Chiral Co(III)–salen complex supported over highly ordered functionalized mesoporous silica for enantioselective aminolysis of racemic epoxides

Abstract

Here we demonstrate the synthesis of a novel chiral Co(III)–salen complex supported functionalized 2D-hexagonal mesoporous silica material Co(III)@AFS-1. This material has shown excellent catalytic activity for the regio- and enantioselective asymmetric ring opening (ARO) of terminal and meso epoxides using various aromatic as well as cyclic amines to produce chiral β-amino alcohols having very good enantioselectivities (ee > 99%) at ambient temperature under solvent-free neat conditions.

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1 Introduction

The enantio- and stereo-selective ring-opening of racemic epoxides by amines as a nucleophilic partner, is a highly convenient and elegant approach in synthetic/medicinal chemistry to construct biologically and medicinally demanding β-amino alcohols.¹ Among numerous applications, β-amino alcohols are widely used as insecticidal agents, chiral auxiliaries and β-adrenergic blockers in chiral synthesis.² A number of synthetic pathways for the generation of β-amino alcohols in excellent yield and enantioselectivity are reported through the ARO of trans, racemic and meso epoxides by aromatic/alkyl amines over different metal-bearing catalysts.³ The S_N2 nucleophilic addition of achiral amines into the activated epoxide produces two adjacent stereogenic centres in a single step. The nature of reactive moieties in the chiral catalyst can directly influence the catalytic activity as well as enantioselectivity of the product. Besides it is the key factor, which determines the requirement of additives, catalyst loading, recyclability of the catalyst etc. Most frequently, the value added chiral β-amino alcohols were synthesized by means of homogeneous chiral salen complexes of various metal ions.⁴ But the immobilization of metal–salen complex into a suitable solid support often become highly desirable as the chiral catalysts are highly expensive. Hence proper designing of the salen-containing chiral heterogeneous catalyst is highly desirable for the ARO reaction of racemic epoxides to obtain outstanding product yield as well as high enantioselectivity. Some research groups have efficiently used chiral Co(III) and Cr(III) salen complexes to synthesize chiral β-amino alcohols to achieve higher yield and enantiomeric excess.⁵ In this circumstance, loading of metal–salen complexes onto the functionalized mesoporous materials is often considered as an useful strategy in designing the highly reactive and robust heterogeneous catalytic systems.⁶ Largespecific surface area, controllable pore dimension, distinct pore arrangement, high hydrothermal and mechanical stability and exceptional recyclability made these materials to be reliable as a solid support. The pore size and linking ability of these materials can be tuned through some chemical reactions, so that, the active sites can be bonded to the functional groups present at the surface of these materials.⁷ It is worth mentioning that, only few mesoporous chiral catalysts with periodic arrangement of pores are reported till date.⁸ Therefore, significant research interest has been paid for designing the organic–inorganic hybrid mesoporous silica bearing immobilized chiral salen complex at its surface. Very recently, our group has explored ARO reaction using mesoporous silica supported Fe(III) Salen complex.^4g Herein, we first report an efficient chemically transformed carboxylic acid functionalized mesoporous silica (AFS-1) supported chiral Co(III)–salen catalyst through functionalization of outer surface of SBA-15 using 3-aminopropyl triethoxysilane (3-APTES) followed by 4-formylbenzoic acid. This chiral catalyst Co(III)@AFS-1 catalyses the ARO of meso and terminal epoxides with aromatic and cyclic-aliphatic amines to the β-amino alcohols in very good yield together with excellent enantiomeric excess at ambient temperature under neat conditions (Scheme 1).

Scheme 1 Enantioselective and regeoselective ARO of epoxides with aminescatalyzed by Co(III)@AFS.

2 Experimental

2.1. Synthesis of Co(III)@AFS-1 catalyst

In Scheme 2 we have shown the schematic pathways for the synthesis of chiral catalyst Co(III)@AFS-1. The most probable structure of the aldehyde (2), chiral Schiff base ligand (5) and homogeneous chiral Co(III)–salen complexes (6) are examined by ¹H NMR and HRMS analysis (ESI). The m/z value obtained for the complex 6 was (C₄₂H₆₈Cl₃CoN₄O₂) 789.39 (M⁺, –Cl), which suggested successful synthetic protocol.

Scheme 2 Synthesis of the chiral Co(III)@AFS-1 catalyst. Reagents and condition: (a) (HCHO)_n, HCl, 313 K, 72 h, 96%; (b) triethylamine, benzene, reflux, 36 h, 95%; (c) (1S,2S)-(+)-1,2-diaminocyclohexane, abs. EtOH, reflux, 8–9 h, 93%; (d) (1) Co(OAc)₂·H₂O, EtOH, reflux, N₂, 8–9 h (2) LiCl, rt, 4 h, 89%; (e) CHCl₃, reflux, N₂, 14 h; (f) MeOH, reflux, 5–6 h; (g) EtOH, 318 K, 10 h.

2.1.1 Synthetic procedure of homogeneous chiral Co(III) salen complex (6). Homogeneous chiral Co(III) salen complex has synthesized through the following steps.
2.1.1.1 Preparation of 3-tert-butyl-5-chloromethyl-2-hydroxybenzaldehyde⁹ (2). In a pre-cooled (5–10 °C) medium of 22 mL concentrated hydrochloric acid, 3-tert-butyl-2-hydroxybenzaldehyde (1) (5.3 g, 30.0 mmol) and paraformaldehyde (2.0 g, 65.7 mmol) were added, and the ensuing mixture was stirred at 313 K for 72 h along with slow and continuous HCl gas bubbling for 6 h. Then the crude mixture was repetitively extracted using diethyl ether (3 × 15 mL), followed by washing with NaHCO₃ (sat. sol.) and brine. The organic part was dried using anhy. Na₂SO₄ and evaporated to obtain 2 as a yellow crystalline solid (96% yield). The ¹H NMR data of the product (ESI) was in agreement with those reported earlier.¹⁰
2.1.1.2 Preparation of the complex 3 (ref. 10). Compound 2 (1.5 mmol, 15 mL) in dry benzene was taken into a 100 mL RB flask. After then, 1.5 mmol of triethylamine, taken in 15 mL dry benzene, was mixed drop wise to the stirred solution of 2. The resulting hazy solution was then refluxed for 36 h. When the reaction was completed, the mixture was cooled to rt and solvent was evaporated using rotary evaporator to furnish quaternary aldehyde 3 (95% yield). The spectroscopic data of the product was in accordance with the reported literatures.¹⁰
2.1.1.3 Synthetic procedure for the chiral Schiff base ligand¹⁰ (5). To the ethanolic (abs.) solution of complex 3 (2 eqvt, 15 mL), ethanolic (abs.) solution of (1S,2S)-(+)-1,2-diaminocyclohexane (1 eqvt, 10 mL) was added drop wise. The entire mixture was heated at 90 °C for 8–9 h (monitored on TLC). Then the solvent was evaporated partially, and the yellow crystalline product (yield 93%) was precipitated by n-hexane. The ¹H NMR data of the product is given in ESI.†
2.1.1.4 Synthesis of homogeneous chiral Co(III) salen complex (6). To an abs. ethanolic solution of the chiral Schiff base ligand 5 (3 mmol, 30 mL), ethanolic solution of Co(OAc)₂·4H₂O (6 mmol, 20 mL), was added drop wise in inert atmosphere. The as-formed dark brown mixture was heated at 90 °C for 8–9 h. Then the mixture was allowed to cool to rt, then 9 mmol solid LiCl was added and stirred further for an extra 4 h under air. Next, the solvent was evaporated and the resulting mass was extracted with DCM (80 mL). The organic phase was washed with deionised water followed by brine, and finally dried using Na₂SO₄. Then, the solution was further reduced to furnish the expected complex as the dark brown solid (yield 89%), which was precipitated from n-hexane. The characterization data Co(III) salen complex is given in ESI.†

2.1.2 Synthesis of the mesoporous support.
2.1.2.1 Synthesis of 3-APTES functionalized SBA-15 material (9). Mesoporous SBA-15 (7) was synthesized according to a literature procedure.¹¹ Then it was functionalized with 3-APTES (1.8 g, 8) by refluxing 1 g of SBA-15 (8) in dry CHCl₃ at rt (25 °C) for 14 h in N₂ atmosphere.¹² The resulting mass was filtered, and washed repetitively with CHCl₃ and dichloromethane, and ultimately dried in air.
2.1.2.2 Synthetic procedure of acid functionalized material AFS-1 ¹². To the methanolic solution (70 mL) of 4-formylbenzoic acid (10) (0.30 g, 2 eqvt), 1.0 g of 3-APTES functionalized SBA-15 (9) was added. The mixture was then heated at 333 K for 5–6 h. The resulting mass was filtered, washed repetitively using hot methanol to remove all of the unreacted carboxylic acid. Finally it was dried in oven at 60 °C.

2.1.3 Synthesis of the heterogeneous chiral Co(III) complex Co(III)@AFS-1 catalyst. The acid functionalized mesoporous support AFS-1 (0.25 g) was added to the ethanolic solution (60 mL) of the homogeneous chiral Co(III) salen complex (6) (0.05 g). It was then stirred at 318 K for 10 h. Then the ensuing brown solid was centrifuged, washed down thoroughly and extracted repetitively with EtOH and DCM on Soxhlet apparatus until the colourless washing was appeared. The amount of cobalt metal in the heterogenized catalyst was determined by using atomic absorption spectrophotometer (ESI†).

3 Results and discussion

3.1. Characterization of catalyst

3.1.1 Atomic absorption spectroscopy (AAS) analysis. The observed Co content in the Co(III)@AFS-1 catalyst is 0.0984 mmol g⁻¹ and that after 5^th catalytic cycle is 0.0923 mmol g⁻¹. Both are determined by atomic absorption spectroscopy analysis. This study clearly indicates that practically no metal leaching occurred during the course of the reaction and our Co(III)@AFS-1 catalyst is truly heterogeneous in nature.

3.1.2 XRD study. The small angle XRD pattern for the as synthesized Co(III)@AFS-1 catalyst is shown in Fig. 1a. The Co(III)–salen incorporated AFS-1 chiral catalyst exhibited one sharp peak at 2θ = 0.82° and a very weak peak at 1.3°. These peaks can be ascribed as strong 100 and weak 110 reflections of the 2D-hexagonal mesoporous structure.¹³ The wide angle XRD pattern for the as synthesized Co(III)@AFS-1 catalyst is also included in ESI Fig. S6.†

Fig. 1 (a) Low angle PXRD pattern of the Co(III)@AFS-1 catalyst. (b) The N₂ adsorption–desorption isotherm of Co(III)@AFS-1. The PSD estimated by NLDFT method is given inset.

3.1.3 N₂ adsorption–desorption isotherms. The N₂-adsorption desorption isotherm of as prepared SBA-15 and Co(III)@AFS-1 chiral catalyst at 77 K are given in Fig. S7 (ESI)† and 1b. The catalyst revealed a typical type-IV isotherm along with large H1 type hysteresis loop in the high pressure region P/P₀ 0.55 to 0.79, which strongly support the mesoporous nature of the material.^11,14 The pore size distribution (PSD) plot obtained through NLDFT (non-local density functional theory) method is given inset of Fig. 1b. The PSD suggested peaks at 2.58 and 10.47 nm, which suggested the mesoporous nature of the Co(III)@AFS-1 chiral catalyst. The BET area of surface of the chiral catalyst is 76 m² g⁻¹.

3.1.4 TEM analysis. High resolution-transmission electron microscopic images of AFS-1 and Co(III)@AFS-1 are shown in Fig. 2. Huge numbers of low electron density spots are appeared all over the material.¹⁵ They are highly ordered in nature and appeared as honeycomb like 2D-hexagonal arrangement. This is further established from the hexagonal array of FFT patterns given in the insets of Fig. 2.

Fig. 2 The UHR-TEM images of Co(III)@AFS-1 (A) and AFS-1 (B) materials. The FFT patterns are shown in the insets.

3.1.5 TGA analysis. To check the thermal stability of freshly prepared Co(III)@AFS-1 material, thermogravimetric analysis has been carried out from room temperature (25 °C) to 800 °C with a temperature ramp of 10 °C min⁻¹ under air flow. The TG/DTA profile diagram of Co(III)@AFS-1 material has been given in Fig. 3. From Fig. 3a it is seen that initially the weight loss up to 102 °C due to the evaporation of surface adsorbed moisture and the second weight loss from 162 to 350 °C temperature could be attributed to the decomposition of organic part of the material. Thus, the thermal analysis data reveals the considerably good thermal stability of Co(III)@AFS-1.

Fig. 3 TG/DTA profile diagram of Co(III)@AFS-1.

3.2. Catalytic activity

To evaluate the catalytic efficiency of the Co(III)@AFS-1 catalyst, a number of reactions are carried out changing reaction parameters like solvent, catalyst loading and time for the corresponding ARO of cyclohexene oxide with aniline as the nucleophilic partner. The results are summarized in Table 1. In absence of catalyst as well as solvent, only a little amount of β-amino alcohol was formed after 24 h at RT (Table 1, Entry 1). Variation in the amount of catalyst loading influences the yield as well as enantioselectivity of the products (Table 1, Entries 2–5). It was found that, solvent-free neat conditions gave the best results in terms of yield and enantioselectivity for this ARO reaction (Table 1, entries 6–8). The highest product yield is obtained after 1.5 h at room temperature under neat condition (Table 1, Entry 3). Appreciably as envisioned, this catalytic reaction condition did not demand an addition of any kind of additives or a base to promote the reaction.

Table 1 Optimization of the reaction condition over Co(III)@AFS-1 catalyst^a

Entry	Catalyst amount (mg)	Solvent	Time (h)	Yield^b (%)	ee^c (%)
a Conditions: cyclohexene oxide (1.0 equvt), aniline (1.0 equvt), Co(III)@AFS-1 (25 mg, 2.46 × 10^−6 mol Co or 0.246 mol% Co), without solvent, RT.b Isolated yield.c Determined by chiral HPLC analysis using Chiralpak OD-H column.
1	—	—	24	Trace	Nd
2	30	—	1.5	97	98
3	25	—	1.5	97	>99
4	20	—	2	89	91
5	15	—	3	85	88
6	25	Toluene	3	67	64
7	25	DCM	3	79	82
8	25	THF	3	65	51

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To investigate the scope and utility for this new chiral catalyst, different kind of racemic terminal epoxides, viz. allylglycidyl ether, styrene oxide, epichlorohydrin, 1,2-epoxy-3-phenoxy propane, propylene oxide etc. and meso epoxide like cyclohexene oxide are employed for aminolysis using aromatic amines like aniline, 3-chloro-aniline, p-anisidine, as well as cyclic amines viz. piperidine, and morpholine to deliver a library of chiral β-amino alcohols. Table 2 shows the summarized results. The catalyst exhibited excellent efficiency in most of the cases in case of yield as well as enantioselectivity. Generally, substrates irrelevant of electron withdrawing or donating group on aromatic ring of the amine counterpart provided the products by means of excellent conversion (91–98%) and high enantioselectivity (87 to >99%). A regioselective cleavage of styrene oxide by aniline provides the corresponding product (Table 2, entry 5) having ‘S’ configuration (ee 98%). This can be explained as the preferential attack by aniline, at the benzylic carbon atom having greater electrophilic nature due to the presence of phenyl ring at that carbon.

Table 2 Enantioselective ARO reaction of various epoxides with different anilines/amines over chiral Co(III)@AFS-1catalyst^a

Entry	Epoxide	Amine	Product^b	Time, h	Yield^c, %	ee^d%	TON^e
a Reaction conditions: epoxide (1 mmol), amines (1 mmol), Co(III)@AFS-1 (25 mg, 2.46 × 10^−6 mol Co or 0.246 mol% Co), neat condition, RT.b Absolute configurations were allocated through comparing the HPLC retention time and the sign of optical rotation with the literature data.c Yields refer to those of isolated pure products.d Enantiomeric excess was determined by HPLC analysis using the Chiralpak OD-H column.e TON = moles of substrate converted per mole of active site.f No product was isolated.
1	Cyclohexene oxide (1a)	Aniline (2a)		1.5	97	>99	394
2	Cyclohexene oxide (1a)	m-Cl-C6H4NH2 (2b)		2	91	98	370
3	Cyclohexene oxide (1a)	Morpholine (2c)		2.5	89	77	362
4	Cyclohexene oxide (1a)	Piperidine (2d)		2.5	87	80	354
5	Styrene oxide (1b)	Aniline (2a)		1.5	96	98	390
6	Allyl glycidyl ether (1c)	Aniline (2a)		2	92	94	374
7	1,2-Epoxy-3-phenoxy-propane (1d)	Aniline (2a)		1.5	98	>99	398
8	Epichloro hydrin (1e)	Aniline (2a)		1.5	96	98	390
9	Propylene oxide (1f)	Aniline (2a)		1.5	98	98	398
10	Propylene oxide (1f)	p-Anisidine (2e)		2	95	87	386
11^f	Cyclohexene oxide (1a)	n-Propyl amine (2f)	(3k)	6	—	—
12^f	Styrene oxide (1b)	n-Butyl amine (2g)	(3l)	6	—	—

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Cyclohexene oxide undergoes ARO reaction with cyclic amines viz. morpholine and piperidine to furnish good yield as well as good enantioselectivity (Table 2, Entry 3, 4). Whereas, aliphatic amines like n-propyl amine and n-butyl amine give no yield (Table 2, Entry 11, 12). It is worthy to mention that in our present study, AFS-1 supported Co(III)–salen complex with (S,S) selectivity furnish the corresponding (R,R)-amino alcohols excluding styrene oxide with excellent regio- and enantioselectivity (Table 2). Kureshy et al. have also reported a parallel observation using chiral macrocyclic Cr(III)–salen complexes.¹⁶ 1,2-Epoxy-3-phenoxy propane and cyclohexene oxide furnished brilliant yield with ee > 99% (Table 2, Entry 1, 7). The data in Table 2 clearly indicate that, all the terminal and meso epoxide gave only one single regioisomer without any bis-adduct.

The isolated products' selectivity and enantiomeric excess are confirmed from the ¹H-NMR and HPLC analyses (ESI†). To examine the recyclability and reusability of the mesoporous Co(III)@AFS-1 catalyst in this ARO reaction, cyclohexene oxide and aniline are taken as model substrate. It can be seen that the catalyst be able to be recollected and reused for five successful cycles devoid of noticeable loss of its activity and enantioselectivity, as shown in Fig. 4. We have compared the catalytic activity of Co(III)@AFS-1 with the related catalysts (ESI†). As seen from this table that Co(III)@AFS-1 showed comparatively better catalytic activity over related heterogeneous systems in the epoxide ring opening reactions.¹⁷ Further, comparable catalytic activity and enantioselectivity is observed in case of homogeneous chiral Co(III)–salen complexes.¹⁸ High recyclability and reusability of our chiral Co(III)@AFS-1 is one of the major advantages in immobilizing the chiral complex on mesoporous support. Thus, Co(III)@AFS-1 chiral catalyst is easy to synthesize and it is highly efficient as well as advantageous for the enantioselective catalysis in the ARO reaction.

Fig. 4 Recyclability chart of chiral Co(III)@AFS-1 catalyst.

3.3. Recyclability test

The easy separation technique, recyclability and reusability are the most important aspect for a heterogeneous catalyst. In our work when the reaction was ended, the entire reaction mixture was dispersed in ethyl acetate then the solution was centrifuged at 10 000 rpm to take apart the solid catalyst from the mixture. The recovered catalyst was washed repeatedly using DCM. Finally, the catalyst was kept in oven to dry at 55 °C for 5 h for reactivation. The catalyst can be reused for five times devoid of appreciable loss in its activity and enantioselectivity (Fig. 4). To know the possibilities of metal leaching, AAS elemental analysis has been performed for Co in the reaction mixture. It indicates that practically no metal leaching occurred during the course of the reaction. The HR-TEM analysis has been performed to check the stability of the hexagonally ordered pores of the reused Co(III)@AFS-1 catalyst. From Fig. 5 it is clearly noticed that the honeycomb-like hexagonal array is arranged uniformly having pore size diameter of 7–9 nm throughout the whole specimen. Further, the FFT pattern shown in the inset of Fig. 5 suggested the two dimensional hexagonal mesoporous channel in Co(III)@AFS-1. Hence the TEM image also indicates that the catalyst remains unaltered in nature after five times recycles (Fig. 5). The IR spectrum and UV-Vis spectrum of the reused catalyst are found to be quite similar with the fresh one (ESI†).

Fig. 5 The HR-TEM images of reused Co(III)@AFS-1. The FFT pattern is shown in the inset of this figure.

4 Conclusions

In conclusion, a convenient synthetic pathway for designing a novel chiral Co(III)–salen implanted on highly ordered mesoporous silica catalyst has been reported. The catalyst is thoroughly characterized and the catalytic activity has been explored in the ARO of racemic epoxides with various aryl as well as cyclic amines to produce chiral β-amino alcohols as the final product. Short reaction time, ambient temperature, neat (solvent-free) reaction condition, very high turnover frequency, good yield, high regio- and enantioselectivity (ee > 99%) of the products and good recyclability of the catalyst are the remarkable advantages of the chiral mesoporous catalyst presented herein and this may open new avenues for asymmetric catalysis in future.

Acknowledgements

SMI acknowledges Department of Science and Technology, (DST-SERB, Project no. SB/S1/PC/107/2012) New Delhi, Govt. of India, University Grant Commission (UGC, Project F. No. 43-180/2014(SR)), 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. MMI thanks Ms Susmita Roy, and acknowledges UGC, New Delhi for MANF. PB and SKK want to thank CSIR, New Delhi for their Fellowship. M. H. conveys cordial thanks to UGC, New Delhi, for providing her SRF. AB is thankful to DST, New Delhi for funding through DST-UKIERI project. We gratefully recognize DST and UGC for necessary funding to the Department of Chemistry, University of Kalyani under FIST, PURSE and SAP program.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization data of chiral catalyst, ¹H NMR and HPLC data. See DOI: 10.1039/c6ra21523h

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