Article pubs.acs.org/IC
Enantioselective Manganese-Porphyrin-Catalyzed Epoxidation and C−H Hydroxylation with Hydrogen Peroxide in Water/Methanol Solutions Hassan Srour, Paul Le Maux, and Gerard Simonneaux* Institute of Sciences Chimiques of Rennes, Ingénierie Chimique et Molécules pour le vivant UMR 6226 CNRS, Campus de Beaulieu 35042 Rennes cedex, France ABSTRACT: The asymmetric epoxidation of alkene and hydroxylation of arylalkane derivatives by H2O2 to give optically active epoxides (enantiomeric excess (ee) up to 68%) and alcohols (ee up to 57%), respectively, were carried out in water/methanol solutions using chiral water-soluble manganese porphyrins as catalysts.
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INTRODUCTION The use of water instead of organic solvents in transition-metal homogeneous catalysis has received remarkable attention,1−3 because water is inexpensive, nontoxic, nonflammable, and environmentally sustainable and, sometimes it allows simple separation and reuse of the catalyst. Moreover, catalysis often permits the use of less-toxic reagents, as in the case of oxidation by hydrogen peroxide. Hydrogen peroxide (H2O2) is widely accepted as a green oxidant, because it is relatively nontoxic and breaks down in the environment into benign byproducts.4,5 Over the last three decades, there have been large advances in the development of catalytic methodology for epoxidation reactions.6 However, asymmetric epoxidations in aqueous solution with H2O2 are still rare,7 although a nice possibility has been recently reported using iron and a chiral nonporphyrin ligand.8 The main obstacle when using H2O2 is the high activity of transition-metal complexes in the catalase reaction and the homolytic cleavage of the peroxidic O−O bond with the formation of the hydroxyl radical.9 A few examples of the use of H2O2 can be found in the literature with manganese porphyrins as catalysts, giving low enantiomeric excess (ee) for epoxidation reaction in organic solvents or in a biphasic medium.10−12 However, to our knowledge, there are no examples of asymmetric hydroxylation by H2O2 in water catalyzed by Mn porphyrins. Our group11 reported the asymmetric oxidation of sulfides into sulfoxides using macroporous resins containing chiral metalloporphyrins, and, more recently, catalytic sulfoxidation reactions with H2O2 in water using a chiral iron porphyrin as a catalyst.12 We now want to describe here chiral epoxidation of alkenes in water/ methanol solutions using H2O2 as an oxidant catalyzed by new optically active water-soluble manganese porphyrins (Figure 1). Because high-valency manganese(IV or V)-oxo porphyrins are capable of activating C−H bonds of alkanes,13,14 we also have considered the possibility of catalytic asymmetric hydroxylation with chiral water-soluble manganese porphyrins, using H2O2 as an oxidant.
was the introduction of four sulfonate groups into an optically active porphyrin with the objective of preparing chiral watersoluble porphyrins. We choose a C2-symmetric group that contains two norbornane groups fused to the central benzene ring, which was previously reported by Halterman and Jan.15 Initially, we planned to introduce first the sulfonic acid in the porphyrin ring and then manganese insertion. The sulfonation of the Halterman porphyrin was performed as we previously reported.16 A 10-fold excess of MnBr2·4H2O was necessary to get the expected metalloporphyrin (Figure 1) in a reasonable yield (80%). The expected compound was characterized by UV−visible spectrum, and mass spectrum (see the Experimental Section). It is easily soluble in methanol and water. Catalytic Asymmetric Epoxidation. Because of the high solubility of the catalyst in methanol, the epoxidation of styrene was first examined in this solvent at room temperature and with imidazole as a co-catalyst. The rate of the epoxidation reaction was very low to give 6% conversion after 1 h and 31% conversion after 4 h (Table 1, entry 1). However, in the presence of 25% phosphate buffer (pH 7), we noticed a large acceleration, since the conversion increases to 68% after 1 h and a quasicompletion after 4 h (93%) (Table 1, entry 2). In a typical oxidation, 1 equiv of the Mn porphyrin catalyst 1, 24 equiv of imidazole, 120 equiv of H2O2, and 40 equiv of substrate were used under anaerobic conditions. The reaction is quasi-complete after 4 h at room temperature and the corresponding epoxide was obtained with good yield (93%) and 47% ee (Table 1, entry 2). This situation was amplified with a 50/50 ratio of methanol/water, giving 100% conversion after 1 h but with a small decrease of the ee (from 47% to 41%). Surprisingly, completion of the reaction in pure water (without methanol) requires 4 h under similar conditions (Table 1, entry 4). Probably, the weak solubility of styrene in water may explain this result. It should be noted that an increase of the epoxidation reaction was reported recently,17 because of the presence of water in saturated CH2Cl2 solution for similar
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RESULTS Preparation of Water-Soluble Chiral Manganese Porphyrins. The starting point of the work described here © 2012 American Chemical Society
Received: February 29, 2012 Published: May 8, 2012 5850
dx.doi.org/10.1021/ic300457z | Inorg. Chem. 2012, 51, 5850−5856
Inorganic Chemistry
Article
Figure 1. Structures of catalyst 1 and 2.
gave a low conversion (38%) with 43% ee. A similar value (52%) was reported by Halterman and co-workers19 in organic solvent (CH2Cl2) using NaOCl as an oxidant. pH-Dependent H2O2 Oxidation. To explore whether the pH affects the yield and the ee at different pH values, we chose styrene as a substrate because it is commonly applied for the catalytic activity of manganese porphyrins. The experiments for pH-dependent styrene oxidation were carried out in the presence of H2O2 at 25 °C with the ratio H2O2/styrene being 3 and the yields and ee values were observed after 4 h. The results are summarized in Table 3. We found that the reaction rate increases with increasing pH showing 100% conversion at pH 10.5 after 1 h (Table 3, entry 4). In contrast, at acidic pH (pH 4), we observed only 12% conversion after 1 h. It is worth noting that the substitution of imidazole by N-methyl imidazole as Mn ligand has a large negative effect, since only 5% conversion is observed after 1 h (Table 3, entry 5). Influence of an Excess of Oxidant. Since the oxide yields in the oxidation were high (95% after 4 h) with 5 equiv of H2O2, we have examined the effect of decreasing the ratio H2O2/styrene on chemical yield. As expected, Table 4 shows that a decrease of the oxidant/styrene ratio from 5 to 1.2 provides a decrease of the epoxidation yield, varying from 83% to 50% after 1 h. However, the yield (64%) is still acceptable after 4 h, according to the high reactivity of the system, in the presence of water. Influence of the Nature of Oxidant. The nature of the active oxygen complex, which is able to transfer its O atom to alkenes and hydrocarbons, although not completely established, is generally considered to be a high-valency Mn-oxo complex, at least formally, a MnV=O species, although a MnIII−OOH could be invoked.20 In order to characterize the oxidizing species, in the chiral Mn system, we have compared the yield and enantiomeric excess using different oxidants (see Table 5). Using an identical concentration of oxidant and styrene (Mn/imidazole/ oxidant/styrene =1/24/40/40), Table 5 shows that there is an increase of the yield when the oxidant is iodobenzene diacetate (from 49% to 75%). However, it is remarkable that, with three different oxidantsmeta-chloroperoxybenzoic acid, H2O2, and PhI(OAc)2the ee values are similar, according to a probable common optically active intermediate. With t-buOOH, only traces of styrene epoxide are observed, as previously reported with oxidation of styrene catalyzed by Mn porphyrins.21
Table 1. Effects of the Amount of H2O, Relative to MeOH on Mn HaltS−H2O2−Imidazole Systema MeOH/H2O
conversion (%)
enantiomeric excess, ee (%)
time (h)
1
1/0
2
3/1
3 4b
1/1 0/1
5b,c
0/1
6 31 68 93 100 7 100 46 100
47 47 47 47 41 37 35 36 36
1 4 1 4 1 1 4 1 4
entry
a
The reaction was conducted with syringe-pump addition of 3 equiv (relative to alkene) of H2O2 (30% in H2O, diluted 5 times in MeOH) and imidazole (20 equiv relative to Mn) at 25 °C to a solution of styrene/imidazole/Mn HaltS mixture (40:4:1) in 0.4 mL of MeOH/ buffer solution (pH 7). bThe reaction was conducted with syringepump addition of 3 equiv (relative to alkene) of H2O2 (30% in H2O, diluted 5 times in H2O and imidazole (20 equiv, relative to Mn) at 25 °C to a solution of styrene/imidazole/Mn HaltS mixture (40:4:1) in 0.4 mL of MeOH/buffer solution (pH 7). cThe reaction was conducted in buffer solution (pH 10.5).
systems with achiral metalloporphyrins. The key role of imidazole in metalloporphyrin-catalyzed oxygenations with H2O2, evidenced by Mansuy et al.18 in olefin epoxidation with iodosyl benzene, is confirmed herein, since only a weak conversion (4%) was detected in the absence of this ligand (see Table 2, entry 3). We also investigated the epoxidation of various substituted styrenes with the same catalyst (1). Results for the catalytic epoxidation of several alkenes are summarized in Table 2. As shown in Table 2, epoxide yields in the range of 45%−100% were obtained with enantiomeric excess (ee) as high as 68% for 1,2-dihydronaphthalene. Small amounts of aldehydes (
