faster, higher reaction temperatures adversely affected the product selectivity (Table 4 and Fig. 3). At near-ambient conditions, complete conversion of styrene oxide and very high selectivity of the B-type regio-isomer (97.6%) were obtained (Table 5).

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Fig. 3. Aminolysis of styrene oxide over Ti-MCM-41 (Si/Ti = 35). (i) Influence of reaction time. Reaction conditions: styrene oxide, 40 mmol; aniline, 40 mmol; Ti-MCM-41, 100 mg; temperature, 308 K. (ii) Influence of temperature. Reaction conditions: styrene oxide, 20 mmol; aniline, 20 mmol; Ti-MCM-41, 50 mg.

Table 4.

Influence of temperature: reaction of styrene oxide with aniline^a

Reaction temperature (K)	Epoxide conversion (%)	Product selectivity (%)		TOF (h−1)
		A	B
273	20.2	4.0	96.0	85
293	31.4	3.8	96.2	132
303	50.9	4.7	95.3	214
308	77.1	5.6	94.4	324
323	88.0	6.5	93.5	370
343	97.6	7.7	92.3	410

^a Reaction conditions: catalyst – Ti-MCM-41 (Si/Ti = 35), 50 mg; styrene oxide, 20 mmol; styrene oxide:aniline (molar ratio), 1:1; no solvent; reaction time, 2 h.

Table 5.

Influence of substrate to catalyst mole ratio: reaction of styrene oxide with aniline^a

Amount of styrene oxide (mmol)	Epoxide conversion (%)	Product selectivity (%)		TOF (h−1)
		A	B
30	76.4	7.1	92.9	240
25	80.0	6.0	94.0	210
20	93.3	4.5	95.5	196
10	97.5	4.3	95.7	102
5	99.5	2.4	97.6	52
10b	6.6	10.3	89.7	–

^a Reaction conditions: catalyst – Ti-MCM-41 (Si/Ti = 35), 50 mg; styrene oxide:aniline (molar ratio), 1:1; no solvent; reaction temperature, 308 K; reaction time, 4 h.
^b With no catalyst.

A range of β-amino alcohols could be synthesized in high yields at ambient conditions over Ti-MCM-41 catalysts. Electronic effects in substrate molecules have affected the conversions but not the products selectivity (Table 6). Aminolysis with aromatic amines yielded higher conversion than with aliphatic n-butyl amine.

Table 6.

Influence substrate structure on amino alcohol synthesis over Ti-MCM-41

Run no.	Epoxide	Amine	Epoxide conversion (%)	Product selectivity (%)		TOF (h−1)
				B	A
1			80.5	93.8	6.2	169
2			70.1	94.2	5.8	147
3			72.9	94.5	5.5	153
4			68.8	95.8	4.2	144
5			50.0	96.2	3.8	105
6			3.6	100	0	7
7			76.6	100	0	161
8			32.7	100	0	68

Reaction conditions: catalyst, 50 mg; epoxide, 20 mmol; epoxide:amine (molar ratio), 1:1; solvent, nil; temperature, 308 K; reaction time, 4 h.

Product yields were significantly higher in case of terminal epoxides than epoxides from cyclic olefins. Cyclohexene oxide is a symmetrical epoxide (Table 6, entry 8). Reaction with aniline results in cis- and trans-diastereomeric products (Scheme 2). Interestingly, only the trans-diastereomer was formed with 100% selectivity over Ti-MCM-41 (Table 6, entry 8).

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Scheme 2. Stereoselective aminolysis of cyclohexene oxide with aniline.

Earlier, in the application of titanosilicate molecular sieves for the synthesis of cyclic carbonates and dialkyl and aryl carbamates [34] we found that Lewis acidic Ti⁴⁺ ions are the sites for adsorption and activation of epoxide and amine molecules. Depending on the basic strength of amine molecules, either the epoxide or amine molecule can adsorb on Ti sites. After activation, the subsequent coupling reaction forms the β-amino alcohol. Ti-MCM-41 having a higher density and strength of accessible Lewis acid sites [30] exhibits a higher catalytic activity.

4. Conclusions

A novel application of titanosilicates in the synthesis of β-amino alcohols via., opening of epoxide ring with amines is reported for the first time. Very high (100%) regio- and stereoselective synthesis of β-amino alcohols has been achieved at mild and solvent-free conditions over mesoporous Ti-MCM-41 catalysts. The catalysts are reusable and a range of β-amino alcohols could be synthesized with high yields.

Acknowledgments

J.K. Satyarthi and L. Saikia acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi for the award of research fellowships.

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