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Immobilized dimeric chiral Mn(III) salen complex on short channel ordered mesoporous silica as an effective catalyst for the epoxidation of non-functionalized alkenes
Tetrahedron 68 (2012) 6314e6322
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Immobilized dimeric chiral Mn(III) salen complex on short channel ordered mesoporous silica as an effective catalyst for the epoxidation of non-functionalized alkenes Tamal Roy, Rukhsana I. Kureshy *, Noor-ul H. Khan, Sayed H.R. Abdi, Arghya Sadhukhan, Hari C. Bajaj Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR), G. B. Marg, Bhavnagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2012 Received in revised form 4 May 2012 Accepted 6 May 2012 Available online 14 May 2012
Chiral dimeric Mn(III) salen complex with 1R, 2R-()-diaminocyclohexane collar was immobilized on short channel large pore sized silica through a long linker of {(CH2)3eNHemelamineepiperazine} to investigate its performance in enantioselective epoxidation of chromenes, indene, styrene and cis b-methyl styrene in the presence of pyridine N-oxide (PyNO) as an axial base using aqueous NaOCl as an oxidant at 0 C. The immobilized catalyst system showed high turnover frequency (TOF) and enantioselectivity for the smaller and bulkier alkenes like styrene, indene, 2,2-dimethylchromene and 6-cyano-2,2-dimethylchromene (ee up to 98%). These results are the best reported for heterogeneous catalyst under biphasic reaction conditions and were comparable to the dimeric Mn(III) salen system under homogeneous condition. The performance of the immobilized catalyst was retained for six reuse experiments. This protocol was extended to the synthesis of an antihypertensive drug (S)-Levchromakalim (ee 98%) at 1 g level. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Ordered mesoporous silica Dimeric chiral Mn(III) salen complex Enantioselective Chiral epoxides Non-functionalized alkenes Levcromakalim
1. Introduction Asymmetric epoxidation of non-functionalized alkenes is an important area of current interest due to its ability to engender up to two stereogenic centres1 at one go with construction of a highly reactive three membered oxirane ring,2 consequently make them immensely valuable as intermediates for several bioactive compounds as well as in pharmaceuticals.3,4 In early 90s Jacobsen and Katsuki got the noteworthy breakthrough by designing of chiral Mn(III) salen complexes, which are still most handy catalysts for the production of epoxides from non-functionalized alkenes in high yield and optical purity.5e7 However, for the sustainability of the process, efficient catalyst recovery and recyclability is required that can be achieved by supporting these catalysts on solids in order to harness the virtues of homogenous as well as heterogeneous catalysis. In this direction, immobilization of chiral Mn(III) salen complexes have been reported by anchoring them on inorganic supports,8e12 encapsulation in zeolite cages,8,13 vander wall’s wrapping in a polysiloxane membrane,14 immobilization on organic polymer,8,15 use of an ionic liquid16 etc. Still in many cases the activity and enantioselectivity are compromised and invariably leaching of active metal complex was observed if it is not covalently * Corresponding author. Fax: þ91 0278 2566970; e-mail addresses: rukhsana93@ yahoo.in, [email protected] (R.I. Kureshy). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2012.05.022
linked with the support.17 In an alternative approach, recyclability of chiral metal complexes under homogeneous condition is reported by means of fine tuning in the solubility of the catalyst resulting in precipitation of the catalyst by the addition of a solvent, in which the catalyst is insoluble in a post catalytic work up step.18 This tactic is particularly interesting due to the creation of more than one reactive centre that often worked in co-operation to improve product yield and enantioselectivity with the catalyst recyclability. However, under homogeneous condition still the separation of the catalyst and the product from the reaction medium is still not -vis heterogeneous condition where the catalyst hassle-free vis-a and the product can be separated conveniently by filtration or centrifugation. In the past few years mesoporous organiceinorganic hybrids have attracted much attention due to their potential application as a good support for immobilization of optically pure metal complex and hence their successive application in chiral catalysis.19 Keeping the above findings in observance we have designed a melamineepiperzaine based dinuclear Mn(III) salen Complex 10 immobilized on aminopropyl functionalized mesoporous silica (AFMS) and successfully used the same in asymmetric epoxidation reaction of non-functionalized alkenes. The purposeful design of such kind of catalyst allowed us to check the well-established fact of co-operative interaction in heterogeneous condition and also the advantage of recyclability protocols. The use of long linker {(CH2)3eNHemalamineepiperazine} to hook
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
the Jacobsen type dimeric Mn(III) salen complex also helped in hinging out the catalytically active site in a reasonably good distance from the silica support.20 This immobilized catalyst was used for enantioselective epoxidation of non-functionalized alkenes viz., styrene, cis b-methyl styrene, indene and chromenes using NaOCl as an oxidant in the presence of PyNO as an axial base at 0 C. Excellent yield (up to >99%) of enantio-pure epoxides along with high chiral induction (ee, up to 98%) in the case of indene and 6cyano-2,2-dimethylchromene were achieved. The present immobilized complex 1 worked very well for six cycles without any significant loss of activity and enantioselectivity. The immobilized
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complex 10 (derived from 1S,2S-(þ)-1,2 diaminocyclohexane collar) was used to catalyze asymmetric epoxidation of 6-cyano-2,2dimethylchromene (1 g scale) for the synthesis of (S)-Levchromakalim (antihypertensive drug) in good yield and 98% ee.
2. Results and discussion Non-C2 symmetrical Mn(III) salen complex covalently bounded on ordered mesoporous silica 1 was synthesized according to the Scheme 1.
Scheme 1. Synthesis of chiral dimeric Mn(III) salen complex immobilized on ordered mesoporous solid support.
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This strategy gives many fold advantages over physically entrapped salen complexes (1) covalent linkage prevents leaching of active complex, (2) presence of melamine-piperazine linker provides sufficient space between the wall of silica material and the active complex, thereby providing flexibility to the active complex akin to homogenous system, (3) allows two Mn(III) salen units close enough to show cooperative interaction, (4) large pore sized silica material with welldefined channels of shorter length minimizes the diffusional constrains for the reactants and product, and (5) the presence of active complex inside the channels may provide confinement effect for higher enantioselectivity in the product. The immobilized chiral Mn(III) salen complex 1 and its precursors AFMS, AFMS-MP were characterized by appropriate physicoechemical methods, such as solid state UVevis spectroscopy (DRS), IR-, solid state NMR (13C and 29 Si), LC-MS, SEM, TEM, XRD, N2 adsorption and desorption, ICP and TGA.
bilized inside the channels of mesoporous silica material without disturbing its periodic nature.10c The PXRD pattern of the mesoporous support material AFMS shows peaks corresponding to the reflection along 100, 110 and 200 planes indicating a uniform hexagonal lattice structure as reported earlier.22 The intensities of these peaks however, gradually diminished on the attachment of linker AFMS-MP followed by Mn(III) salen complex (see supplementary data, Fig. S2), which further supported successful loading of the Mn(III) salen complex 1 through linker inside the channels of the mesoporous material keeping the mesoporosity intact. SEM images of AFMS show hexagonal disc like morphology (1 to <2 mm), which remain intact on further functionalization with linker and Mn(III) salen in AFMS-MP and complex 1, respectively, implying thereby that the silica material has robust mechanical, thermal and chemical stability (Fig. 2, AeC). TEM images (Fig. 2, DeF) for all these samples distinctly show the presence of hexagonal pore structure when viewed along the short axis (average length of the channels >100 nm to <300 nm22) of the material and equidistant parallel fringes from side view. The presence of these equidistant parallel fringes and uniform hexagonal lattice structure23 is in agreement with the finding of PXRD. The loading of the homogeneous Mn(III) salen complex in the supported complex 1 was found to be 21 mg/100 mg as calculated by UVevis spectroscopic measurement, ICP and TGA analysis.
2.1. Characterization of the support and the catalyst 2.1.1. Textural and chemical properties of the supported material (AFMS), AFMS with melamine piperazine linker (AFMS-MP) and immobilized Mn(III) salen complex 1. The N2 adsorptionedesorption isotherms (Fig. 1) of the AFMS exhibited representative type IV isotherms,21 with hysteresis loops typically identical to that of ordered mesoporous materials with narrow pore size distribution centred around 10.5 nm similar to periodic hexagonal mesoporous silica material. Upon functionalization with linker followed by the attachment of unsymmetrical Mn(III) salen complex D, BET surface area and pore diameter (Fig. 1) decreased in the order AFMS>AFMS-MP>complex 1 (Table 1), but isotherm properties remained unchanged (see supplementary data, Fig. S1). This clearly suggests that Mn(III) salen complex has been successfully immo-
FTIR spectra of the AFMS show characteristic bands at 2931 and 1630 cm1 corresponding to n(CH2) of the propyl arm of the silylating agent and OeH bending vibration band, respectively. The additional bands at 1546, 1449 and 1361 cm1 for melamine
0.25
1000
Pore Volume (cm /g nm)
0.20
800
3
Volume Adsorbed (cm /g STP)
2.2. FTIR studies on AFMS, AFMS-MP and immobilized catalyst 1
600
0.15
0.10
0.05
0.00 0
400
20
40
60
80
100
120
140
160
180
200
Pore Width (nm)
200
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P ) Fig. 1. N2 adsorptionedesorption isotherm and pore width distribution profile (inset) of mesoporous support material (AFMS).
functionality and at 2952 cm1 for piperazine n(CH2) indicate successful attachment of linker in AFMS-MP. These peaks were accentuated when AFMS-MP was allowed to react with D showing the successful immobilization of Mn(III) salen complex 1 (Fig. 3).
Table 1 Textural parameters for AFMS, AFMS-MP and complex 1 Compound
AFMS AFMS-MP Complex 1
BJH pore width ( A)
Total pore volume (cm3/g)
BET surface area (m2/g)
Homogeneous Mn(III) salen complex loading (mg/100 mg)
75 66 55
1.21 1.04 0.85
603 543 433
d d 21
2.3. UVevis. DRS studies on AFMS, AFMS-MP and immobilized catalyst 1 Fig. 4 shows the UVevis. DRS spectra of AFMS (a), linker modified silica AFMS-MP (b) and the immobilized complex 1 (c).
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
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Fig. 2. SEM and TEM images of AFMS (A, D), AFMS-MP (B, E) and complex 1 (C, F).
2359
1983
1863
681
a
3789
1630 2931
3419
b
2360
2866
798
959
c
2361
2952 1449
%T
1361
803 462
3425
2866 2953
1546
1361 1447
804
1504
3421
1081
1545
458
1076 1078
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
459
1000
800
600
400.0
cm-1 Fig. 3. FTIR spectra of AFMS (a), AFMS-MP (b) and complex 1 (c).
UVevis spectrum of AFMS showed no distinct band in the entire UV-visible region, while AFMS-MP showed bands around 248 and 309 nm due to the p/p* and n/p* transitions of melamine functionality. In catalyst 1, additional ligand to metal charge transfer (LMCT) band centred at 436 nm and d/d transition band around 509 nm demonstrated successful immobilization of Mn(III) salen complex onto mesoporous silica material.10e,12c
2.4. NMR spectra of AFMS-MP Due to the paramagnetic nature of Mn(III) salen complex a direct confirmation for its grafting on silica material by NMR analysis was not possible. However, its precursor AFMS-MP in 13C{1H} CPMAS (Fig. 5a) showed a peak at 166.7 ppm due to the aromatic carbons of the melamine units, and 44.3 ppm due to the piperazine
Absorbance (a.u)
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1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 200
2.5. LC-MS spectra of AFMS-MP and complex 1 To confirm the presence of dimeric salen complex onto the solid mesoporous support through the linker molecule, LC-MS was taken for both AFMS-MP and complex 1 by means of HF treatment followed by extraction of the organic compound with the help of DCM. After evaporation of DCM the solid obtained was tested on LC-MS. Peak at m/z value of 391.46 and 1394.29 confirms the presence of melamineepiperazine linker AFMS-MP and dimeric salen ligand C on solid support (see supplementary data, Fig. S3 and Fig. S4).
c b a
2.6. Enantioselective epoxidation of non-functionalized alkenes using NaOCl as an oxidant 300
400
500
600
700
800
Wavelength (nm) Fig. 4. UVevis (DRS) spectra of AFMS (a), AFMS-MP (b) and complex 1 (c).
Fig. 5. Solid state CP-MAS
13
C NMR (a) and
29
Si-NMR (b) spectra of AFMS-MP.
units. The two peaks at 23.17 and 10.06 ppm were assigned to the aliphatic carbon atom of the aminopropyl linker while its third peak due to CeN was merged with CeN peaks of piperazine at w44 ppm24 These signal clearly provided evidence for the incorporation of the linker group on silica matrix. The 29Si-MAS spectra of AFMS-MP shows three sets of broad peak in between 102 and 112 ppm consisting of Q2, Q3 and Q4 silicon centres, which indicates that the silane is successfully grafted to the surface (Fig. 5b).
After successful characterisation of the dimeric Mn(III) salen complex 1 immobilized on mesoporous silica support, was used as heterogeneous catalyst for asymmetric epoxidation of nonfunctionalized alkenes viz, styrene, cis b-methyl styrene, indene and various chromenes in the presence of PyNO as an axial base and NaOCl as an oxidant at 0 C in dichloromethane. The data in Table 2 show that the immobilized catalyst 1 efficiently promoted the enantioselective reaction of styrene (STR), cis b-methyl styrene (MSTR), indene (IND), 2,2-dimethylchromene (CHR), 6-cyano-2,2-dimethylchromene (CN-CHR), 6-nitro-2,2dimethylchromene (NO2-CHR), 6-methoxy-2,2-dimethylchromene (MeO-CHR) and spiro[cyclohexane-1,2-[2H][1]chromene] (CyCHR) (conversion, 90/99%; Table 2) to give respective epoxides in good to excellent enantioselectivity (ee, 70e98%) in 8e9 h. Significantly, with catalyst 1, the enantioeinduction and TOF values (Table 2) were high with all the alkenes used in the present study, which is comparable to the results obtained with our earlier reported dimeric salen in a macrocycle that showed cooperativity under homogeneous condition18a,18b and better than earlier immobilized monomeric Mn(III) salen complexes.8,9c,10e,12a,12b,20 These results suggests that the two Mn(III) salen units in the catalyst 1 are in close proximity but dangling reasonably away from the support walls, thereby creating an environment akin to homogeneous system. The salen units in the catalyst 1 still require PyNO to stabilize Mnv]O intermediate in order to show high conversions and enantioselectivities10e,12b as in its absence, the catalyst 1 epoxidized indene (Table 2, entry 3) with low conversion (45%) and ee (72%). A distinct advantage of the present mesoporous silica over other mesoporous silica reported earlier to support Mn(III) salen for the epoxidation of non-functionalized alkenes lies in higher conversions and ees even for bulkier alkenes (e.g., indene and chromenes),8,9c,10e,12a,12b,20 possibly due to large space available inside the short channels. Control experiments with mesoporous silica supported material AFMS and AFMS-MP with indene as representative substrate showed negligible catalytic activity to give epoxide in trace amount (Table 2, entry 4, 5). Hence, it can be safely concluded that the catalytic activity is entirely due to the chiral dimeric Mn(III) salen complex 1 immobilized on the mesoporous silica. To study the leaching of the chiral metal complex or de-coordinated Mn from the immobilized catalyst 1, the post epoxidation reaction mixture was analyzed by ICP, which showed no trace of manganese. Moreover, after the first catalytic run of epoxidation reaction of indene as representative substrate, the catalyst was separated by centrifugation. Fresh reactants are added to the supernatant. Gas chromatographic analysis of the reaction mixture showed no further formation of the product, indene epoxide. The recyclability of the immobilized catalyst 1 was carried for the epoxidation of indene as a representative substrate. After the first run of the epoxidation reaction, the catalyst was separated by centrifugation. The separated catalyst was washed thoroughly with dichloromethane, dried, and subjected to another cycle with fresh reactants under similar epoxidation conditions. The recovered
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Table 2 Product yield and ee data for epoxidation of non-functionalized alkenes catalysed by complexes 1e4 in presence of PyNO as an axial base with NaOCl as an oxidanta
Entry 1 2 3i 4 5 6 7 8 9 10 11
Catalyst Complex 1 Complex 1 Complex 1 AFMS AFMS-MP Complex 1 Complex 1 Complex 1 Complex 1 Complex 1 Complex 10
Substrate f
STY INDg INDg IND IND CHRh Cy-CHRh CN-CHRh MeO-CHRh MSTYg CN-CHRl
% Yieldb >99 >99 45 2 4 99 92 90 98 >99k 90
Time (h) 8 9 9 9 9 9 9 9 9 9 10
Ee (%) c
70 92d 72d d d 96e 90e 98e 91 85 98
TOF103 s1j 1.74 1.54 0.69 d d 1.52 1.42 1.39 1.51 1.54 1.25
Reaction was performed using (S,S)-form of the catalyst in CH2Cl2 (40 mL) with catalyst (0.2 mmol), substrate (10 mmol), PyNO (1.3 mmol), aqueous NaOCl (27.5 mmol) at 0 C. a Reactions were performed in CH2Cl2 (4 mL) with catalyst (0.02 mmol), substrate (1.00 mmol), PyNO (0.13 mmol), aqueous NaOCl (2.75 mmol) at 0 C. b Based on GC. c Chiral capillary column GTA-type. d Chiral HPLC OB column. e Chiral HPLC OD Column. f Epoxide configuration R. g Epoxide configuration: 1R,2S. h Epoxide configuration 3R,4R. i Reaction conducted in absence of PyNO. j Turn over frequency is calculated by the expression [product]/[catalyst]time (s1). k Ratio of cis and trans epoxide is 85:15. l Epoxide configuration 3S,4S.
catalyst was used for six repeated cycles with no observable loss in its activity and enantioselectivity (Fig. 6). This indicates that the chiral dimeric Mn(III) salen complex is strongly bonded to the ordered short channel mesoporous silica through the apical coordination.
3. Conclusions Chiral dimeric Mn(III) salen complex immobilized into disc like mesoporous silica of short channel was used for the enantioselective epoxidation of various non-functionalized alkenes, which showed activity and enantioselectivity comparable with dimeric macrocyclic homogeneous catalysts. The present support due to large pore size and short channel length was apparently responsible for least diffusional constrains in accessing the supported Mn(III) salen sites, thereby giving high activity and enantioselectivity even for relatively bulkier substrates, such as indene and Cy-CHR and 6eCNe2,2-dimethylchromene and the catalyst was easily recycled six times without any loss in its performance. This protocol with catalyst 10, derived from 1S,2S-(þ)-1,2 diaminocyclohexane collar was used for the synthesis of (S)-levchromakalim, a potent hypertensive drug in single step in good yield and enantioselectivity by using 6-cyano 2,2 dimethylchromene oxide. 4. Experimental methods
Fig. 6. Recycling of complex 1 using indene as the test substrate.
The characterization of the recycled catalyst (after one cycle) was accomplished by FTIR, ICP, and CHN analysis, which suggested no significant changes in its structure barring some entrapment of reactants within the silica matrix.20 In all the catalytic reaction the configuration of the product epoxide was the same as that of the catalyst. As a synthetic relevance of asymmetric epoxidation protocol, we have synthesized immobilized complex 10 with 1S,2S-(þ)-1,2 diaminocyclohexane collar and used it for the synthesis of valuable antihypertensive drug (S)-Levchromakalim25 via asymmetric epoxidation of 6-cyano-2,2 dimethylchromene with overall good yield (54%) (Scheme 2) (For details please see the supplementary data).
4.1. General Pluronic P123 (Aldrich), sodium metasilicate (Qualigens, India), HCl (Ranbaxy, India), APTES (Fluka), aqueous NaOCl (12%) (National Chemicals, India), Manganese acetate (SD Fine Chemicals, India) and PyNO (Fluka) were used as received. Indene and styrene were passed through a pad of neutral alumina before use. All the chromenes were synthesized by reported method.26,27 A highly ordered hexagonal disc like aminopropyl functionalized silica (AFMS) was synthesized as per reported procedure22 using a MAS-II microwave. All of the solvents used in the present study were purified by known methods.28 The product epoxides were purified by carrying out flash chromatography using silica gel 60e200 mesh purchased from SD Fine Chemicals Limited, India. The purity of the solvents, alkenes and the product epoxide were checked on gas
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Scheme 2. Synthesis of (S)-levcromakalim.
chromatography (GC) using a Shimadzu GC 14B instrument equipped with a stainless-steel column (2 m long, 3 mm inner diameter, 4 mm outer diameter) packed with 5% SE 30 (mesh size, 60e80) and having a flame ionization detector (FID). Ultrapure N2 was used as a carrier gas (rate 30 mL/min) with the injection port temperature set at 200 C. The column temperature for analysis of epoxidation product of styrene, cis b-methyl styrene and indene was in the range of 70e140 C, whereas for chromenes the isothermal temperature condition was maintained at 140 C. The racemic epoxides were prepared by the epoxidation of respective alkenes with 3-chloroperbenzoic acid in CH2Cl2 and were used to determine conversions by relating the height and area of the GC peaks. The ees of styrene epoxide was determined by GC on a chiral capillary column (Chiraldex GTA), whereas epoxides obtained from epoxidation of indene and chromenes were determined on HPLC (Shimadzu SCL-10AVP) using a Chiralcel column (OB/OD). 1H & 13C NMR spectra were recorded on Bruker 200 MHz or 500 MHz spectrometer at ambient temperature. The IR spectra were recorded on a PerkineElmer Spectrum GX spectrophotometer in KBr/ Nujol mull. High-resolution mass spectra were recorded using a LCMS (Q-TOFF) LC (Waters), MS (Micromass) instrument. Diffuse reflectance spectra (DRS) were obtained from UVevis-NIR scanning spectrophotometer UV-3101 PC. Microanalysis of the complex, intermediates was done on a PerkineElmer 2400 CHNS analyzer. Estimation of Mn content in the loaded complex was done by using inductively coupled plasma (ICP) spectrometer (PerkineElmer, Optima 2000 DV) and by using TGA (Mettler Toledo). Powder X-ray diffraction (XRD) patterns of the samples were recorded on a PhiA) radiation lips X’Pert MPD diffractometer using Cu-Ka (l¼1.5405 with a step size of 0.02 2q and a step time of 5 s of curved Cu-Ka monochromator under identical conditions. BET surface area was determined using N2 sorption data measured at 77 K using a volumetric adsorption setup (Porous Materials; PMI system DET sorptometer). The pore diameter of the samples were determined from the desorption branch of the N2 adsorption isotherm using the BJH method. Transmission electron microscopy (TEM) images were taken with the help of a Philips Tecnai 20 instrument. Scanning
electron microscopy (SEM) analysis of the sample was done with a LEO 1430 VP. 4.2. Synthesis of aminopropyl functionalized mesoporous silica AFMS Highly ordered aminopropyl group functionalized mesoporous silica (AFMS) was synthesized according to the procedure reported by Park et al.22 In a typical synthetic procedure, 10 g of triblock, poly(ethylene oxide)epoly(propylene oxide)epoly(ethylene oxide) (EO20ePO70eEO20) (Pluronic P123, mw 5800) was dissolved in 156 g of double distilled water. To this solution, a solution of Na2SiO3, 9H2O (27.34 g in 100 g of double distilled water) was added in a drop-wise manner. The mixed solution was stirred at 40 C until the entire solution became homogenous and transparent that followed the addition of 3-aminopropyl triethoxysiliane (APTES) (1.73 g). This mixture was stirred further for 30 min, to which conc. HCl (81 g) was added all at once. The mixture after stirring for 1 h was subjected to microwave irradiation under static condition at 100 C temperature and 300 W operating power for 2 h. The white crystalline product thus obtained was filtered off, washed with warm deionized water (4500 mL) and dried at 60 C under vacuum. The template was removed by soxhlet extraction with ethanol over a period of 24 h. Finally the solid was dried at 60 C under vacuum. The characterization was accomplished by means of IR-, diffuse reflectance UVevis spectroscopy, XRD, microanalysis, SEM, TEM and N2 adsorptionedesorption studies. 4.3. Synthesis of AFMS-MP Melamineepiperazine-mesoporous silica composite (AFMSMP) was prepared according to the procedure reported by Shantz et al.24 In a typical synthetic procedure 1 g of AFMS was added to a solution comprising of cyanuric chloride (CC) (1.25 g, 6.78 mmol) and diisopropylethylamine (DIPEA) (8 mmol) in THF (25 mL) under an inert atmosphere. The above slurry was stirred for a period of 24 h and filtered. The residue thus obtained was washed
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
sequentially with 50 mL portions of methanol, dichloromethane (DCM) and THF and transferred to a RBF containing piperazine (0.86 g, 10 mmol) in dry THF (25 mL) under inert atmosphere. The resulting mass was stirred for 24 h, filtered and washed in the same manner as described above. Finally the functionalized silica material was dried at RT under vacuum for 6 h (Scheme 1) (1.1 g); nmax (KBr) 458, 803, 1076, 1361, 1449, 1546, 2952, 3425 cm1; lmax 248, 309 nm; dC{dH} NMR (125 MHz) 10.1, 11.6, 23.2, 25.4, 44.3, 166.7. 4.4. Synthesis of unsymmetrical Mn(III) salen complex D The synthesis of a non-C2-symmetrical Mn(III) salen complex D was accomplished by our previously reported method.12a The synthetic procedure involved 1:1 condensation of 3,5-di-tert-butyl salicylaldehyde wih (1R,2R)-(-)-cyclohexane diamine to get chiral half unit N-(2-hydroxy-3,5-di-tert-butylbenzaldehyde)-1-amino-2cyclohexaneimine B, which (331 mg, 1 mmol) was allowed to react with 5-chloromethyl 3-tert-butylsalicylaldehyde (227 mg, 1 mmol) in dry methanol to get unsymmetrical chiral salen ligand C. The ligand C (431 mg, 0.8 mmol) subsequently on complexation with Mn(CH3COO)2.4H2O (294 mg, 1.2 mmol) followed by aerobic oxidation in the presence of LiCl (102 mg, 2.4 mmol) gave the unsymmetrical Mn(III) salen complex D (Scheme 1). Ligand C: (0.458 g, 85%); [Found: C, 73.48; H, 8.76; N, 5.17. C33H47ClN2O2 requires C, 73.54; H, 8.73; N, 5.20%]; nmax (KBr) 3411, 2957, 2863, 1629, 1597, 1470, 1441, 1390, 1361, 1272, 1251, 1204, 1172, 1095,1044, 878, 827, 712, 644, 590 cm1; dH (200 MHz CDCl3) 1.23 (s, 18H), 1.42 (s, 9H), 1.47e2.10 (m, 8H), 3.30e3.48 (m, 2H), 4.56 (s, 2H), 6.85 (d, 1H, J 1.9 Hz), 7.05 (d, 2.0 Hz), 7.43 (d, 1H, J 2.2 Hz), 7.52 (d, 1H J 2.2 Hz), 8.32 (s,1H), 8.44 (s,1H),13.50 (b s,1H),14.48 (b s,1H); dC 24.2, 24.8, 26.2, 29.3, 29.6, 29.8, 33.9, 34.1, 35.2, 45.8, 72.3, 78.1, 116.8, 118.2, 122.4, 126.4, 127.4, 127.6, 136.2, 139.8, 157.9, 161.5, 164.8, 165.7. Complex D: (0.452 g, 90%); [Found: C, 63.21; H, 7.25; N, 4.41. C33H45Cl2MnN2O2 requires C, 63.16; H, 7.23; N, 4.46%]; ½a30 D ¼þ663 (c 0.04, CH2Cl2); nmax (KBr) 3434, 2955, 2866, 1613, 1536, 1436, 1389, 1252, 1202, 1029, 836, 568 cm1; lmax (CH2Cl2) 284, 416, 422, 399, 320, 284 nm. 4.5. Synthesis of complex 1 The silica composite AFMS-MP (1 g) was added to a solution of unsymmetrical Mn(III) salen complex D (314 mg, 0.5 mmol) in dry toluene (10 mL) under inert atmosphere and the reaction mixture was refluxed for 24 h. The immobilized complex 1 was washed with toluene and solvent ether followed by extraction with DCM and methanol on a soxhlet extractor until the washings become colourless. All the washings were collected together, solvents were evaporated and the residue was dissolved in 10 mL of dry toluene in order to measure the difference between the initial and final concentration of Mn(III) salen complex by UVevis spectroscopy. This difference gave the amount of Mn(III) salen complex immobilized on solid support (Scheme 1). Complex 1 (1.02 g); nmax (KBr) 459, 804, 1078, 1361, 1447, 1504, 1545, 2953, 3421 cm1; lmax 247, 310, 436, 509 nm. Complex 10 (1.04 g); nmax (KBr) 458, 806, 1080, 1362, 1449, 1506, 1543, 2955, 3420 cm1; lmax 248, 312, 437, 510 nm (Scheme 2). 4.6. Enantioselective epoxidation of non-functionalized alkenes Enantioselective epoxidation reactions were performed with catalyst 1 (0.02 mmol) using styrene (STY), cis b-methyl styrene (MSTY), indene (IND), 2,2-dimethylchromene (CHR), 6-cyano-2,2dimethylchromene (CN-CHR), 6-methoxy-2,2-dimethylchromene (MeO-CHR), spiro[cyclohexane-1,2-[2H][1]chromene] (Cy-CHR)
6321
(1 mmol) as substrates in dichloromethane (3 mL) in the presence of PyNO (0.13 mmol) as an axial base with aqueous buffered NaOCl (12%, 2.75 mmol; pH¼11.3) as an oxidant. The NaOCl was added in five equal portions at 0 C, and the reaction mass was stirred using a mechanical stirrer at 90020 rpm. The course of the epoxidation reaction was monitored by GC with n-tridecane (0.1 mmol) as a GLC internal standard for product quantification. After completion of the reaction, the immobilized catalyst 1 was separated by centrifugation, washed thoroughly with dichloromethane, and dried in vacuum for subsequent catalytic runs. Acknowledgements Authors are thankful to the CSIR-Network project for supporting this work. T.R. is thankful to CSIR for the award of SRF. Supplementary data Additional characterization data including PXRD, N2 adsorptionedesorption profile, IR-, LC-MS, characterization data for product epoxides and synthetic procedure of (S)-levcromakalim along with characterization data are included in Supplementary data. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tet.2012.05.022. References and notes 1. (a) Song, F.; Wang, C.; Lin, W. Chem. Commun. 2011, 8256; (b) Faveri, G. D.; Ilyashenko, G.; Watkinson, M. Chem. Soc. Rev. 2011, 40, 1722; (c) Song, F.; Wang, C.; Falkowski, J. M.; Ma, L.; Lin, W. J. Am. Chem. Soc. 2010, 132, 15390; (d) Liao, S.; List, B. Angew. Chem., Int. Ed. 2010, 49, 628; (e) Tanaka, H.; Nishikawa, H.; Uchida, T.; Katsuki, T. J. Am. Chem. Soc. 2010, 132, 12034. 2. Arends, I. W. C. E. Angew. Chem., Int. Ed. 2006, 45, 6250. 3. Jacobsen, E. N.; Wu, M. H. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999. 4. Katsuki, T. In Comprehensive Coordination Chemistry II; McCleverty, J., Ed.; Elsevier Science: Oxford, 2003; vol. 9. 5. McGarrigle, E. M.; Gilheany, D. G. Chem. Rev. 2005, 105, 1563. 6. Katsuki, T. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York, NY, 2000; p 287. 7. Jacobsen, E. N. In Asymmetric Epoxidation of Un-functionalized Olefins; Ojima, I., Ed.; VCH: New York, NY, 1993; ch. 4.2, and references therein. 8. Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R.; Jasra, R. V. In Catalysis Research at the Cutting Edge; Bevy, L. P., Ed.; Nova Science: New York, NY, 2005; pp 115e133; References therein. 9. (a) Huang, J.; Fu, X.; Miao, Q. Appl. Catal., A 2011, 407, 163; (b) Meng, X.; Qin, C.; Wang, X. L.; Su, Z. M.; Li, B.; Yang, Q. H. Dalton Trans. 2011, 40, 9964; (c) Lou, L.L.; Jiang, S.; Yu, K.; Gu, Z.; Ji, R.; Dong, Y.; Liu, S. Microporous Mesoporous Mater. 2011, 142, 214; (d) Zou, X.; Fu, X.; Li, Y.; Tu, X.; Fu, S.; Luo, Y.; Wu, X. Adv. Synth. Catal. 2010, 352, 163; (e) Tangestaninejad, S.; Moghadam, M.; Mirkhani, V.; Baltork, I. M.; Saeedi, M. S. Appl. Catal., A 2010, 381, 233; (f) Piaggio, P.; McMorn, P.; Langham, C.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Hutchings, G. J. New J. Chem. 1998, 22, 1167; (g) Piaggio, P.; Langham, C.; McMorn, P.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Sly, C.; Hutchings, G. J. J. Chem. Soc., Perkin Trans. 1 2000, 2, 143; (h) Piaggio, P.; McMorn, P.; Murphy, D.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Sly, C.; Kerton, O. J.; Hutchings, G. J. J. Chem. Soc., Perkin Trans. 1 2000, 2, 2008. 10. (a) Biernacka, K.; Silva, A. R.; Carvalho; Pires, A. P. J.; Freire, C. Catal. Lett. 2010, 134, 63; (b) Tan, R.; Yin, D.; Yu, N.; Zhao, H.; Yin, D. J. Catal. 2009, 263, 284; (c) Gong, B.; Fu, X.; Chen, J.; Li, Y.; Zou, X.; Tu, X.; Ding, P.; Ma, L. J. Catal. 2009, 262, 9; (d) Tang, X.; Tang, Y.; Xu, G.; Wei, S.; Sun, Y. Catal. Commun. 2008, 10, 317; (e) Zhang, H.; Wang, Y. M.; Zhang, L.; Gerritsen, G.; Abbenhuis, H. C. L.; Van Santen, R. A.; Li, C. J. Catal. 2008, 256, 226. 11. (a) Choudary, B. M.; Ramani, T.; Maheswaran, H.; Prashant, L.; Ranganath, K. V. ~o, A.; Garcia, H. S.; Kumar, K. V. Adv. Synth. Catal. 2006, 348, 493; (b) Baleiza Chem. Rev. 2006, 106, 3987. 12. (a) Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.; Pathak, K.; Jasra, R. V. J. Catal. 2006, 238, 134; (b) Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.; Singh, S.; Pandia, P. H.; Jasra, R. V. J. Catal. 2005, 235, 28; (c) Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.; Singh, S.; Jasra, R. V. J. Catal. 2004, 221, 234. 13. Silva, M.; Freire, C.; de Castro, B.; Figueiredo, J. L. J. Mol. Catal. A: Chem. 2006, 258, 327. 14. Jannsen, K. B. M.; Laquire, I.; Dehaen, W.; Parton, R. F.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron: Asymmetry 1997, 8, 3481. 15. Reger, T. S.; Janda, K. D. J. Am. Chem. Soc. 2000, 122, 6929. 16. Song, C. E.; Roh, E. J.; Yu, B. M.; Chi, D. Y.; Kim, S. C.; Lee, K. J. Chem. Commun. 2000, 837. 17. Zhang, H.; Xiang, S.; Li, C. Chem. Commun. 2005, 1209.
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24. (a) Acosta, E. J.; Carr, C. S.; Simanek, E. E.; Shantz, D. F. Adv. Mater. 2004, 16, 985; (b) Yoo, S.; Lunn, J. D.; Gonzalez, S.; Ristich, J. A.; Simanek, E. E.; Shantz, D. F. Chem. Mater. 2006, 18, 2935. 25. (a) Bulman Page, P. C.; Buckley, B. R.; Heaney, H.; Blacker, A. J. Org. Lett. 2005, 7, 375; (b) Jacobsen, E. N.; Zhang, W.; Deng, L. PCT Int. Appl. 1993, WO 9303838 AI 19930304. 26. Bergmann, R.; Gericke, R. J. Med. Chem. 1990, 33, 492. 27. Godfrey, J. D., Jr.; Mueller, R. H.; Sendergran, T. C.; Soundararanjan, N.; Colandrea, V. J. Tetrahedron Lett. 1994, 35, 6405. 28. Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; Pergamon: New York, NY, 1981.
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Immobilized dimeric chiral Mn(III) salen complex on short channel ordered mesoporous silica as an effective catalyst for the epoxidation of non-functionalized alkenes Tamal Roy, Rukhsana I. Kureshy *, Noor-ul H. Khan, Sayed H.R. Abdi, Arghya Sadhukhan, Hari C. Bajaj Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR), G. B. Marg, Bhavnagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2012 Received in revised form 4 May 2012 Accepted 6 May 2012 Available online 14 May 2012
Chiral dimeric Mn(III) salen complex with 1R, 2R-()-diaminocyclohexane collar was immobilized on short channel large pore sized silica through a long linker of {(CH2)3eNHemelamineepiperazine} to investigate its performance in enantioselective epoxidation of chromenes, indene, styrene and cis b-methyl styrene in the presence of pyridine N-oxide (PyNO) as an axial base using aqueous NaOCl as an oxidant at 0 C. The immobilized catalyst system showed high turnover frequency (TOF) and enantioselectivity for the smaller and bulkier alkenes like styrene, indene, 2,2-dimethylchromene and 6-cyano-2,2-dimethylchromene (ee up to 98%). These results are the best reported for heterogeneous catalyst under biphasic reaction conditions and were comparable to the dimeric Mn(III) salen system under homogeneous condition. The performance of the immobilized catalyst was retained for six reuse experiments. This protocol was extended to the synthesis of an antihypertensive drug (S)-Levchromakalim (ee 98%) at 1 g level. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Ordered mesoporous silica Dimeric chiral Mn(III) salen complex Enantioselective Chiral epoxides Non-functionalized alkenes Levcromakalim
1. Introduction Asymmetric epoxidation of non-functionalized alkenes is an important area of current interest due to its ability to engender up to two stereogenic centres1 at one go with construction of a highly reactive three membered oxirane ring,2 consequently make them immensely valuable as intermediates for several bioactive compounds as well as in pharmaceuticals.3,4 In early 90s Jacobsen and Katsuki got the noteworthy breakthrough by designing of chiral Mn(III) salen complexes, which are still most handy catalysts for the production of epoxides from non-functionalized alkenes in high yield and optical purity.5e7 However, for the sustainability of the process, efficient catalyst recovery and recyclability is required that can be achieved by supporting these catalysts on solids in order to harness the virtues of homogenous as well as heterogeneous catalysis. In this direction, immobilization of chiral Mn(III) salen complexes have been reported by anchoring them on inorganic supports,8e12 encapsulation in zeolite cages,8,13 vander wall’s wrapping in a polysiloxane membrane,14 immobilization on organic polymer,8,15 use of an ionic liquid16 etc. Still in many cases the activity and enantioselectivity are compromised and invariably leaching of active metal complex was observed if it is not covalently * Corresponding author. Fax: þ91 0278 2566970; e-mail addresses: rukhsana93@ yahoo.in, [email protected] (R.I. Kureshy). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2012.05.022
linked with the support.17 In an alternative approach, recyclability of chiral metal complexes under homogeneous condition is reported by means of fine tuning in the solubility of the catalyst resulting in precipitation of the catalyst by the addition of a solvent, in which the catalyst is insoluble in a post catalytic work up step.18 This tactic is particularly interesting due to the creation of more than one reactive centre that often worked in co-operation to improve product yield and enantioselectivity with the catalyst recyclability. However, under homogeneous condition still the separation of the catalyst and the product from the reaction medium is still not -vis heterogeneous condition where the catalyst hassle-free vis-a and the product can be separated conveniently by filtration or centrifugation. In the past few years mesoporous organiceinorganic hybrids have attracted much attention due to their potential application as a good support for immobilization of optically pure metal complex and hence their successive application in chiral catalysis.19 Keeping the above findings in observance we have designed a melamineepiperzaine based dinuclear Mn(III) salen Complex 10 immobilized on aminopropyl functionalized mesoporous silica (AFMS) and successfully used the same in asymmetric epoxidation reaction of non-functionalized alkenes. The purposeful design of such kind of catalyst allowed us to check the well-established fact of co-operative interaction in heterogeneous condition and also the advantage of recyclability protocols. The use of long linker {(CH2)3eNHemalamineepiperazine} to hook
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
the Jacobsen type dimeric Mn(III) salen complex also helped in hinging out the catalytically active site in a reasonably good distance from the silica support.20 This immobilized catalyst was used for enantioselective epoxidation of non-functionalized alkenes viz., styrene, cis b-methyl styrene, indene and chromenes using NaOCl as an oxidant in the presence of PyNO as an axial base at 0 C. Excellent yield (up to >99%) of enantio-pure epoxides along with high chiral induction (ee, up to 98%) in the case of indene and 6cyano-2,2-dimethylchromene were achieved. The present immobilized complex 1 worked very well for six cycles without any significant loss of activity and enantioselectivity. The immobilized
6315
complex 10 (derived from 1S,2S-(þ)-1,2 diaminocyclohexane collar) was used to catalyze asymmetric epoxidation of 6-cyano-2,2dimethylchromene (1 g scale) for the synthesis of (S)-Levchromakalim (antihypertensive drug) in good yield and 98% ee.
2. Results and discussion Non-C2 symmetrical Mn(III) salen complex covalently bounded on ordered mesoporous silica 1 was synthesized according to the Scheme 1.
Scheme 1. Synthesis of chiral dimeric Mn(III) salen complex immobilized on ordered mesoporous solid support.
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This strategy gives many fold advantages over physically entrapped salen complexes (1) covalent linkage prevents leaching of active complex, (2) presence of melamine-piperazine linker provides sufficient space between the wall of silica material and the active complex, thereby providing flexibility to the active complex akin to homogenous system, (3) allows two Mn(III) salen units close enough to show cooperative interaction, (4) large pore sized silica material with welldefined channels of shorter length minimizes the diffusional constrains for the reactants and product, and (5) the presence of active complex inside the channels may provide confinement effect for higher enantioselectivity in the product. The immobilized chiral Mn(III) salen complex 1 and its precursors AFMS, AFMS-MP were characterized by appropriate physicoechemical methods, such as solid state UVevis spectroscopy (DRS), IR-, solid state NMR (13C and 29 Si), LC-MS, SEM, TEM, XRD, N2 adsorption and desorption, ICP and TGA.
bilized inside the channels of mesoporous silica material without disturbing its periodic nature.10c The PXRD pattern of the mesoporous support material AFMS shows peaks corresponding to the reflection along 100, 110 and 200 planes indicating a uniform hexagonal lattice structure as reported earlier.22 The intensities of these peaks however, gradually diminished on the attachment of linker AFMS-MP followed by Mn(III) salen complex (see supplementary data, Fig. S2), which further supported successful loading of the Mn(III) salen complex 1 through linker inside the channels of the mesoporous material keeping the mesoporosity intact. SEM images of AFMS show hexagonal disc like morphology (1 to <2 mm), which remain intact on further functionalization with linker and Mn(III) salen in AFMS-MP and complex 1, respectively, implying thereby that the silica material has robust mechanical, thermal and chemical stability (Fig. 2, AeC). TEM images (Fig. 2, DeF) for all these samples distinctly show the presence of hexagonal pore structure when viewed along the short axis (average length of the channels >100 nm to <300 nm22) of the material and equidistant parallel fringes from side view. The presence of these equidistant parallel fringes and uniform hexagonal lattice structure23 is in agreement with the finding of PXRD. The loading of the homogeneous Mn(III) salen complex in the supported complex 1 was found to be 21 mg/100 mg as calculated by UVevis spectroscopic measurement, ICP and TGA analysis.
2.1. Characterization of the support and the catalyst 2.1.1. Textural and chemical properties of the supported material (AFMS), AFMS with melamine piperazine linker (AFMS-MP) and immobilized Mn(III) salen complex 1. The N2 adsorptionedesorption isotherms (Fig. 1) of the AFMS exhibited representative type IV isotherms,21 with hysteresis loops typically identical to that of ordered mesoporous materials with narrow pore size distribution centred around 10.5 nm similar to periodic hexagonal mesoporous silica material. Upon functionalization with linker followed by the attachment of unsymmetrical Mn(III) salen complex D, BET surface area and pore diameter (Fig. 1) decreased in the order AFMS>AFMS-MP>complex 1 (Table 1), but isotherm properties remained unchanged (see supplementary data, Fig. S1). This clearly suggests that Mn(III) salen complex has been successfully immo-
FTIR spectra of the AFMS show characteristic bands at 2931 and 1630 cm1 corresponding to n(CH2) of the propyl arm of the silylating agent and OeH bending vibration band, respectively. The additional bands at 1546, 1449 and 1361 cm1 for melamine
0.25
1000
Pore Volume (cm /g nm)
0.20
800
3
Volume Adsorbed (cm /g STP)
2.2. FTIR studies on AFMS, AFMS-MP and immobilized catalyst 1
600
0.15
0.10
0.05
0.00 0
400
20
40
60
80
100
120
140
160
180
200
Pore Width (nm)
200
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P ) Fig. 1. N2 adsorptionedesorption isotherm and pore width distribution profile (inset) of mesoporous support material (AFMS).
functionality and at 2952 cm1 for piperazine n(CH2) indicate successful attachment of linker in AFMS-MP. These peaks were accentuated when AFMS-MP was allowed to react with D showing the successful immobilization of Mn(III) salen complex 1 (Fig. 3).
Table 1 Textural parameters for AFMS, AFMS-MP and complex 1 Compound
AFMS AFMS-MP Complex 1
BJH pore width ( A)
Total pore volume (cm3/g)
BET surface area (m2/g)
Homogeneous Mn(III) salen complex loading (mg/100 mg)
75 66 55
1.21 1.04 0.85
603 543 433
d d 21
2.3. UVevis. DRS studies on AFMS, AFMS-MP and immobilized catalyst 1 Fig. 4 shows the UVevis. DRS spectra of AFMS (a), linker modified silica AFMS-MP (b) and the immobilized complex 1 (c).
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
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Fig. 2. SEM and TEM images of AFMS (A, D), AFMS-MP (B, E) and complex 1 (C, F).
2359
1983
1863
681
a
3789
1630 2931
3419
b
2360
2866
798
959
c
2361
2952 1449
%T
1361
803 462
3425
2866 2953
1546
1361 1447
804
1504
3421
1081
1545
458
1076 1078
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
459
1000
800
600
400.0
cm-1 Fig. 3. FTIR spectra of AFMS (a), AFMS-MP (b) and complex 1 (c).
UVevis spectrum of AFMS showed no distinct band in the entire UV-visible region, while AFMS-MP showed bands around 248 and 309 nm due to the p/p* and n/p* transitions of melamine functionality. In catalyst 1, additional ligand to metal charge transfer (LMCT) band centred at 436 nm and d/d transition band around 509 nm demonstrated successful immobilization of Mn(III) salen complex onto mesoporous silica material.10e,12c
2.4. NMR spectra of AFMS-MP Due to the paramagnetic nature of Mn(III) salen complex a direct confirmation for its grafting on silica material by NMR analysis was not possible. However, its precursor AFMS-MP in 13C{1H} CPMAS (Fig. 5a) showed a peak at 166.7 ppm due to the aromatic carbons of the melamine units, and 44.3 ppm due to the piperazine
Absorbance (a.u)
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1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 200
2.5. LC-MS spectra of AFMS-MP and complex 1 To confirm the presence of dimeric salen complex onto the solid mesoporous support through the linker molecule, LC-MS was taken for both AFMS-MP and complex 1 by means of HF treatment followed by extraction of the organic compound with the help of DCM. After evaporation of DCM the solid obtained was tested on LC-MS. Peak at m/z value of 391.46 and 1394.29 confirms the presence of melamineepiperazine linker AFMS-MP and dimeric salen ligand C on solid support (see supplementary data, Fig. S3 and Fig. S4).
c b a
2.6. Enantioselective epoxidation of non-functionalized alkenes using NaOCl as an oxidant 300
400
500
600
700
800
Wavelength (nm) Fig. 4. UVevis (DRS) spectra of AFMS (a), AFMS-MP (b) and complex 1 (c).
Fig. 5. Solid state CP-MAS
13
C NMR (a) and
29
Si-NMR (b) spectra of AFMS-MP.
units. The two peaks at 23.17 and 10.06 ppm were assigned to the aliphatic carbon atom of the aminopropyl linker while its third peak due to CeN was merged with CeN peaks of piperazine at w44 ppm24 These signal clearly provided evidence for the incorporation of the linker group on silica matrix. The 29Si-MAS spectra of AFMS-MP shows three sets of broad peak in between 102 and 112 ppm consisting of Q2, Q3 and Q4 silicon centres, which indicates that the silane is successfully grafted to the surface (Fig. 5b).
After successful characterisation of the dimeric Mn(III) salen complex 1 immobilized on mesoporous silica support, was used as heterogeneous catalyst for asymmetric epoxidation of nonfunctionalized alkenes viz, styrene, cis b-methyl styrene, indene and various chromenes in the presence of PyNO as an axial base and NaOCl as an oxidant at 0 C in dichloromethane. The data in Table 2 show that the immobilized catalyst 1 efficiently promoted the enantioselective reaction of styrene (STR), cis b-methyl styrene (MSTR), indene (IND), 2,2-dimethylchromene (CHR), 6-cyano-2,2-dimethylchromene (CN-CHR), 6-nitro-2,2dimethylchromene (NO2-CHR), 6-methoxy-2,2-dimethylchromene (MeO-CHR) and spiro[cyclohexane-1,2-[2H][1]chromene] (CyCHR) (conversion, 90/99%; Table 2) to give respective epoxides in good to excellent enantioselectivity (ee, 70e98%) in 8e9 h. Significantly, with catalyst 1, the enantioeinduction and TOF values (Table 2) were high with all the alkenes used in the present study, which is comparable to the results obtained with our earlier reported dimeric salen in a macrocycle that showed cooperativity under homogeneous condition18a,18b and better than earlier immobilized monomeric Mn(III) salen complexes.8,9c,10e,12a,12b,20 These results suggests that the two Mn(III) salen units in the catalyst 1 are in close proximity but dangling reasonably away from the support walls, thereby creating an environment akin to homogeneous system. The salen units in the catalyst 1 still require PyNO to stabilize Mnv]O intermediate in order to show high conversions and enantioselectivities10e,12b as in its absence, the catalyst 1 epoxidized indene (Table 2, entry 3) with low conversion (45%) and ee (72%). A distinct advantage of the present mesoporous silica over other mesoporous silica reported earlier to support Mn(III) salen for the epoxidation of non-functionalized alkenes lies in higher conversions and ees even for bulkier alkenes (e.g., indene and chromenes),8,9c,10e,12a,12b,20 possibly due to large space available inside the short channels. Control experiments with mesoporous silica supported material AFMS and AFMS-MP with indene as representative substrate showed negligible catalytic activity to give epoxide in trace amount (Table 2, entry 4, 5). Hence, it can be safely concluded that the catalytic activity is entirely due to the chiral dimeric Mn(III) salen complex 1 immobilized on the mesoporous silica. To study the leaching of the chiral metal complex or de-coordinated Mn from the immobilized catalyst 1, the post epoxidation reaction mixture was analyzed by ICP, which showed no trace of manganese. Moreover, after the first catalytic run of epoxidation reaction of indene as representative substrate, the catalyst was separated by centrifugation. Fresh reactants are added to the supernatant. Gas chromatographic analysis of the reaction mixture showed no further formation of the product, indene epoxide. The recyclability of the immobilized catalyst 1 was carried for the epoxidation of indene as a representative substrate. After the first run of the epoxidation reaction, the catalyst was separated by centrifugation. The separated catalyst was washed thoroughly with dichloromethane, dried, and subjected to another cycle with fresh reactants under similar epoxidation conditions. The recovered
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
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Table 2 Product yield and ee data for epoxidation of non-functionalized alkenes catalysed by complexes 1e4 in presence of PyNO as an axial base with NaOCl as an oxidanta
Entry 1 2 3i 4 5 6 7 8 9 10 11
Catalyst Complex 1 Complex 1 Complex 1 AFMS AFMS-MP Complex 1 Complex 1 Complex 1 Complex 1 Complex 1 Complex 10
Substrate f
STY INDg INDg IND IND CHRh Cy-CHRh CN-CHRh MeO-CHRh MSTYg CN-CHRl
% Yieldb >99 >99 45 2 4 99 92 90 98 >99k 90
Time (h) 8 9 9 9 9 9 9 9 9 9 10
Ee (%) c
70 92d 72d d d 96e 90e 98e 91 85 98
TOF103 s1j 1.74 1.54 0.69 d d 1.52 1.42 1.39 1.51 1.54 1.25
Reaction was performed using (S,S)-form of the catalyst in CH2Cl2 (40 mL) with catalyst (0.2 mmol), substrate (10 mmol), PyNO (1.3 mmol), aqueous NaOCl (27.5 mmol) at 0 C. a Reactions were performed in CH2Cl2 (4 mL) with catalyst (0.02 mmol), substrate (1.00 mmol), PyNO (0.13 mmol), aqueous NaOCl (2.75 mmol) at 0 C. b Based on GC. c Chiral capillary column GTA-type. d Chiral HPLC OB column. e Chiral HPLC OD Column. f Epoxide configuration R. g Epoxide configuration: 1R,2S. h Epoxide configuration 3R,4R. i Reaction conducted in absence of PyNO. j Turn over frequency is calculated by the expression [product]/[catalyst]time (s1). k Ratio of cis and trans epoxide is 85:15. l Epoxide configuration 3S,4S.
catalyst was used for six repeated cycles with no observable loss in its activity and enantioselectivity (Fig. 6). This indicates that the chiral dimeric Mn(III) salen complex is strongly bonded to the ordered short channel mesoporous silica through the apical coordination.
3. Conclusions Chiral dimeric Mn(III) salen complex immobilized into disc like mesoporous silica of short channel was used for the enantioselective epoxidation of various non-functionalized alkenes, which showed activity and enantioselectivity comparable with dimeric macrocyclic homogeneous catalysts. The present support due to large pore size and short channel length was apparently responsible for least diffusional constrains in accessing the supported Mn(III) salen sites, thereby giving high activity and enantioselectivity even for relatively bulkier substrates, such as indene and Cy-CHR and 6eCNe2,2-dimethylchromene and the catalyst was easily recycled six times without any loss in its performance. This protocol with catalyst 10, derived from 1S,2S-(þ)-1,2 diaminocyclohexane collar was used for the synthesis of (S)-levchromakalim, a potent hypertensive drug in single step in good yield and enantioselectivity by using 6-cyano 2,2 dimethylchromene oxide. 4. Experimental methods
Fig. 6. Recycling of complex 1 using indene as the test substrate.
The characterization of the recycled catalyst (after one cycle) was accomplished by FTIR, ICP, and CHN analysis, which suggested no significant changes in its structure barring some entrapment of reactants within the silica matrix.20 In all the catalytic reaction the configuration of the product epoxide was the same as that of the catalyst. As a synthetic relevance of asymmetric epoxidation protocol, we have synthesized immobilized complex 10 with 1S,2S-(þ)-1,2 diaminocyclohexane collar and used it for the synthesis of valuable antihypertensive drug (S)-Levchromakalim25 via asymmetric epoxidation of 6-cyano-2,2 dimethylchromene with overall good yield (54%) (Scheme 2) (For details please see the supplementary data).
4.1. General Pluronic P123 (Aldrich), sodium metasilicate (Qualigens, India), HCl (Ranbaxy, India), APTES (Fluka), aqueous NaOCl (12%) (National Chemicals, India), Manganese acetate (SD Fine Chemicals, India) and PyNO (Fluka) were used as received. Indene and styrene were passed through a pad of neutral alumina before use. All the chromenes were synthesized by reported method.26,27 A highly ordered hexagonal disc like aminopropyl functionalized silica (AFMS) was synthesized as per reported procedure22 using a MAS-II microwave. All of the solvents used in the present study were purified by known methods.28 The product epoxides were purified by carrying out flash chromatography using silica gel 60e200 mesh purchased from SD Fine Chemicals Limited, India. The purity of the solvents, alkenes and the product epoxide were checked on gas
6320
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Scheme 2. Synthesis of (S)-levcromakalim.
chromatography (GC) using a Shimadzu GC 14B instrument equipped with a stainless-steel column (2 m long, 3 mm inner diameter, 4 mm outer diameter) packed with 5% SE 30 (mesh size, 60e80) and having a flame ionization detector (FID). Ultrapure N2 was used as a carrier gas (rate 30 mL/min) with the injection port temperature set at 200 C. The column temperature for analysis of epoxidation product of styrene, cis b-methyl styrene and indene was in the range of 70e140 C, whereas for chromenes the isothermal temperature condition was maintained at 140 C. The racemic epoxides were prepared by the epoxidation of respective alkenes with 3-chloroperbenzoic acid in CH2Cl2 and were used to determine conversions by relating the height and area of the GC peaks. The ees of styrene epoxide was determined by GC on a chiral capillary column (Chiraldex GTA), whereas epoxides obtained from epoxidation of indene and chromenes were determined on HPLC (Shimadzu SCL-10AVP) using a Chiralcel column (OB/OD). 1H & 13C NMR spectra were recorded on Bruker 200 MHz or 500 MHz spectrometer at ambient temperature. The IR spectra were recorded on a PerkineElmer Spectrum GX spectrophotometer in KBr/ Nujol mull. High-resolution mass spectra were recorded using a LCMS (Q-TOFF) LC (Waters), MS (Micromass) instrument. Diffuse reflectance spectra (DRS) were obtained from UVevis-NIR scanning spectrophotometer UV-3101 PC. Microanalysis of the complex, intermediates was done on a PerkineElmer 2400 CHNS analyzer. Estimation of Mn content in the loaded complex was done by using inductively coupled plasma (ICP) spectrometer (PerkineElmer, Optima 2000 DV) and by using TGA (Mettler Toledo). Powder X-ray diffraction (XRD) patterns of the samples were recorded on a PhiA) radiation lips X’Pert MPD diffractometer using Cu-Ka (l¼1.5405 with a step size of 0.02 2q and a step time of 5 s of curved Cu-Ka monochromator under identical conditions. BET surface area was determined using N2 sorption data measured at 77 K using a volumetric adsorption setup (Porous Materials; PMI system DET sorptometer). The pore diameter of the samples were determined from the desorption branch of the N2 adsorption isotherm using the BJH method. Transmission electron microscopy (TEM) images were taken with the help of a Philips Tecnai 20 instrument. Scanning
electron microscopy (SEM) analysis of the sample was done with a LEO 1430 VP. 4.2. Synthesis of aminopropyl functionalized mesoporous silica AFMS Highly ordered aminopropyl group functionalized mesoporous silica (AFMS) was synthesized according to the procedure reported by Park et al.22 In a typical synthetic procedure, 10 g of triblock, poly(ethylene oxide)epoly(propylene oxide)epoly(ethylene oxide) (EO20ePO70eEO20) (Pluronic P123, mw 5800) was dissolved in 156 g of double distilled water. To this solution, a solution of Na2SiO3, 9H2O (27.34 g in 100 g of double distilled water) was added in a drop-wise manner. The mixed solution was stirred at 40 C until the entire solution became homogenous and transparent that followed the addition of 3-aminopropyl triethoxysiliane (APTES) (1.73 g). This mixture was stirred further for 30 min, to which conc. HCl (81 g) was added all at once. The mixture after stirring for 1 h was subjected to microwave irradiation under static condition at 100 C temperature and 300 W operating power for 2 h. The white crystalline product thus obtained was filtered off, washed with warm deionized water (4500 mL) and dried at 60 C under vacuum. The template was removed by soxhlet extraction with ethanol over a period of 24 h. Finally the solid was dried at 60 C under vacuum. The characterization was accomplished by means of IR-, diffuse reflectance UVevis spectroscopy, XRD, microanalysis, SEM, TEM and N2 adsorptionedesorption studies. 4.3. Synthesis of AFMS-MP Melamineepiperazine-mesoporous silica composite (AFMSMP) was prepared according to the procedure reported by Shantz et al.24 In a typical synthetic procedure 1 g of AFMS was added to a solution comprising of cyanuric chloride (CC) (1.25 g, 6.78 mmol) and diisopropylethylamine (DIPEA) (8 mmol) in THF (25 mL) under an inert atmosphere. The above slurry was stirred for a period of 24 h and filtered. The residue thus obtained was washed
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
sequentially with 50 mL portions of methanol, dichloromethane (DCM) and THF and transferred to a RBF containing piperazine (0.86 g, 10 mmol) in dry THF (25 mL) under inert atmosphere. The resulting mass was stirred for 24 h, filtered and washed in the same manner as described above. Finally the functionalized silica material was dried at RT under vacuum for 6 h (Scheme 1) (1.1 g); nmax (KBr) 458, 803, 1076, 1361, 1449, 1546, 2952, 3425 cm1; lmax 248, 309 nm; dC{dH} NMR (125 MHz) 10.1, 11.6, 23.2, 25.4, 44.3, 166.7. 4.4. Synthesis of unsymmetrical Mn(III) salen complex D The synthesis of a non-C2-symmetrical Mn(III) salen complex D was accomplished by our previously reported method.12a The synthetic procedure involved 1:1 condensation of 3,5-di-tert-butyl salicylaldehyde wih (1R,2R)-(-)-cyclohexane diamine to get chiral half unit N-(2-hydroxy-3,5-di-tert-butylbenzaldehyde)-1-amino-2cyclohexaneimine B, which (331 mg, 1 mmol) was allowed to react with 5-chloromethyl 3-tert-butylsalicylaldehyde (227 mg, 1 mmol) in dry methanol to get unsymmetrical chiral salen ligand C. The ligand C (431 mg, 0.8 mmol) subsequently on complexation with Mn(CH3COO)2.4H2O (294 mg, 1.2 mmol) followed by aerobic oxidation in the presence of LiCl (102 mg, 2.4 mmol) gave the unsymmetrical Mn(III) salen complex D (Scheme 1). Ligand C: (0.458 g, 85%); [Found: C, 73.48; H, 8.76; N, 5.17. C33H47ClN2O2 requires C, 73.54; H, 8.73; N, 5.20%]; nmax (KBr) 3411, 2957, 2863, 1629, 1597, 1470, 1441, 1390, 1361, 1272, 1251, 1204, 1172, 1095,1044, 878, 827, 712, 644, 590 cm1; dH (200 MHz CDCl3) 1.23 (s, 18H), 1.42 (s, 9H), 1.47e2.10 (m, 8H), 3.30e3.48 (m, 2H), 4.56 (s, 2H), 6.85 (d, 1H, J 1.9 Hz), 7.05 (d, 2.0 Hz), 7.43 (d, 1H, J 2.2 Hz), 7.52 (d, 1H J 2.2 Hz), 8.32 (s,1H), 8.44 (s,1H),13.50 (b s,1H),14.48 (b s,1H); dC 24.2, 24.8, 26.2, 29.3, 29.6, 29.8, 33.9, 34.1, 35.2, 45.8, 72.3, 78.1, 116.8, 118.2, 122.4, 126.4, 127.4, 127.6, 136.2, 139.8, 157.9, 161.5, 164.8, 165.7. Complex D: (0.452 g, 90%); [Found: C, 63.21; H, 7.25; N, 4.41. C33H45Cl2MnN2O2 requires C, 63.16; H, 7.23; N, 4.46%]; ½a30 D ¼þ663 (c 0.04, CH2Cl2); nmax (KBr) 3434, 2955, 2866, 1613, 1536, 1436, 1389, 1252, 1202, 1029, 836, 568 cm1; lmax (CH2Cl2) 284, 416, 422, 399, 320, 284 nm. 4.5. Synthesis of complex 1 The silica composite AFMS-MP (1 g) was added to a solution of unsymmetrical Mn(III) salen complex D (314 mg, 0.5 mmol) in dry toluene (10 mL) under inert atmosphere and the reaction mixture was refluxed for 24 h. The immobilized complex 1 was washed with toluene and solvent ether followed by extraction with DCM and methanol on a soxhlet extractor until the washings become colourless. All the washings were collected together, solvents were evaporated and the residue was dissolved in 10 mL of dry toluene in order to measure the difference between the initial and final concentration of Mn(III) salen complex by UVevis spectroscopy. This difference gave the amount of Mn(III) salen complex immobilized on solid support (Scheme 1). Complex 1 (1.02 g); nmax (KBr) 459, 804, 1078, 1361, 1447, 1504, 1545, 2953, 3421 cm1; lmax 247, 310, 436, 509 nm. Complex 10 (1.04 g); nmax (KBr) 458, 806, 1080, 1362, 1449, 1506, 1543, 2955, 3420 cm1; lmax 248, 312, 437, 510 nm (Scheme 2). 4.6. Enantioselective epoxidation of non-functionalized alkenes Enantioselective epoxidation reactions were performed with catalyst 1 (0.02 mmol) using styrene (STY), cis b-methyl styrene (MSTY), indene (IND), 2,2-dimethylchromene (CHR), 6-cyano-2,2dimethylchromene (CN-CHR), 6-methoxy-2,2-dimethylchromene (MeO-CHR), spiro[cyclohexane-1,2-[2H][1]chromene] (Cy-CHR)
6321
(1 mmol) as substrates in dichloromethane (3 mL) in the presence of PyNO (0.13 mmol) as an axial base with aqueous buffered NaOCl (12%, 2.75 mmol; pH¼11.3) as an oxidant. The NaOCl was added in five equal portions at 0 C, and the reaction mass was stirred using a mechanical stirrer at 90020 rpm. The course of the epoxidation reaction was monitored by GC with n-tridecane (0.1 mmol) as a GLC internal standard for product quantification. After completion of the reaction, the immobilized catalyst 1 was separated by centrifugation, washed thoroughly with dichloromethane, and dried in vacuum for subsequent catalytic runs. Acknowledgements Authors are thankful to the CSIR-Network project for supporting this work. T.R. is thankful to CSIR for the award of SRF. Supplementary data Additional characterization data including PXRD, N2 adsorptionedesorption profile, IR-, LC-MS, characterization data for product epoxides and synthetic procedure of (S)-levcromakalim along with characterization data are included in Supplementary data. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tet.2012.05.022. References and notes 1. (a) Song, F.; Wang, C.; Lin, W. Chem. Commun. 2011, 8256; (b) Faveri, G. D.; Ilyashenko, G.; Watkinson, M. Chem. Soc. Rev. 2011, 40, 1722; (c) Song, F.; Wang, C.; Falkowski, J. M.; Ma, L.; Lin, W. J. Am. Chem. Soc. 2010, 132, 15390; (d) Liao, S.; List, B. Angew. Chem., Int. Ed. 2010, 49, 628; (e) Tanaka, H.; Nishikawa, H.; Uchida, T.; Katsuki, T. J. Am. Chem. Soc. 2010, 132, 12034. 2. Arends, I. W. C. E. Angew. Chem., Int. Ed. 2006, 45, 6250. 3. Jacobsen, E. N.; Wu, M. H. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999. 4. Katsuki, T. In Comprehensive Coordination Chemistry II; McCleverty, J., Ed.; Elsevier Science: Oxford, 2003; vol. 9. 5. McGarrigle, E. M.; Gilheany, D. G. Chem. Rev. 2005, 105, 1563. 6. Katsuki, T. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York, NY, 2000; p 287. 7. Jacobsen, E. N. In Asymmetric Epoxidation of Un-functionalized Olefins; Ojima, I., Ed.; VCH: New York, NY, 1993; ch. 4.2, and references therein. 8. Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R.; Jasra, R. V. In Catalysis Research at the Cutting Edge; Bevy, L. P., Ed.; Nova Science: New York, NY, 2005; pp 115e133; References therein. 9. (a) Huang, J.; Fu, X.; Miao, Q. Appl. Catal., A 2011, 407, 163; (b) Meng, X.; Qin, C.; Wang, X. L.; Su, Z. M.; Li, B.; Yang, Q. H. Dalton Trans. 2011, 40, 9964; (c) Lou, L.L.; Jiang, S.; Yu, K.; Gu, Z.; Ji, R.; Dong, Y.; Liu, S. Microporous Mesoporous Mater. 2011, 142, 214; (d) Zou, X.; Fu, X.; Li, Y.; Tu, X.; Fu, S.; Luo, Y.; Wu, X. Adv. Synth. Catal. 2010, 352, 163; (e) Tangestaninejad, S.; Moghadam, M.; Mirkhani, V.; Baltork, I. M.; Saeedi, M. S. Appl. Catal., A 2010, 381, 233; (f) Piaggio, P.; McMorn, P.; Langham, C.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Hutchings, G. J. New J. Chem. 1998, 22, 1167; (g) Piaggio, P.; Langham, C.; McMorn, P.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Sly, C.; Hutchings, G. J. J. Chem. Soc., Perkin Trans. 1 2000, 2, 143; (h) Piaggio, P.; McMorn, P.; Murphy, D.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Sly, C.; Kerton, O. J.; Hutchings, G. J. J. Chem. Soc., Perkin Trans. 1 2000, 2, 2008. 10. (a) Biernacka, K.; Silva, A. R.; Carvalho; Pires, A. P. J.; Freire, C. Catal. Lett. 2010, 134, 63; (b) Tan, R.; Yin, D.; Yu, N.; Zhao, H.; Yin, D. J. Catal. 2009, 263, 284; (c) Gong, B.; Fu, X.; Chen, J.; Li, Y.; Zou, X.; Tu, X.; Ding, P.; Ma, L. J. Catal. 2009, 262, 9; (d) Tang, X.; Tang, Y.; Xu, G.; Wei, S.; Sun, Y. Catal. Commun. 2008, 10, 317; (e) Zhang, H.; Wang, Y. M.; Zhang, L.; Gerritsen, G.; Abbenhuis, H. C. L.; Van Santen, R. A.; Li, C. J. Catal. 2008, 256, 226. 11. (a) Choudary, B. M.; Ramani, T.; Maheswaran, H.; Prashant, L.; Ranganath, K. V. ~o, A.; Garcia, H. S.; Kumar, K. V. Adv. Synth. Catal. 2006, 348, 493; (b) Baleiza Chem. Rev. 2006, 106, 3987. 12. (a) Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.; Pathak, K.; Jasra, R. V. J. Catal. 2006, 238, 134; (b) Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.; Singh, S.; Pandia, P. H.; Jasra, R. V. J. Catal. 2005, 235, 28; (c) Kureshy, R. I.; Ahmad, I.; Khan, N. H.; Abdi, S. H. R.; Singh, S.; Jasra, R. V. J. Catal. 2004, 221, 234. 13. Silva, M.; Freire, C.; de Castro, B.; Figueiredo, J. L. J. Mol. Catal. A: Chem. 2006, 258, 327. 14. Jannsen, K. B. M.; Laquire, I.; Dehaen, W.; Parton, R. F.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron: Asymmetry 1997, 8, 3481. 15. Reger, T. S.; Janda, K. D. J. Am. Chem. Soc. 2000, 122, 6929. 16. Song, C. E.; Roh, E. J.; Yu, B. M.; Chi, D. Y.; Kim, S. C.; Lee, K. J. Chem. Commun. 2000, 837. 17. Zhang, H.; Xiang, S.; Li, C. Chem. Commun. 2005, 1209.
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