Unusual Behavior of a Novel Heterogeneous Chiral Dimer Cr(III

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Unusual Behavior of a Novel Heterogeneous Chiral Dimer Cr(III)-Salen Complex in the Epoxidation/Epoxide Ring-Opening Reaction of trans-Methylcinnamate Ester† Loredana Protesescu,‡ Madalina Tudorache,‡ Stefan Neatu,‡ Maria Nicoleta Grecu,| Erhard Kemnitz,⊥ Petru Filip,§ Vasile I. Parvulescu,*,‡ and Simona M. Coman*,‡ Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, UniVersity of Bucharest, Bdul Regina Elisabeta, 4-12, Bucharest 030016, Romania, “Costin D. Nenit¸escu” Institute of Organic Chemistry of the Romanian Academy, Spl. Independentei 202B, 71141 Bucharest, Romania, National Institute of Materials Physics, Atomistilor Str. 105 bis, PO Box MG 7, Magurele-Bucharest, Bucharest 077125, Romania, and Humboldt-UniVersita¨t zu Berlin, Institut fu¨r Chemie, Brook-Taylor-Straβe 2, D-12489 Berlin, Germany ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: NoVember 22, 2010

A dimer chromium(III)-salen complex immobilized on modified silica is an effective catalyst for the epoxidation/epoxide ring-opening reaction of trans-methylcinnamate ester and gave significantly high ee in methyl (2R,3S)-2,3-dihydroxy-3-phenylpropionate. For the first time, it is shown that a donor ligand (e.g., Et3N) can be involved into the enantioselective mechanism of the product configuration. It is suggested that the additive is interacting with the reaction intermediate in a manner that prolongs its lifetime, thus affording a free rotation of the C-C single bond in this species and a selective collapse to the cis-epoxide product. In the presence of water, this epoxide led to methyl (2S,3R)-2,3-dihydroxy-3-phenylpropionate. Alternatively, the donor additives may give rise to a new chromium-based oxidant that effects epoxidation and its consecutive ring opening to diols via an unusual pathway. I. Introduction (2R,3S)-2,3-Dihydroxy-3-phenylpropionate is a valuable building block in the production of pharmaceuticals, agrochemicals, and other fine chemicals. For example, the C13 side chain of drugs as such as Taxol1,2 can be prepared from this valuable chiral compound. At the industrial level, its production consists of the kinetic resolution of racemates (()-phenyl glycinate through a lipase-catalyzed asymmetric hydrolysis.3 Unfortunately, this method suffers from different drawbacks, the most important being the poor yield (42%) of (2R,3S)-phenyl glycinate as well as the high amounts of formed waste (ca. 200 t/year) arising from the unwanted enantiomer (2S,3R)- phenyl glycinate. A catalytic asymmetric synthesis of the chiral glycidic acid derivatives involving (2R,3S)-phenyl glycinate has therefore been highly desirable. Thus, over the last 20 years, the efficient synthesis of the enantiopure C13 side chain (N-benzoyl-(2R,3S)3-phenylisoserine) has attracted attention both from the academic and industry communities,4–15 the synthesis employing as a key step the asymmetric epoxidation (AE) or the asymmetric dihydroxylation (AD) of cis-methylcinnamates to (2R,3S)-phenyl glycinate. Sharpless et al.16 developed a process for the Taxol side chain through AD (asymmetric dihydroxylation) of cismethylcinnamate that led to 23% overall yield, but the diol needs to be recrystallized to enrich the ee. On the other hand, the asymmetric dihydroxylation with AD-mix-β of cis-methylcinnamate gave the diol in 36% yield (7% optical purity). The commercially available and inexpensive trans-methylcinnamate †

Part of the “Alfons Baiker Festschrift”. * Corresponding authors. E-mail: [email protected]; [email protected]. ‡ University of Bucharest. § “Costin D. Nenit¸escu” Institute of Organic Chemistry of the Romanian Academy. | National Institute of Materials Physics. ⊥ Institut fu¨r Chemie.

was also subjected to asymmetric dihydroxylation as above with AD-mix-β, when the diol was obtained in 72% yield.17 Choi and co-workers18 reported very high ee in (2R,3S)-phenyl glycinates (>90%) by osmium-catalyzed AD of trans-cinnamate derivatives under high-pressure conditions. While the asymmetric dihydroxylation (AD) of the chiral glycidic acid derivatives has so far been extensively investigated, an approach based on catalytic asymmetric epoxidation of cinnamates is considered to be the most efficient one since the chiral glycidates can be obtained in a single step from cinnamates. In this context, it has been shown that while manganese-salen complexes show high enantioselectivities in the epoxidation of Z (cis) alkenes, chromium-salen complexes show the opposite characteristics for the olefin asymmetric epoxidation.19 On the other hand, the difficulties in the preparation of cis-cinnamic esters are well-known.20 These facts prompted us to search for methods to synthesize the (2R,3S)-phenyl glycinates via the catalytic asymmetric epoxidation of the commercially available and inexpensive trans-methylcinnamate. Moreover, given that chiral salen complexes are remarkably effective catalysts for the asymmetric epoxidation of simple olefins,21–23 it was speculated whether similar systems could be useful in epoxide ring-opening reactions. Thus, the elements of stereochemical communication between substrate and ligand in olefin epoxidation may apply to epoxide activation by a metal (salen) complex. Because of the inherent polarity and ring strain of the epoxy group, the oxirane ring represents a highly reactive moiety. Therefore, epoxides can undergo stereospecific ring-opening reactions with a variety of nucleophiles to form 1,2-difunctional compounds.24 Diols, for example, can be produced when water is present as a nucleophile, and the important finding therefore is the hydrolysis reaction taking place with a high enantioselectivity. The hydrolytic kinetic resolution (HKR) takes place

10.1021/jp106281z  2011 American Chemical Society Published on Web 01/04/2011

Heterogeneous Chiral Dimer Cr(III)-Salen Complex

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SCHEME 1: Synthesis of the Optically Active Dimeric (R,R)-Salen Ligand

with high ee in the presence of a slight excess of water and with trivalent cobalt-salen complexes as catalysts.25 The industrial need of enantiomerically pure compounds in drug synthesis has motivated in the past decade a tremendous interest in the search for suitable solid supports for heterogeneous stereoselective synthesis.26 This interest derives from the simpler experimental workup, easier separation from reagents and reaction products, catalyst reuse, and the possibility to design continuous-flow processes for the heterogeneous version of a highly enantioselective catalyst. This is particularly the case of chiral metal(salen) complexes.27,28 However, a major problem generally encountered in this type of work is the lowering in the degree of the asymmetric induction ability of the complexes supported on a solid. On the other hand, the presence of adventitious uncomplexed metal ions present in the solid is undesirable because these ions would introduce sites that would decrease the overall enantiomeric excess. So, alternative methodologies consisting of the synthesis of conveniently functionalized metal(salen) complexes that would be subsequently anchored on a solid surface would be worth exploring. Although this methodology is not new, the enantiomeric excesses achieved up to now are frequently rather modest compared to the homogeneous phase without having a clear understanding of the reasons for this. Clearly, more efforts are needed in this field until a general and reliable way to obtain enantioselective catalysts anchored on a metal oxide or silicate surface can be established. In this context, the chiral salen-based catalysts for the enantioselective ring opening of meso-epoxides and kinetic resolution of terminal epoxides29 were found to be good candidates for the immobilization on solid support. For example, covalent attachment of chiral Co(salen) complexes to polystyrene and silica led to efficient and highly enantioselective catalysts for the hydrolytic kinetic resolution (HKR) of terminal epoxides. Mechanistic studies showed a dramatic correlation between the degree of catalyst site isolation and reaction rate indicating that for these cobalt catalysts a cooperative bimetallic mechanism occurs.29,30 This behavior was confirmed by independent studies of the Shibasaki group,31 showing that the cooperative effect is also present in the asymmetric epoxide

ring opening and probably in other reactions as well. Indeed, it was then confirmed that in a similar way to the case of HKR reactions, the mechanism of the asymmetric ring opening (ARO) of epoxides involves a cooperative effect between chiral catalyst units.32 Bimetallic activation offers an appealing explanation for the mechanism of stereoinduction in ARO reactions, as the two reactants are sandwiched between two chiral catalyst units. As we are interested in the improvement of the Taxol sidechain synthesis also from the environmental point of view, the first interest was to search for a new alternative green catalytic route in the asymmetric synthesis of the methyl (2R,3S)-2,3dihydroxy-3-phenylpropionate intermediate. Thus, here we report a novel heterogeneous optically active dimer Cr(III)-salen complex as a highly active and enantioselective catalyst in the epoxidation/epoxide ring-opening reaction of the readily cheap and available trans-methylcinnamate ester to methyl (2R,3S)2,3-dihydroxy-3-phenylpropionate. The applied reaction is notable not only for its high enantioselectivity and the synthetic utility of its products but also for its remarkable efficiency as a catalytic process. For comparison, an optically active dimer Mn(III)-salen complex was also prepared and tested in the reaction. II. Experimental Section II.1. Catalyst Preparation. II.1.1. Synthesis of the Optically ActiWe Dimer (R,R)-Salen Ligand. The synthesis of the dimer salen (Scheme 1) followed a modified procedure of Jacobs and co-workers.33 Accordingly, we used 3-tert-butyl-5-chloromethyl salicilaldehyde (1) with the scope to covalently dock the salen structure through 3-aminopropyl tethers to functionalized silica. Additionally, the complexation of the dimer salen was made with chromium acetate(III) instead of CrCl2. In the first step, the Schiff base (2) was obtained by the condensation of (1R,2R)-(-)-1,2-diaminecyclohexane with 3-tertbutyl-5-chloromethyl salicilaldehyde (1). The dialdehyde (3) was synthesized through the reaction of the 3-tert-butyl salicilaldehyde with trioxane under acidic conditions. Then the dimer salen (4) was obtained from the reaction of dialdehyde (3) by coupling with two Schiff base (2) units.

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SCHEME 2: Synthesis of N-(2-Hydroxy-3-tert-butyl-5-chloromethyl Benzaldehyde)-1-amino-2-cyclohexenimine (2)

4.32 (s, 2H, CH2-Cl); 6.96 (d, 2H, Ar-H); 7.20 (d, 2H, Ar-H); 8.26 (s, 2H, CHdN); 13.83 (s, 1H, OH). 1 H NMR spectra are given in the Supporting Informations (Figure 1S). II.1.1.3. Synthesis of the 5,5-Methylene-di-tert-butyl Salicilaldehyde (3). 3-tert-Butyl salicilaldehyde (0.25 mol) was treated with a trioxane solution (6.0 g) in concentrated acetic acid (20 mL) and sulfuric acid (0.2 mL) under a protected atmosphere. The resulted mixture was heated to 90 °C under vigorous stirring for 24 h. After that, the mixture was added to 500 mL of cold water and stirred for another 12 h. The obtained brown product was then extracted in dichloromethane (3 × 40 mL), and the solvent was removed under vacuum. A dark brown compound was obtained that was purified on a silica column using hexane/ethyl acetate (90/10) as eluent. After the removal of the solvent, the pure dialdehyde was obtained as an orangebrown oil which progressively transformed into a light yellow solid with raising temperature (yield ) 61%). 1H NMR (CDCl3; 300 MHz): δ (ppm) 1.40 (s, 9H, tert-butyl); 3.93 (s, 2H, methylene); 7.15 (d, 2H, Ar-H); 7.36 (d, 2H, Ar-H); 9.81 (s, 2H, CHO); 11.70 (s, 2H, OH) (cf. Figure 2S from Supporting Information). II.1.1.4. Synthesis of 5,5-Methylene Di-[(R,R)-(N-3-tert-butylsalicilidine)-N′-(3′-tert-butyl-5′-chloromethyl Salicilidine)-1,2cyclohexanediamine] (4). After the separation of compound 5 the pure N-(2-hydroxy-3-tert-butyl-5-chloromethyl benzaldehyde)-1-amino-2-cyclohexenimine (2) was used for the synthesis of 5,5-methylene di-[(R,R)-(N-3-tert-butylsalicilidine)-N′-(3′tert-butyl-5′-chloromethyl salicilidine)-1,2-cyclohexane diamine] (4), in the following conditions: 2 equiv of the compound (2) in dichloromethane with an equivalent of the compound (3) in ethanol was mixed under reflux for 8 h. After the evaporation of the reaction mixture, the dimeric salen ligand precipitate was separated with a yield of 85%. The obtained dimer salen was characterized by NMR, DRIFT, and Raman spectroscopy (see Figures 3S-5S from Supporting Information). Both the NMR and DRIFT spectra are in agreement with ref 33. 1 H NMR (CDCl3; 300 MHz): δ (ppm) 1.23 (s, 18H, tertbutyl); 1.80 (s, 36H, tert-butyl); 1.80-2.0 (m, 16H, cyclohexane); 2.84 (m, 4H, CH-N); 3.93 (s, 2H, methylene); 4.42 (s, 4H, CH2-Cl); 6.97 (s, 4H, Ar-H); 7.12 (s, 4H, Ar-H); 8.41 (s, 4H, CHdN); 13.89 (s, 4H, OH) (Figure 3S from Supporting Information). Infrared (IR) and Raman (R) spectroscopy are essential tools for the study and elucidation of the molecular structures of organic and inorganic materials. The DRIFT spectra of dimer salen evidenced the ν(C-H) vibrations of the -CH2- groups in the 2800-3000 cm-1 range and the -OH characteristic bands in the 3200-3500 cm-1 range and two strong bands at 1360 and 1380 cm-1. The high intense signal at 863 cm-1 is characteristic for the tert-butyl, while the strong band from 1626 cm-1 was assigned to the stretching vibrations of azomethine groups (H-CdN). Moreover, the stretching vibrations of the -CH2-Cl groups of the dimer salen complex appear at 710 cm-1 (Figure 4S, Supporting Informations). Moreover, the -CdN- band can be seen in the Raman spectrum at approximately 1605 cm-1 with another strong band at 1380 cm-1 (Figure 5S, Supporting Informations). II.1.2. Synthesis of the Dimer Mn(III)-salen Complex. To 0.031 g (0.126 mmol) of manganese acetate(II) in absolute ethanol/CH2Cl2 (1/1) (16 mL) was added 0.061 g (0.063 mmol) of (R,R)-4 under a protected atmosphere. The obtained dark brown solution was further stirred under an argon atmosphere for 12 h and then in air for another 12 h. The obtained solution

II.1.1.1. Synthesis of the 2-Hydroxy-3-tert-butyl-5-chloromethyl Benzaldehyde (1). 3-tert-Butyl-2-hydroxybenzaldehyde (2.7 g, 15.2 mmol) reacted with para-formaldehyde (1.0 g, 33.3 mmol) and tetrabutylammonium bromide (0.47 g, 1.46 mmol) in 11.0 mL of conc. hydrochloric acid under vigorous stirring for 96 h, at 45 °C. The obtained products were extracted several times with diethyl ether (3 × 15 mL). Then the organic phase was separated, washed with a 5% NaHCO3 (2 × 10 mL) solution, saturated with NaCl aqueous solution (2 × 10 mL), dried on MgSO4, and concentrated to a yellow crystalline solid (3.4 g, yield ) 99%). 1H NMR (CDCl3, 300 MHz): δ (ppm) 1.43 (s, 9H, tert-butyl), 4.59 (s, 2H, -CH2-Cl), 7.44 (d, 1H, Ar-H), 7.53 (d, 1H, Ar-H), 9.87 (s, 1H, -CHO), 11.87 (s, 1H, -OH). II.1.1.2. Synthesis of the N-(2-Hydroxy-3-tert-butyl-5-chloromethyl Benzaldehyde)-1-amino-2-cyclohexenimine (2). To a solution of 3-tert-butyl-5-chloromethyl salicilaldehyde (0.12 g; 0.5 mmol) in ethanol (7 mL) was added a solution of (1R,2R)(-)-1,2-diaminocyclohexane (0.57 g; 5 mmol) in ethanol (3 mL). The mixture was vigorously stirred for 12 h at room temperature. After 12 h the mixture was concentrated under vacuum, dissolved in dichloromethane, and extracted with water to remove the excess of unreacted diaminocyclohexane. The extract was dried with Na2SO4. After solvent removal, the product was obtained as a yellow solid. The obtained product was monitored by TLC chromatography and purified by silica gel column chromatography using hexane/ethyl acetate (90/10) as eluent. Both products (e.g., before and after purification on silica) were analyzed by 1H and 13C NMR. The 1H NMR spectra of the product obtained in the synthesis of the N-(2-hydroxy-3-tert-butyl-5-chloromethyl benzaldehyde)1-amino-2-cyclohexenimine (2) before and after the silica purification showed during the synthesis two compounds were formed, namely, N-(2-hydroxy-3-tert-butyl-5-chloromethyl benzaldehyde)-1-amino-2-cyclohexenimine (2) and a monomeric salen complex (5) in a molar ratio of 3:2 (Scheme 2). N-(2-Hydroxy-3-tert-butyl-5-chloromethyl benzaldehyde)-1amino-2-cyclohexenimine (2). 1H NMR (CDCl3, 300 MHz): δ (ppm) 1.42 (s, 9H, tert-butyl); 1.50-1.95 (m, 8H, cyclohexane); 2.20 (m, 2H, NH2); 2.85 (m, 1H, (R,R)-CH-N); 4.42 (s, 1H, CH2-Cl); 7.11 (d, 1H, Ar-H); 7.28 (d, 1H, Ar-H); 8.40 (s, 1H, CHdN); 13.89 (s, 1H, OH) 1 H NMR (CDCl3, 300 MHz) (5): δ (ppm): 1.42 (s, 18H, tertbutyl); 1.50-1.95 (m, 8H, cyclohexane); 2.85 (m, 2H, CH-N);

Heterogeneous Chiral Dimer Cr(III)-Salen Complex was diluted with 250 mL of tert-butyl-methyl ether (TBME) and washed with a saturated solution of NH4Cl (3 × 150 mL) and a saturated solution of NaCl (3 × 150 mL). The organic phase was separated and dried on Na2SO4, and the solvent was removed under reduced pressure. A brown solid was obtained with a yield of 75%. II.1.3. Synthesis of the Dimer Cr(III)-salen Complex. To 0.1605 g (0.266 mmol) of chromium acetate(III) in CH2Cl2/ EtOH 1/1 (16 mL) was added 0.1294 g (0.133 mmol) of (R,R)-4 under an air atmosphere. The obtained light green solution was further stirred under an air atmosphere for 24 h. The obtained solution was diluted with 250 mL of tert-butyl-methyl ether (TBME) and washed with a saturated solution of NH4Cl (3 × 150 mL) and a saturated solution of NaCl (3 × 150 mL). The organic phase was separated and dried on Na2SO4, and the solvent was removed under reduced pressure. A green-brown solid was obtained with a yield of 42%. The successful complexation of the dimer salen with chromium acetate was demonstrated by DRIFT and Raman spectroscopy (cf. Figures 6S and 7S from Supporting Information). A comparison of the DRIFT spectra of the dimer salen (spectra B, Figure 6S, Supporting Information) and the Cr(III) dimer-salen complex (spectra A, Figure 6S, Supporting Information) shows the disappearance of -OH characteristic bands from the 3200-3500 cm-1 range and a strong lowering of the two strong bands from 1360 and 1380 cm-1. This behavior can be attributed to the appearance of new bonds between Cr species and -OH groups with the disappearances of the last. IR spectra of Cr(III) acetate (spectra C, Figure 6S, Supporting Information) show the characteristic bands of the COO- group at 1550 and 1450 cm-1, corresponding to the asymmetrical and symmetrical stretching vibrations, respectively. The gap of 100 cm-1 is characteristic to the presence of bidentate OAc- groups.34 Moreover, the presence of -OH and H2O lead to a broad band in the 3000-3500 cm-1 region. As an effect of the dimer salen complexation with Cr(III) acetate, this broad band disappeared. Unfortunately, the characteristic bands from Cr(III) acetate and the stretching vibrations of azomethine groups (H-CdN) are superposing (spectra A and C, Figure 6S, Supporting Information) making difficult a clear attribution of the changes in this band value to other wavenumbers. This difficulty is somehow normal if we take into consideration that by complexation of the Cr(III) acetate with dimer salen only one Cr(III) will enter in coordination with the salen complex, and the other Cr(III) ions from the trinuclear acetate species will remain unchanged. Raman spectroscopy is the one of the few analytical techniques which can positively identify and characterize both elements and molecules. Figure 7S (cf. Supporting Informations) shows the Raman spectra of the chromium salen and of the free dimer salen. A fluorescence background, however, is induced both in the Cr(III) acetate and Cr(III)-salen dimer which overshadows the Raman features of the chromium species which complicates the collection of Raman spectra. Nevertheless, the Raman spectrum of the Cr(III)-salen dimer shows a Cr-Ostr. vibration at around 550 cm-1, a band located at around 310 cm-1, which can be attributed to Cr-Nstr. vibration in agreement with ref 35, and two intense bands located at 1350-1400 cm-1 atributted to C-C vibration. II.1.4. Grafting of the (3-Aminopropyl)triethoxysilane. First, 0.53 g (2.4 mmol) of (3-aminopropyl)triethoxysilane was dissolved in 20 mL of anhydrous toluene, and 2 g of silica was added. The slurry was heated under reflux for 12 h and then cooled at room temperature, at which point the solid was

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1115 SCHEME 3: Synthesis of Aminopropylated Silica

separated by filtration. The solid filtrate was washed with 50 mL of pentane, 100 mL of acetonitrile, and 100 mL of diethyl ether and after drying under vacuum resulted in the aminopropylated silica (Scheme 3). II.1.5. Grafting of the Optically ActiWe Dimer Me(III) (Me ) Mn, Cr)-salen Complex. The immobilization of the dimer Mn(III)-salen complex was carried out by contacting it with the previously functionalized silica support under refluxed ethanol/CH2Cl2 (1/1) conditions (Scheme 4). Thereby, 0.028 g of dimer Mn(III)-salen complex per gram of the hybrid organic-inorganic silica material was added into a round-bottom flask. The mixture was stirred and heated at reflux temperature in air for 24 h. The solid was separated by filtration and dried for 12 h at 60 °C. The grafting of the optically active dimer Cr(III)-salen complex was carried out in a similar way (0.020 g of dimer Cr(III)-salen complex/1 g of functionalized silica) following the same procedure as for the case of the grafting of the dimer Mn(III)-salen complex. II.2. Catalyst Characterization. II.2.1. Characterization of the DeriWed Complexes. 1H and 13C NMR Spectroscopy. Bruker AV 400 spectrometer, in CDCl3 solvent and Me4Si as the internal standard. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT). Spectra were collected with a Thermo 4700 spectrometer (200 scans with a resolution of 4 cm-1) on a domain of 600-4000 cm-1. Raman Spectra. They were collected with an Horiba Jobin Yvon - Labram HR UV-visible-NIR (200-1600 nm) Raman microscope spectrometer, at room temperature. Raman spectra were excited by a 633 nm laser at a resolution of 2 cm-1 in the range between 200 and 4000 cm-1. DR-UV-Vis Spectroscopy. Optical spectra in the UV-vis region were recorded using a diffuse reflectance technique with a Specord 250 spectrometer from Analytik Jena. II.2.1. Characterization of the Anchored Catalysts. X-ray Diffraction (XRD). Powder X-ray diffraction (XRD) was made using a Siemens D5000 X-ray diffractometer with nickel filtered Cu KR radiation (λ ) 1.5418 Å) at a scanning rate of 0.1 min-1 in the 2θ range of 10-80°. DR-UV-Vis Spectroscopy. Optical spectra in the UV-vis region were recorded using the diffuse reflectance technique with a Specord 250 spectrometer from Analytik Jena. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT). Spectra were collected with a Thermo 4700 spectrometer (200 scans with a resolution of 4 cm-1) on a domain of 600-4000 cm-1. Electron Paramagnetic Resonance (EPR). EPR spectra were recorded on Compact Adani CMS 8400 and Jeol upgraded JESME X-band EPR spectrometers, with 100 kHz field modulation. Measurements were performed on powder samples at room (17 °C) and variable temperatures (-173 °C to 77 °C). DPPH was used as a field standard marker (g ) 2.0036). The microwave frequency was measured using a digital frequency-meter Pendulum CNT-90.

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SCHEME 4: Grafting of the Optically Active Dimer Mn(III)-Salen Complex

ThermograVimetric Analysis (TG-DTA). Thermogravimetric analysis was performed using a SDT Q600 instrument supplied by TA Instruments. The samples were placed in an alumina sample holder and heated at a rate of 10 °C min-1 from room temperature to 900 °C under a N2 flow with a rate of 20 mL min-1. II.3. General Procedure for the Epoxidation/Epoxide Ring-Opening Reaction of trans-Methylcinnamate Ester. The reaction was carried out in different solvents. To a solution of trans-methylcinnamate ester (1 mmol (0.1622 g) in 6 mL of solvent (e.g., EtOH, EtOH:H2O (5:1), acetone:H2O (9:1), acetone:acetonitrile (1:1), acetone:CH2Cl2 (1:1), and dioxane)) were added the heterogeneous dimer Me(III)-salen complex (Me ) Mn, Cr) (3.2 mg or 8.1 mg corresponding to 2 mol % and 5 mol %, respectively), Et3N (0.1 g, 1 mmol), and the cooxidant (NMO or hydrogen peroxide) (1.5 mmol). When NMO was used as co-oxidant (1.5 mmol, 0.1757 g), the total amount was added to the slurry from the beggining of the reaction in one single charge, while when using H2O2 (1.5 mmol, 0.2 mL) this was dropwise added in a period of 30 min from the beggining of the reaction. The reaction time varied between 4 and 24 h and the reaction temperature between 25 and 70 °C. Some tests by adding NaN3 in the reaction were also done. Leaching Measurements. The percentage of activity due to the complex leached from the solid and present in the solution was determined by performing the reactions at 70 °C for 12 h. At this time, half the volume was filtered, and the resulting clear solution was allowed to react for another 12 h. The percent of leaching was estimated by comparing the conversions with and without solid. Product Analysis. The reaction products were analyzed by UPLC (Thermo Scientific apparatus, column: Astec CYCLOBOND I 2000, 25 cm × 4.6 mm, 5 µm, eluent: n-hexane/ethanol )99/1, Q ) 1 mL min-1, UV-vis detection, λ ) 215 nm; 5.7 min (2R,3S), 6.0 min (2S,3R), cf. Figures 8S and 9S from Supporting Information) and by an HPLC (KNAUER apparatus, column: CHIRALCEL OD-H, n-hexane/ethanol ) 99/1, Q ) 1 mL min-1, UV-vis detection, λ ) 215 nm; 6.9 min (2S,3R), 14.7 min (2R,3S)), while their structures were confirmed by 1H and 13C NMR (Bruker AV 400 spectrometer, in CDCl3 solvent and Me4Si as internal standard) and DRIFT spectroscopy. III. Results and Discussion The catalytic screening experiments showed that the dimer Mn(III)-salen complex (homogeneous or heterogeneous) is not emerging as an effective catalyst in the asymmetric dihydroxy-

lation (AD) of the trans-methylcinnamate ester. As was expected, this complex coordinated the trans-methylcinnamate ester to any measurable extent. Therefore, this catalyst was unlikely to serve as a Lewis acid catalyst for the epoxide ringopening reaction. Moreover, it is well-known that when hydrogen peroxide is used as an co-oxidant the addition of a donor ligand is indispensable probably because the coordination of an axial ligand is crucial to the O-O bond cleavage of the intermediate hydroperoxide species [HO-O-MnIII].36 However, using triethylamine (Et3N) as a donor ligand, both the reactivity and enantioselectivity of the dimer Mn(III)-salen complex were very poor irrespective of its presence or absence in the reaction medium (the convensions of trans-methylcinnamate were below 2%). While the dimer Mn(III)-salen complex displayed very poor reactivity and negligible enantioselectivity, the screen revealed rather exciting results in the presence of the dimer Cr(III)-salen complex which concerns both the reactivity and the enantioselectivity. Literature data reports that in the homogeneous catalysis the epoxide ring opening follows a second-order reaction in relation to the catalysts that involve two metal salen molecules in the transition state complex. Accordingly, the dimeric salen species led to reaction rates of one or two orders of magnitute higher than the monomeric catalyst.29,31 Such a behavior can question the catalytic behavior of dimeric salen species complexed not with mononuclear metallic species but with trinuclear ones as is the case for Cr(III) acetate. Cr(III) acetate, used for the salen-based complex preparation, has a trimeric structure (Scheme 4), in which the three chromium ions are bridged by three pairs of acetate ligands and form an equilateral triangle. The three metal ions are connected by a triply bridging oxygen, and three water molecules act as the terminal ligands.37 The structure of Cr(III) acetate, under acidic pH conditions, is referred to as a cyclic chromium trimer.38 Aqueous solutions of Cr(III) acetate contain green cyclic chromium trimer. According to Tackett,38 Cr(III) acetate undergoes hydrolysis in solution. During this process, hydroxyl groups may replace the central oxygen and some of the acetates leading to linear structures as shown in Scheme 5. A complex mixture of chromium structures can be present in chromium acetate solutions.38 However, the chemistry of chromium in aqueous solution is complex,39 involving different mechanisms. The identification of the grafted organic chains was achieved by DRIFT and UV-vis spectroscopic measurements by comparison of the spectra of modified silica solid with those of the



SCHEME 5: Cr(III) Acetate Structures

corresponding molecules in solution. The UV-vis spectra of Cr(III) acetate, the dimer Cr(III)-salen complex, and the heterogenized Cr(III)-salen complex are displayed in Figure 1. Since formation of the chromium-salen complex was performed in the homogeneous phase in the absence of the silica support, the presence of uncomplexed chromium(III) species deposited on the solid surface should be negligible. According to Vargas-Vasquez et al.,40 fresh Cr(III) acetate solutions are characterized by a green color that indicates the presence of the cyclic chromium acetate structure, with wavelengths of 460 nm for the first peak and 597 nm for the second peak (Figure 1A). During the complexation with the dimer salen, the solution becomes blue, indicating that chromium acetate solutions undergo hydrolysis under the temperature effect. While the complex dimer Cr(III)-salen starts to be formed, the color of the final complex precipitate changed to green-brown which indicates a supplementary change of the chromium complex conformation. However, the possibility of the existence of other oxidation states of Cr ions can not be ruled out. The recorded spectra for the final complex have shown that the wavelength of the first peak shifts to 420 nm and that of the second peak shifts to 572 nm (Figure 1B). These shifts are in a good concordance with data reported previously by Tackett.37,38 Moreover, a third peak appears at 357 nm that could be attributed to the charge transfer transition of the salen ligand. Thus, the spectroscopic changes

Figure 1. UV-vis spectra of Cr(III) acetate (A), the dimer Cr(III)-salen complex (B), and the grafted dimer Cr(III)-salen complex (C).

Figure 2. X-band EPR spectra of Cr(III) acetate (A) and dimer Cr(III)-salen (B) at room temperature.

observed in the Cr(III) acetate solutions can be explained by a hydrolytic process and a paralleled complexation of chromium with the dimeric salen ligand under reflux conditions. The presence of the same bands in the spectra recorded for the solid catalyst confirms the incorporation of the chiral complex in the support (Figure 1C). The lowered intensity of the absorption spectra of the solid catalyst is in accordance with the low amount of dimer Cr-salen complex immobilized onto the silica surface. The band from 580 nm completely disappeared, while a blue shift from 440 to 430 nm and from 357 to 336 nm indicated an additional interaction between the dimer Cr(III)-salen complex and the support (Figure 1C). EPR investigation gave more information on the structure of the chromium catalysts. Figure 2 gives the EPR spectra of the Cr(III) acetate and dimer Cr(III)-salen precursors. The EPR signals of both complex precursors are usually broadened due to dipolar interactions. The presence of salen enhances the exchange interaction between paramagnetic molecules resulting in a narrowing of the line. Usually a single line is observed with a line width determined by the competition of the two (dipolar and exchange) interactions. It is evident that for the dimer Cr(III)-salen complex the exchange interaction is stronger, and the line width is more narrow (∆Hpp(1) ) 64 mT and ∆Hpp(2) ) 36 mT). This is supported also by the Lorentzian line shape for the dimer Cr(III)-salen complex. The deposition of the Cr(III) acetate complex corresponds to a significant decrease of the intensity. The line width and geff ) 1.99 are practically unchanged which may account for a small dispersion of the complex on the silica support. However, the deposition of the dimer Cr(III)-salen complex has a completely different behavior (Figure 3). The spectrum is more complex, and the spectral characteristics are changed: geff is increased to 2.00, the line width is much broader, and the line intensity is correspondingly much lower (Figure 3B). These data are consistent with a large dispersion of the complex, so that the exchange interaction between molecules is no more efficient. These data are in concordance with the XRD measurements which shown only a broad diffraction line characteristic to the amorphous silica. No additional diffraction lines appeared after the dimer Cr(III)-salen complex grafting indicating a very high dispersion of the complex on the silica surface and no complex decomposition and segregation after the complexation. Again, the existence of Cr ions in other oxidation states can not be excluded. In addition, for the supported complex the line is split, and a quite narrow signal with geff ) 2.2 is observed (Figure 3B). With decreasing temperature, a signal shift toward lower fields with gradually decreased intensity can be observed. Below 120

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Figure 3. X-band EPR spectra of the dimer Cr(III)-salen complex (A) and dispersed on silica (B) at room temperature.

K the line disappears. These changes are however reversible. Such a behavior is characteristic for the antiferromagnetic coupled ions and may account for a reorganization of the complex after its immobilization. Figure 4 shows the representative DRIFT spectra in the scan range of 1000-4000 cm-1 for the silica support, the 3-aminopropyltriethoxysilane-modified support, and the heterogeneous dimer Cr(III)-salen complex catalyst. Due to the low concentration of the organic modifier part on the surface, the intensity of the new bands probing the presence of -CH2- groups for the functionalized silica is weak (Figure 4c). Almost no differences in the 2800-3000 cm-1 range were observed, where ν(C-H) vibrations of the -CH2- groups are evidenced on the surface of the functionalized silica (Figure 4a in comparison with Figure 4c). However, the intensities of these bands increased with the immobilization of the chiral dimer Cr(III)-salen complex (Figure 4b). The infrared spectra of the heterogeneous dimer Cr(III)-salen complex exhibited the characteristic signals of CrIII complex molecules, respectively, in addition to the C-H vibration bands of the propyl chains (1420 and 1389 cm-1). Moreover, after the immobilization of the dimeric Cr(III)-salen complex, two new characteristic IR bands appeared at 1540 and 1450 cm-1, which were assigned to the stretching vibrations of azomethine groups (H-CdN) and the deformation vibrations of the C-H bond, respectively. The high intense signal at 950 cm-1 is characteristic for the tert-butyl which belongs to the salen structure (C8 and C8′). The most important IR band indicating that the dimer Cr(III)-salen complex was indeed grafted onto the functionalized silica via the condensation reaction between aminopropyl groups and the side chain -CH2-Cl of the dimer salen complex is located at 1050 cm-1. It accounts for the bond between the silica and the salen complex as long as the stretching vibrations of the second -CH2-Cl group of the dimer salen complex appear at 800 cm-1. The band located at 1200-1300 cm-1 can be assigned to the acetate groups from the trinuclear Cr(III) acetate. In conclusion, the DRIFT spectra confirmed the successful immobilization of chiral dimer Cr(III)-salen complex. The anchoring of the complexes was also checked by combined TG-DTA measurements. The decomposition of the free dimer Cr(III)-salen and of the anchored complexes has been compared to understand the effect of anchoring (Figure 5). The free complex decomposes during heating until 900 °C in several well-defined steps. This behavior indicates that the dimer Cr(III)-salen complex decomposes in a relatively welldefined manner and releases defined fragments, which readily

Protesescu et al. decomposed. An endothermic weight loss appears at temperatures below 350 °C, assigned to the successive cleavage of the tert-butyl and methyl groups. Above this temperature, several endothermal and exothermal processes take place, another part of organic moieties being decomposed up to 900 °C. The functionalized aminopropyl silica also decomposes in a relatively well-defined manner and releases defined fragments (Figure 6). The mass loss together with the enthalpy effects are given in Table 1. The main mass loss (4.12%) corresponds to the decomposition of the aminopropyl entities. This percentage indicates that 63.3% from the total amount of (3-aminopropyl)triethoxysilane used during the functionalization process was grafted on the silica surface. The anchored dimer Cr(III)-salen complex showed a different decomposition behavior due to the covalent bonding established between the aminopropyl functions and the -CH2-Cl side chain of the dimer Cr(III)-salen complex (Figure 7). The combined TG-DTA curves of the silica anchored dimer Cr(III)-salen complex show well-defined steps of weight loss and endothermic peaks. Two well-defined endothermic DTA peaks and a shoulder were observed between 390 and 580 °C. Decomposition of the complex is complete at 580 °C. The shift of the onset temperature of decomposition of the anchored dimer Cr(III)-salen complex in comparison with the free dimer Cr(III)-salen complex may be attributed to the covalent bonding of the complex on the functionalized silica support. From these TG measurements, results that the nonvolatile residue remained after the sample’s decomposition correspond to a molar ratio dimer salen complex/Cr(III) acetate of 1/1, meaning that each dimer salen molecule coordinates to only one molecule of trinuclear Cr(III) acetate. On the basis of the characterization data, several features of the dimer Cr(III)-salen complex and its grafting onto the silica surface could be concluded: (i) each dimer salen molecule interacts with only one molecule of trinuclear Cr(III) acetate (TG measurements); (ii) the grafting of the dimer Cr(III)-salen complex takes place indeed through the condensation between one of the salen Cl-CH2- side chain units and the aminopropyl NH2-CH2- unit from the silica surface (DRIFT measurements); (iii) the deposition of the complex stabilizes a certain configuration of the complex (EPR). On the basis of these data, the structure model depicted in Scheme 6B (1:1 acetate to dimer molar ratio) is more probable than the structure models despicted in Scheme 6A (2:1 acetate to dimer molar ratio) and Scheme 6C (1:2 acetate to dimer molar ratio). The preliminary catalytic results obtained in the epoxidation/ epoxide ring opening of trans-methylcinnamate (Scheme 7) are given in Table 2. Among the used solvents (see Experimental Section), the most appropriate one was the EtOH:H2O mixture, all reactions made in a different solvent taking place with only less than 1% conversion of trans-methylcinnamate substrate. As Table 2 shows, hydrogen peroxide is an appropriate cooxidant for the formed catalytic active species. In the presence of NMO (entry 10), the conversion of the trans-methylcinnamate was only 1% after 48 h. A higher temperature did not improve the conversion of the cinnamate substrate (not shown in the Table 2). However, using hydrogen peroxide, the conversion of trans-methylcinnamate was improved from 3% to 60% at reaction temperatures in the range from room temperature to 70 °C. As expected, oppositely, the selectivity to diol decreased with the increase of the reaction temperature from 78% to 52%.



Figure 4. DRIFT spectra of the silica (a), aminopropyl silica (c), and heterogeneous dimer Cr(III)-salen complex (b).

Figure 5. TG-DTA curves for the free dimer Cr(III)-salen complex.

The enantioselectivity paralleled the chemoselectivity, and increases of the reaction temperature led also to a decrease of the ee in the (2R,3S)-diol configuration with more than 20%. The presence of water as a solvent component has a detrimental effect upon the trans-methylcinnamate conversion, but the selectivity to the diol product was significantly increased (Table

2, entries 2 and 3). The ee was only slightly influenced by the water presence. The optimum reaction conditions for the higher selectivities in the diol product and the higher ee in the (2R,3S) configuration are: temperature, 70 °C; solvent, EtOH:H2O ) 5:1 (vol:vol); and co-oxidant, H2O2. All experiments were carried out with a

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Figure 6. TG-DTA curves for the functionalized silica with aminopropyl groups.

TABLE 1: Main Effects during the Decomposition of the Aminopropyl Functions Grafted on the Silica Surface step

temperature, °C

mass losses, mg/%

enthalpy (∆H), J

I II III

74-264 271-439 451-721

0.466/2.54 0.761/4.12 0.738/3.99

10.52 J/570 J/g 70.48 J/3.82 kJ/g 2.05 J/110.9 J/g

2 wt % heterogeneous catalyst. An increase of the catalyst amount led to an increase of the trans-methylcinnamate conversion. A very interesting feature of this system that is somehow in contradiction with the general experimental observations41 is that the presence of the donor ligand (Et3N) does not necessarily correspond to an acceleration reaction rate effect, as was expected (Table 2, entries 1 and 2). Therefore, in the presence of the donor ligand (Et3N), the conversion of trans-methylcinnamate is considerably lower than in its absence. On the other hand, the donor ligand (Et3N) improves the selectivity to diol, and more interestingly, it induced a preference for the (2S,3R) configuration of the formed diol (most probably via the ring opening of a cis-epoxide intermediate) ruling out the possibility that the additive effect is due to its coordination to the chromium-salen complex. Jacobsen and co-workers42 found an interesting influence of chiral quaternary ammonium salts by inducing a high selectivity for the formation of trans-epoxide from terminal CdC alkenes, but these salts do not appear to exert any influence on the enantioselectivity of epoxidation. Therefore, the epoxidations with achiral (salen)Mn catalysts in the presence of these chiral additives afforded only racemic epoxide products. In each case, the enantiomeric composition of the trans-epoxide was similar (80-85% ee) in reactions using the (salen)Mn complex, and the absolute configuration of the major epoxide product depended only on the stereochemistry of the salen complex. Neither cis-olefin nor cis-epoxide was observed to undergo isomerization under the conditions of epoxidation. On the other hand, Brandt et al.19 discovered an unusual experimental effect of the salen complex counterions. Therefore, the replacement of the NO3- counterion in a chromium salen complex with BARF ([B(3,5-(CF3)2C6H4)4] led to a clean conversion of trans-olefins to a 1:1 mixture of cis and trans epoxides. The authors suggested that this effect was due to the formation of a [(salen)CrIVdO] species by one-electron reduc-

tion of the chromium complex in the presence of a tetraarylborate. Theoretical calculation of the reaction intermediates showed that their stability allows the rotation around the C-C bond to give a mixture of products, which approaches a thermodynamic distribution of cis- and trans-epoxides. Nevertheless, the same authors19 showed that only in a limited number of cases the intermediate is stable enough to undergo a C-C bond rotation prior to the epoxide ring closure, and therefore only a small amount of trans-epoxide (ca. 10%) is produced from cis-alkene. At this stage, no mechanism that accounts for the selective formation of cis-epoxides (and consequently for the enantioselection to the (2S,3R)-diol by a consecutive ring opening of the formed epoxide in the presence of water) from trans-methylcinnamate in the presence of additives as Et3N has been proposed. It is very probable that the additive is interacting with the reaction intermediate in a manner that extends its lifetime, thus affording a free rotation of the C-C single bond in this species and a selective collapse to the cis-product. Alternatively, the donor additive (e.g., Et3N) may give rise to a new chromiumbased oxidant in which the other two chromium ions from the trinuclear species are involved that effects epoxidation and its ring opening via an unusual pathway. The presence of NaN3 in the reaction mixture, a stronger nucleophile than water, led to the formation of the azido alcohol. This behavior is additional proof that the reaction takes place through two consecutive sequences (e.g., epoxidation/epoxide ring opening) and not through a direct dihydroxylation reaction of trans-methylcinnamate ester. The formation of azido-alcohol was proved by the presence of a C-N band in DRIFT spectra (2040 cm-1) and GC chromatograph (cf. Figures 10S and 11S from Supporting Informations). Although this work was successful in demonstrating that the trans-methylcinnamate may be epoxidated/epoxide ring-openned to the key intermediate methyl (2R,3S)-2,3-dihydroxy-3-phenylpropionate for the synthesis of Taxol, the mechanism of substrate activation and enantioselection by the surface-type dimer Cr(III)-salen species remains to be elucidated. The remarkable activity and enantioselectivity together with the different behavior of the reaction system in the presence of different additives (e.g., a donor ligand as Et3N) could mean both a different trajectory of approach of alkene and a different nature of reactive intermediates resulting in a different mech-



Figure 7. TG-DTA curves for the grafted dimer Cr(III)-salen complex.

SCHEME 6: Proposed Structures of the Grafted Dimer Cr(III)-Salen Complexes onto the Functionalized Silica

SCHEME 7: Epoxidation/Ring-Opening Reaction Sequences of trans-Methylcinnamate Ester

anism of the general reaction in comparison with the well-known monomeric and also dimeric mononuclear Cr(III)-salen complexes. Then, the fact that the ee’s in (2R,3S)-diol are quite high (81-100%) shows that there is a bimetallic cooperative effect

of the catalytic species, in agreement with the ring-opening mechanisms of epoxides. Therefore, it is clear that somehow the other two chromium cations from the trinuclear Cr species are involved in the general behavior of the catalytic species,

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TABLE 2: Activity and Enantioselectivity of the Grafted Chiral Dimer Cr(III)-Salen Complex in the Epoxidation/ Epoxide Ring Opening of trans-Methylcinnamate Estera

entry

reaction conditions

1b 2 3c 4c 5 6b 7b 8 9b 10d 11e 12f

24 h, 70 °C 24 h, 70 °C 24 h, 70 °C 24 h, 50 °C 24 h, 50 °C 24 h, 70 °C 12 h, rt 48 h, rt 48 h, rt 48 h, rt 24 h, 70 °C 24 h, 70 °C

selectivity to ((R,S) + (S,R)) ee, % conversion, % diols, % (configuration) 26 50 60 17 16 5 6 3 6 1 4 57

89 68 52 77 77 92 100 78 100 100 0 97

14 (2S,3R) 81 (2R,3S) 84 (2R,3S) 83 (2R,3S) 66 (2R,3S) 22 (2S,3R) >99 (2S,3R) >99 (2R,3S) >99 (2S,3R) >99 (2R,3S) n.d. n.d.

a

Reaction conditions: co-oxidant H2O2, catalyst heterogeneous dimer Cr(III)-salen complex (2%). Solvent: EtOH:H2O ) 5:1 (vol:vol). b Et3N (0.1 g). c Solvent: EtOH. d Co-oxidant NMO. e NaN3 (0.07 g); co-oxidant H2O2. Solvent: EtOH:H2O ) 5:1 (vol:vol). f NaN3 (0.07 g); co-oxidant H2O2. Solvent: MeCN:H2O ) 5:1 (vol:vol).(n.d. ) nondetermined).

but it has to be elucidated how. To our best knowledge, this is the first example when such additives gave an inversion in the enantioselection during the epoxidation/ring opening of epoxides reaction sequence. Moreover, the leaching experiments show that the prepared catalysts are robust materials, no active complex species being leached in the solvent during the reaction. IV. Conclusion In conclusion, we have shown a new concept of a highly active and enantioselective dimer Cr(III)-salen complex prepared with a simple method and from cheaper chromium salts. The applied characterization methods have shown that each dimer-salen was complexed with one molecule of trinuclear Cr(III) acetate. The covalent heterogenization of this complex on the functionalized silica was successful. These heterogenized complexes are highly active and enantioselective for the transformation of trans-methylcinnamate to methyl (2R,3S)-2,3dihydroxy-3-phenylpropionate through an epoxidation/epoxide ring-opening consecutive sequence reaction. The presence of a donor ligand such as Et3N has an unexpected effect upon the enantioselection: the configuration inversion from methyl (2R,3S)2,3-dihydroxy-3-phenylpropionate to methyl (2S,3R)-2,3-dihydroxy-3-phenylpropionate. The presence of NaN3 leads to the formation of azido-alcohol product, showing once again that the reaction took place through epoxidation/ring opening of epoxide with the present nucleophile (e.g., H2O or NaN3) in the reaction system. The mechanism of substrate activation and enantioselectivity by the surface-type dimer Cr(III)-salen species as well as the role of the other two dangling uncomplexed chromium cations from the trinucler Cr(III) acetate remain to be elucidated. Acknowledgment. This work was supported by CNCSIS (PNCDI II 40/2007 and PNII TE 91/2010 projects). Supporting Information Available: Figures 1S-11S. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Unusual Behavior of a Novel Heterogeneous Chiral Dimer Cr(III

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