A new recyclable functionalized mesoporous SBA-15 catalyst grafted with chiral Fe(III) sites for the enantioselective aminolysis of racemic epoxides under solvent free conditions

Abstract

An efficient and enantioselective strategy for the catalytic ring opening of racemic meso and terminal epoxides with aromatic as well as cyclic amine derivatives has been reported over a new mesoporous SBA-15 catalyst grafted with a chiral Fe(III) complex. The catalyst generates the expected chiral β-amino alcohols in over all better yields along with brilliant enantioselectivity (ee upto >99%) under solvent-free reaction condition at RT within 2–4 h reaction time. The catalyst is characterized in detail by PXRD, BET, HR-TEM, EPR, AAS, CHN analysis, IR and UV-Vis spectroscopic analysis etc. The catalyst is easily recoverable from the reaction system and can be used for five consecutive times without considerable loss of its catalytic activity as well as selectivity, which suggest sustainability of this chiral mesoporous catalyst.

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Introduction

Day by day, the scope and utility of chiral technologies have gained very high impact. The synthesis of fine chemicals, drugs and non-linear optical materials cannot be possible without optically as well as enantiomerically pure molecules. Several approaches are available to generate chiral compounds, counting chemical derivatization, chiral resolution and asymmetric catalysis.¹ Asymmetric catalytic transformations over chiral transition-metal catalysts are one of the most resourceful methods for obtaining optically pure products. Synthetic organic chemists show a prime interest to the transition metal catalysed asymmetric ring-opening (ARO) of epoxides using different types of amines because of its unprecedented activity to furnish various kinds of biologically and also commercially significant, enantiomerically pure β-amino alcohols.² In this context, a number of homogeneous catalysts have been synthesized which can efficiently catalyze the ARO of terminal and meso-epoxides by aromatic as well as cyclic amines.^3–5 However, most of these catalytic conditions have need of elevated temperature, long time, stoichiometric amounts of catalyst and hazardous solvent and furnish the product with low enantioselectivity. As a result, designing of stable, mild, low-priced, eco-friendly, and recyclable heterogeneous catalyst for the synthesis of enantiopure β-amino alcohols continue to attract the attention of researchers.

In this context, due to the ease of product separation, easy recovery of the high-priced chiral catalysts, heterogeneous chiral catalytic systems have attracted a wide attention.^6,7 Mesoporous materials have witnessed a remarkable growth in academic and industry because of their unusually huge surface area, tuneable pore size distribution, ease of functionalization with reactive organic groups/metal complexes and of course high hydrothermal as well as mechanical stability.⁸ As a consequence, synthesis of various mesoporous silica supported chiral metal complexes and their application in various organic transformations as an asymmetric heterogeneous catalyst have been an active area of research.⁹ However, the design and successful use of chiral mesoporous catalysts is still quite limited.¹⁰ Among various ordered mesoporous silica materials, SBA-15 with 2D-hexagonally ordered pore channels is considered an excellent support as it is convenient to functionalize, bearing high surface area and good thermal stability.¹¹

Herein, we have synthesized a new chiral iron complex of (S)-amino alcohol-supported SBA-15 material Fe@SBEP via condensation of (S)-epichlorohydrine with aminopropyl trialkoxysilane followed by grafting this chiral organosilane species throughout the surface of SBA-15 material, then epoxide ring opening by aniline and finally complexation with Fe(III)-salt (Scheme 1).

Scheme 1 Functionalization of SBA-15 to chiral Fe@SBEP.

Catalytic activity of Fe@SBEP has been explored in the field of ARO of meso and terminal epoxides with various aromatic as well as cyclic amines to produce highly regio- and enantio-selective β-amino alcohols in over all better yields (up to 98%) along with brilliant enantioselectivity (up to >99%) at RT in solvent free condition (Scheme 2). The mesoporous chiral catalyst has been recycled five repeating reaction cycles and it showed very good retention of product yields as well as enantioselectivity.

Scheme 2 Asymmetric ring opening reactions over Fe@SBEP.

Results and discussion

Characterization

Powder X-ray diffraction (PXRD). Low angle PXRD patterns of SBA-15 and silica-supported iron(III) complex of (S)-amino alcohol (Fe@SBEP) are shown in the Fig. 1. As shown in the inset of Fig. 1, SBA-15 displayed 2D-hexagonal mesostructure with three distinctive peaks at the corresponding values of 2θ = 0.97, 1.65 and 1.89, which can be ascribed as the 100 (strong), 110 (weak) and 200 (weak) reflections hexagonal mesophase.¹¹ When SBA-15 is functionalized with organic (S)-amino alcohol moiety and then with Fe(III) complex, a substantial drop in the peak intensity is observed, and also the peak positions are shifted in to higher 2θ value while 2D-hexagonal ordering has been remained unaltered (Fig. 1). This reduction in the peak intensities and the shifting in peak positions to higher 2θ value reveal the successful incorporation of that organic moiety into the pore walls of the mesoporous SBA-15 material.¹²

Fig. 1 Small angle PXRD of Fe@SBEP and SBA-15 (inset).

HR-TEM analysis. The HR-TEM images of Fe@SBEP chiral catalyst are shown in Fig. 2. These images show that three types of honeycomb like pores of ca. 3.2, 6.0 and 8.9 nm have been arranged throughout the material. This fact is further authenticated by the FFT diffractogram (Fig. 2D). The Fig. 2C (at right angles to the pore axis) demonstrates the parallel orientation of channel directions to the thickness of the plate. Hence, it can be conclude from all these TEM images that, the chiral catalyst is composed with highly ordered 2D-hexagonal mesopores which remain unaltered after organic functionalization and complexation.

Fig. 2 HR-TEM images of Fe(III)-grafted chiral heterogeneous Fe@SBEP catalyst ((A and B) are parallel to pore axis, and (C) is vertical to pore axis), FFT pattern is shown in (D).

BET surface area. The BET isotherm of the SBA-15-supported chiral iron complex Fe@SBEP at 77 K is given in the Fig. 3. The material exhibits a typical type IV isotherm along with a large hysteresis loop in the P/P_o region of 0.6 to 0.8. The surface areas of the pure SBA-15 and Fe(III) incorporated chiral heterogeneous catalysts are 705 and 240 m² g⁻¹, respectively. This substantial drop off of the BET surface area from pure SBA-15 to Fe@SBEP clearly suggests the covalent insertion of organic moieties at the surface of material and also Fe(III) sites have been successfully anchored on the surface of mesopores. The PSD of the Fe(III) grafted chiral heterogeneous catalyst has been evaluated by utilizing non-local density functional theory (NLDFT) (inset, Fig. 3), which showed trimodal porosity having peaks positions at 2.9 nm, 6.3 nm and 9.2 nm. Possibly, three types of pores are observed because all the pores are not functionalized with organic moieties simultaneously. Here it can be seen that, the pore dimension is decreased considerably, which also clearly suggests the incorporation of organic moieties into the pore channels of that mesoporous material. The pore volume of chiral Fe(III) loaded organically functionalized SBA-15 is 0.3665 cm³ g⁻¹ at P/P_o = 0.9994.

Fig. 3 N₂ adsorption/desorption isotherm of the Fe@SBEP sample at 77 K and its pore size distribution is shown in inset.

EPR data. Electron paramagnetic resonance (EPR) spectrum of Fe@SBEP is taken at room temperature in solid state to know the oxidation state of iron. The EPR spectrum of our catalyst is shown in Fig. 4. It exhibits a significant signal at lower magnetic field at 161 mT with g = 4.18 attributed to high spin d⁵ state of Fe(III) complex.¹³

Fig. 4 EPR of SBA-15 supported chiral iron complex Fe@SBEP.

Catalytic activity

To examine the catalytic activity of this catalyst, the first goal is to optimize the reaction condition, accompanied by reaction time, temperature, solvent and the catalyst amount. For this epoxide ring opening reaction, cyclohexene oxide and aniline as a nucleophilic partner, are used as model substrate to test the activity of Fe@SBEP and the results are shown in Table 1. Although, the ARO reactions were performed using different solvents like DCM, toluene, and H₂O (Table 1, entries 2–4), the most superior result in term of yield and enantioselectivity has been observed under solvent-free condition. The reaction is quite sensitive to the temperature. It was quite evident from the table that with increase in temperature (40 °C) enantioselectivity of the product decreased (Table 1, entry 5), while below room temperature also caused the same (Table 1, entry 9). We have optimised the catalyst loading and reaction time (Table 1, entry 6–11). It is found that 0.5 mol% (15 mg) of catalyst loading provides best performance within 2 h (entry 8). In terms of yield (96%) and enantioselectivity (97%), the best result is obtained by performing the reaction without any solvent at RT for 2 h using only 15 mg (0.5 mol%) of the chiral Fe@SBEP catalyst (Table 1, entry 8). In the absence of catalyst, no product is obtained under these conditions (Table 1, entry 1).

Table 1 Optimization table for ARO reaction using Fe@SBEP^a

Entry	Catalyst amt (mg)	Solvent	Temp. (°C)	Time	Yield^b (%)	ee^c (%)
a Condition: cyclohexene oxide (1 mmol), aniline (1 mmol), Fe@SBEP (15 mg, 0.5 mol%), without solvent, rt.b Isolated yield.c Determined by HPLC using Chiralpak OD-H column.
1	—	—	40	24	—	nd
2	20	DCM	40	4	80	71
3	20	Toluene	40	4	65	69
4	20	H2O	40	4	85	81
5	20	—	40	4	94	82
6	20	—	rt	4	96	95
7	15	—	rt	3	96	97
8	15	—	rt	2	96	97
9	15	—	10	2	78	93
10	10	—	rt	2	85	97
11	15	—	rt	1	81	94

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After optimizing the reaction condition, we examined other substrates with Fe@SBEP catalyst for the ARO reaction of different kinds of meso and terminal epoxides alike cyclohexene oxide (1a), allyl glycidyl ether (1b), 1,2-epoxy-3-phenoxy propane (1c), epichlorohydrine (1d), propylene oxide (1e) styrene oxide (1f) and glycidyl isopropyl ether (1g) with aniline (2a), substituted anilines [4-methoxy aniline (2b), 3-chloro aniline (2c), 2-iodo aniline (2d)] and cyclic amines [pipyridine (2d), morpholine (2e)] (Table 2). Irrespective of electron withdrawing or donating group attached with phenyl ring of the aniline derivatives all substrates gave the asymmetric amino alcohols as the final products with excellent conversion (87–98%) along with high enantioselectivity (72 to >99%) within 2–4 h reaction time. The less hindered carbon atom of the epoxide ring of allyl glycidyl ether was attacked by aniline under regioselective manner to form the respective product with moderate enantioselectivity (ee, 72%) having S configuration (Table 2, entry 1). Styrene oxide also under goes regioselective cleavage by aniline to furnish S-configured β-amino alcohol (Table 2, entry 10). Except allyl glycidyl ether and styrene oxide, all of the listed terminal epoxides gave the analogous amino alcohols having R configuration with good regio- and enantioselectivity. It is quite evident from Table 2, that all of the terminal epoxides gave only one single regioisomer without any traces of bis-adduct. Glycidyl isopropyl ether reacts with aniline to give the corresponding product (Table 2, entry 15) with good yield and enantioselectivity. Meso epoxide like cyclohexene oxide gave the corresponding (1R,2R)-amino alcohols with different substituted anilines with high yield (yield 90–96%) and enantioselectivity (ee, 98–99%) (Table 2, entry 5–7). Pipyridine (2d), morpholine (2e) react with cyclohexene oxide efficiently to give the corresponding amino alcohols in good yield (86–88%) and enantioselectivity (76–82%) (Table 2, entry 8 and 9). While, aliphatic amines like n-propyl amine and n-butyl amine gave no traces of product in the similar condition (Table 2, entry 10 and 11).

Table 2 ARO reaction over chiral Fe@SBEP catalyst^a

Entry	Epoxide	Amine	Time (h)	Product^b	Yield^c (%)	ee^d (%)	TOF (h^−1)
a Conditions: epoxide (1 mmol), amine (1 mmol), Fe@SBEP (15 mg, 0.5 mol%), without solvent, RT.b Absolute configurations were determined by comparing with the literature reports.c Isolated yield.d Determined by HPLC using Chiralpak OD-H column.
1	1b	2a	2.5		93	72	74
2	1c	2a	2		98	93	98
3	1d	2a	2		95	>99	95
4	1e	2b	3		87	88	58
5	1a	2a	2		96	97	96
6	1a	2b	4		95	>99	47
7	1a	2c	3		90	>99	60
8	1a	2d	3.5		88	82	50
9	1a	2e	3.5		86	76	49
10	1f	2a	2		95	98	95
11	1a	n-Propyl amine	8	—	—	—	—
12	1a	n-Butyl amine	8	—	—	—	—
13	1e	2a	2		97	89	97
14	1b	2d	3		87	78	58
15	1g	2a	3		88	95	59

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To explain the efficiency and applicability of our present catalytic system, we have summarized the comparative results with reference to the literature reports in Table S1.† From this table, it is obvious that our Fe@SBEP catalyst provides a better activity and enantioselectivity for the corresponding ARO reaction over the reported systems.

Recycling of catalyst

Ease of separation, recoverability and reusability are the most important part for a heterogeneous catalyst. The reusability of the Fe@SBEP catalyst was inspected for the representative ARO reaction between 1,2-epoxy-3-phenoxy propane and aniline. After the completion of the reaction, the entire part was centrifuged to collect the solid catalyst from the mixture. It was then washed thoroughly and repetitively with double distilled water followed by DCM and finally dried in oven at 90 °C for 6 h for reuse. Fig. 5 efficiently indicates that the catalyst can be recycled and reused for five times successfully without appreciable diminish in product yield and enantioselectivity. This fact is confirmed by the EPR spectrum of the reused catalyst which indicates that the oxidation state of iron centres in the catalyst remains unchanged after recycle (Fig. 6).

Fig. 5 Recyclability chart of chiral Fe@SBEP catalyst.

Fig. 6 EPR spectrum of reused Fe@SBEP catalyst.

Conclusion

We have synthesized chiral Fe@SBEP catalyst and its catalytic activity was examined in the field of enantioselective asymmetric ring opening (ARO) of different kinds of meso- and terminal epoxides with various aromatic as well as cyclic amines. Generally, the expected chiral β-amino alcohols are formed in good to very good yields with excellent regio- and enantioselectivity (generally 72 to >99% ee) without any solvent and within short reaction time at RT. The mesoporous chiral catalyst has 2D-hexagonally ordered porous structure, high BET surface area and good thermal stability. Further, low catalyst loading, environmentally benign reaction pathway and high recycling efficiency make the process green and cost effective. Thus we believe that this protocol for designing and successful use of chiral catalyst will find a useful application in green synthesis of enantiopure chiral β-amino alcohols.

Experimental section

Experimental procedure for synthesis of Fe@SBEP catalyst

The synthetic protocol of the chiral ligand (5) and the catalyst Fe@SBEP (6) were out lined in the Scheme 1.

Synthesis of chiral (2′S)-N-(2′,3′-epoxypropyl)-3-(aminopropyl)-triethoxysilane (3)¹⁰. In a typical experimental procedure, (S)-(+)-epichlorohydrine (1) (0.24 ml, 3 mmol) and 3-aminopropyltriethoxysilane (2) (0.7 ml, 3 mmol) were added to a stirred solution of anhy. K₂CO₃ (0.827 g, 5.98 mmol) in dry THF (4 ml) at RT. The entire content was refluxed at 65–70 °C for 14 h under inert atmosphere. Then after filtering, and the solvent was allowed to remove from the filtrate by dry N₂-draft (yield 93%). Due to very high moisture sensitivity of compound 3, throughout the reaction dry and inert condition was maintained and then without any purification the material was used directly for the synthesis of compound 4. The compound 3 was characterized by NMR spectra (ESI) which was in agreement with those reported earlier.

Synthesis of (S)-amino epoxy-SBA-15 (4). Mesoporous SBA-15 was prepared following the literature method.¹⁴ Then the calcined SBA-15 (2 g) was functionalized with compound 3 (0.776 g, 2.80 mmol) by refluxing in dry toluene for 48 h under an inert atmosphere. The resulting white solid was filtered, washed thoroughly with dry toluene and then dried under vacuum. Finally the material was allowed to Soxhlet-extraction with toluene for 10 h and dried under vacuum.

Synthesis of SBA-15 supported (S)-amino alcohol (5). (S)-Amino epoxy-SBA-15 (3.0 g) was added in dry toluene (20 ml). Then aniline (7 mmol) was added to this suspension under dry and inert atmosphere. The resulting mixture was heated at 60–65 °C for 13 h. Then the mixture was cooled, filtered and the separated solid was washed thoroughly by dry toluene and extracted with the Soxhlet extractor using toluene and isopropanol (70 : 30) for 10 h. Finally the sample was dried under vacuum at 45 °C.

Synthesis of SBA-15 supported Fe(III) complex of (S)-amino alcohol (Fe@SBEP). To the ethanolic solution of the chiral ligand 5 (1 g, 10 ml), FeCl₃·6H₂O (15 mmol) was added. The resulting mixture was stirred at RT for 8 h under inert atmosphere. The green solid 6 was collected by filtration and then washed with ethanol and isopropanol respectively. Finally the complex was dried under vacuum at 40 °C for 8 h. The Fe metal content in the heterogenized catalyst 6 was determined by AAS technique (1.8 wt% of Fe). The characterization data of Fe@SBEP is given in ESI.†

Acknowledgements

SMI acknowledges Department of Science and Technology, (DST-SERB, Project No. SB/S1/PC-107/2012 dated 10.06.2013), New Delhi, Govt of India, University Grant Commission (UGC, Project F. No. 43-180/2014(SR) dated 25^th July 2015), New Delhi, Govt of India and Department of Science and Technology, Govt of West Bengal (WB-DST, Project Sanction F. No. 811(sanc.)/ST/P/S & T/4G-8/2014, Dt-04/01/2016) for financial support. M. H. and M. M. I. acknowledges UGC for their fellowships. S. R. acknowledges Kalyani University for providing her fellowship (URS). We gratefully acknowledge DST and UGC, New Delhi, Govt of India for providing fund to the Dept of Chemistry, University of Kalyani through PURSE, FIST and SAP programme. SMI also acknowledges University of Kalyani for providing personal research assistance grant (PRG). We are thankful to Prof. Asim Bhaumik, Department of Materials Science, IACS, Kolkata for providing necessary characterization facility for this research work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23289b

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