Chirally pure 3-phenylglycidol is a key intermediate in the manufacture of a range of drugs. In order to scale up the reaction in an ionic liquid (IL)/supercritical carbon dioxide (scCO2) miscibility switch system, a study was started to select the optimum IL in this case. The vanadium-catalyzed epoxidation reaction of cinnamyl alcohol to 3-phenylglycidol in the presence of imidazolium-type liquids and tert-butyl hydroperoxide (TBHP) as the oxidant was studied experimentally. Three ILs, i.e., (i) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), (ii) 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][TfO]), and (iii) 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]), ranging from hydrophobic to hydrophilic, were selected as suitable candidates in terms of viscosity and stability, and were studied as solvents in the epoxidation reaction. In accordance with previously studied catalytic oxidations in ILs, the most hydrophobic ionic liquid [bmim][Tf2N] results in the best yield and selectivity. Activities and product profiles are similar to those obtained in toluene, the benchmark solvent for these reactions. The maximum selectivity towards 3-phenylglycidol is 65%, with cinnamaldehyde and benzaldehyde as side products. The optimum reaction conditions require 1.5 equivalent of TBHP as the oxidant and 3 mol% of vanadyl acetylacetonate as the catalyst at 25°C. Therefore, [bmim][Tf2N], is a potential solvent for epoxidation reactions under miscibility switch conditions.
Epoxides are key intermediates in the manufacture of functionalized fine chemicals and pharmaceutics. Due to their high susceptibility for nucleophilic attack, and their asymmetric nature, they offer an easy route towards the synthesis of a wide range of important chiral products [1,2]. Recently, for large-scale production of epoxides, kinetic resolution of epoxides using chiral cobalt complexes has proven to be a valuable technology . However, from a viewpoint of atom efficiency, the oxidation of a prochiral alkene into a chiral epoxide in one step is preferred. In this way, 100% yield towards the desired product can be obtained, and potentially two chiral centers are created with one catalyst. For asymmetric epoxidation, a large catalytic toolbox has been developed to perform these reactions on a small scale. This toolbox is dominated by chiral transition metals . In addition, organocatalytic methods, and notably the Shi epoxidation employing oxone as the oxidant and a fructose-derived catalyst, have been shown to be successful . For metal catalysis, a range of oxidants have been probed, but from a viewpoint of atom efficiency, the use of hydrogen peroxide (H2O2) or alkyl hydroperoxide (R-OOH) is preferred [6,7]. Notably, late transition metals, i.e., titanium (Ti), vanadium (V), molybdenum (Mo) and tungsten (W), which can all effectively activate peroxides via the so-called peroxometal pathway leading to MOOR type oxidants, are the candidates of choice .
In the present study, we were interested in studying large-scale catalytic concepts for the production of 3-phenylglycidol from the corresponding allylic alcohol, cinnamyl alcohol. (2R,3R)- and (2S,3S)-phenylglycidol and substituted phenylglycidols, serve as chiral precursors for notable drugs such as reboxetine, tomoxetine, fluoxetine and the taxol side chain [9–]. Vanadium, as a metal, has a key advantage as an epoxidation catalyst for allylic alcohols, because of its high alcohol binding affinity [8, 12]. In 1977, Michaelson et al. demonstrated that a combination of vanadyl acetylacetonate [VO(acac)2] and a chiral hydroxamic acid afforded optically active epoxides from the corresponding allylic alcohols and tert-butyl hydroperoxide (TBHP), with up to 50% enantiomeric excess (ee) . Then, a few years later, the well-known titanium-tartrate method was discovered by Katsuki and Sharpless, which turned out to be a versatile and reliable method for epoxidation both for allylic alcohols and for non-functionalized alkenes . However, in terms of catalyst loading and stability, the use of vanadium still offers many advantages  and over the last decade, major progress has been made in vanadium-catalyzed asymmetric epoxidation using novel chiral hydroxamic acids as ligands . Nowadays, two new protocols are available. Zhang et al. and Barlan et al. reported a C2-symmetric bishydroxamic acid, which, in combination with 1 mol% VO(O-iPr)3 and TBHP as the oxidant, leads to 97% ee for 3-phenylglycidol [16, ]. Malkov and coworkers developed amino-acid-derived hydroxamic acids as chiral ligands, which also showed considerable potential for the vanadium-catalyzed asymmetric epoxidation .
In order to advance the use of vanadium-catalyzed epoxidation technology towards up-scaling, we focused on a method to replace the solvent in these reactions, which is typically dichloromethane or toluene, with an ionic liquid (IL) process solvent [19–]. According to the guidelines of green chemistry , it is of the utmost importance to use less volatile, less flammable and less toxic reaction solvents in the course of chemical reactions and processes, to achieve new attractive industrial processes . The last decade of studies on ILs have taught us that ILs will have a potential as sustainable solvents, because of their low volatility, low flammability, high thermal stability and excellent solubilizing properties, but their benefit has to be clearly demonstrated for each individual case because of their cost, potential toxicity and difficulties in separation afterwards [24–]. In our case, an IL will be used as a process solvent, solubilizing the catalyst, and will be part of the process set-up. It will be applied in such a way that it will not leave the factory, nor end up in the product. The process is based on the miscibility switch phenomenon for carrying out a chemical reaction and separation in the presence of an IL and supercritical carbon dioxide (scCO2) that was proposed recently . When using an IL as a solvent for the reaction, in combination with scCO2 as a co-solvent, the reaction can be performed in a homogenous phase by selection of a suitable pressure, temperature and CO2 concentration. After completion of the reaction, changing the conditions such that CO2 acts as an anti-solvent will result in a multi-phase system. One of the phases is an IL, and one of the other phases is a CO2-rich phase, from which the product is recovered substantially free of IL, because ILs do not dissolve in CO2 .
The use of ILs in oxidation catalysis has been recently highlighted and many examples have been published [29–]. The first epoxidation reactions carried out using ILs as solvents were performed in 2000 . This study involved the epoxidation of alkenes and allylic alcohols to their corresponding epoxides using urea H2O2 as the oxidant, methyltrioxorhenium (MTO) as the catalyst and 1-ethyl-3-methylimidazolium tetrafluoroborate, [emim][BF4], as the IL. The experiment resulted in fairly high chemical yields for different olefin substrates and allowed efficient product recovery by extraction with an organic solvent. Later on, vanadium complexes were also used as epoxidation catalysts in ILs, resulting in good reaction yields and selectivities [33, ].
These latter studies motivated us to explore the vanadium-catalyzed epoxidation reaction of allylic alcohols in the presence of miscibility-switch compatible ILs as solvents. Therefore, in this study, the catalytic epoxidation of cinnamyl alcohol to 3-phenylglycidol in oxidatively stable imidazolium-type liquids was studied with the aim of applying this novel process concept. Once successful, this technology can be extended to a chiral one, by involving a chiral ligand.
Cinnamyl alcohol with a purity of 98%, TBHP solution of ~5 m in decane and trans-cinnamaldehyde with a purity of 98%, were purchased from Sigma-Aldrich, Zwijndrecht, Netherlands. (2S,3S)-(-)-3-Phenylglycidol (99%), benzaldehyde (>99.5%), 2-propanol (99%) and tetradecane (>99.5%, GC standard) were purchased from Fluka (supplier Sigma-Aldrich, Netherlands). H2O2 was purchased as a 30% weight solution in water from Merck (Amsterdam, Netherlands). The quenching agent triphenylphosphine (99%) was used as received from Aldrich. The ILs, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) (>98%) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][TfO]) (>98%) were purchased from Aldrich, and 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) (>95.0%) was purchased from Fluka. They were dried in a vacuum desiccator in the presence of phosphorus pentoxide before use. The first catalyst studied, VO(acac)2, was purchased from Aldrich with a purity of 98%. The second catalyst studied, vanadyl (Salophen) trifluoromethanesulfonate (VO(Salophen)CF3SO3), was kindly provided by Prof. Valeria Conte from the Department of Science and Chemical Technology, University of Rome Tor Vergata .
The catalytic epoxidation experiments were carried out in a round bottom flask equipped with a magnetic stirrer, in a thermostatic water bath. The standard reaction procedure consisted of dissolving 4 mmol of cinnamyl alcohol and a variable amount (1–5 mol%) of catalyst in 3 ml of IL. Once the solution had reached the desired temperature, the oxidizing agent was added to the solution to start the reaction. The type and amount of catalyst and oxidizing agent were subject to change in different sets of reactions to find the optimum conditions.
Samples were taken after 10, 20 and 30 min, and thereafter every hour (up to 5 h) in order to determine the reaction time. Triphenylphosphine was used to quench the remaining TBHP from the solution. Quantitative analyses to calculate the chemical yields and selectivities were carried out using gas chromatography (GC) from CHROMPACK equipped with a CP-Sil 5 CB column and using He as the carrier gas, with tetradecane as the internal standard. Molar responses are calibrated against commercial samples.
3 Results and discussion
3.1 Effect of solvent
The chiral vanadium-catalyzed epoxidation reactions of allylic alcohols have been attracting a lot of attention after the outstanding publication by Yamamoto et al. in 1999 . The main advantages of using vanadium catalysts are their stability against moisture and the lower required catalyst loads compared to other catalysts . With the aim of developing a green process approach for vanadium-catalyzed epoxidation, as described above, a suitable IL had to be found which is able to (i) dissolve and retain vanadium in the IL process solvent, (ii) be stable under oxidative conditions with TBHP and (iii) be compatible in the miscibility-switch region. Imidazolium-based ILs are interesting, due to the fact that they display low melting points , moreover, shorter alkyl chain length cations result in a lower viscosity and lower toxicity compared to longer alkyl chain cations . Previously, it was found that the solubility of CO2 in an IL is mainly caused by the strong interactions between the CO2 molecules and the anions of ILs [38–].
Based on these criteria, three different anions for bmim-based ILs, i.e., (i) [bmim][Tf2N] 1, (ii) [bmim][TfO] 2 and (iii) [bmim][NO3] 3, were applied as solvents during the experiments, varying from hydrophobic 1, to hydrophilic 2, to more hydrophilic 3. In our initial studies, we performed the epoxidation reaction of cinnamyl alcohol at room temperature and with 3 mol% VO(acac)2 as the catalyst. Figures 1 and 2 show the effect of the type of IL on the conversion and chemical yields of the epoxidation reaction of cinnamyl alcohol vs. time, respectively. The reaction stalls after 1 h and is finished after 2 h in all cases. For solvents 1 and 3, the maximum conversion is close to 90%, while in the case of solvent 2, only 62% conversion is reached. Apart from the differences in conversion rate, the solvents have a large impact on the selectivity of the reaction (Tables 1 and 2). The hydrophobic 1 gives the highest selectivity towards the desired product 3-phenylglycidol 65%. Solvents 2 and 3 lead to 42% and 8% selectivity, respectively. The two competing pathways are the oxidation of the alcoholic group leading to cinnamaldehyde, and the oxidative cleavage of the double bond, leading to benzaldehyde. These two reactions are commonly catalyzed by homolytic, radical-type reaction pathways .
1Selectivity is defined as the molar ratio of desired product to the total amount of products.
|Solvent||Yield (%)||Selectivity to 3-phenylglycidol (%)|
These results can be explained by the common notice that polar, non-coordinating solvents endow the best properties for electrophilic epoxidations . The proposed mechanism for vanadium-catalyzed epoxidation of cinnamyl alcohol is shown in Figure 3. Peroxovanadates can also endow competing homolytic radical-type pathways, depending on the nature of the substrate and the solvent. In this case, hydrophilic solvents apparently favor these pathways.
The epoxidation reaction of cinnamyl alcohol was also performed in toluene as a benchmark, which is the conventional organic solvent for this reaction (Table 2). When using toluene as a solvent, the chemical yield and selectivity to 3-phenylglycidol are 63% and 67%, respectively. These results are comparable to those in [bmim][Tf2N], indicating that ILs can be suitable substitutes for organic solvents in epoxidation reactions. However, comparing the yields for the IL 1 and toluene (Figure 4), we noticed that the 3-phenylglycidol tended to decompose in toluene, while it was stable in the IL 1. This means that the rate of product loss due to decomposition is lower in 1 compared to toluene. Therefore, the stability of the product in 1 makes this solvent an attractive alternative to toluene.
In an attempt to improve yield and selectivity, the temperature was increased, using 1 as the solvent (Table 3). From Table 3, it can be observed that the rate of reaction increases with increasing temperature. However, an almost linear decrease of selectivity from 65% at room temperature, to 39% at 55°C was observed. Therefore, in order to minimize the decomposition and to increase the stability of 3-phenylglycidol, it is advisable to carry out the epoxidation reaction at a moderate temperature of 25°C.
|Run||Temperature(C)||Total conversion(%)||Selectivity to3-phenylglycidol (%)|
3.2 Effect of catalyst
Loadings from 1 to 5 mol% catalyst at 25°C and 1.5 equivalent of TBHP were studied (Figure 5). The results show that 3 mol% of VO(acac)2 is an efficient amount of catalyst. A further increase of the load of VO(acac)2 does not improve the chemical yield significantly, while at lower loadings, a decrease of yield to 40% is observed. This seems to be due to the lower activity and thus stability of the catalysts. Apart from VO(acac)2, the use of VO(Salophen)(TfO) as a stable vanadium catalyst  was studied for the epoxidation of cinnamyl alcohol (Figure 6 and Table 4). The yield at 1 mol% catalyst in both cases is almost similar, i.e., around 39–40%. However, when comparing VO(Salophen)(TfO) and VO(acac)2 at higher loadings, in the latter case, the yield (from 40 to 61%) goes up, while for VO(Salophen) (TfO), no increase in yield was observed. The reason for this is not clear yet and requires further study.
3.3 Effect of oxidant
Instead of using TBHP, hydrogen peroxide was also tested as an oxidant. When using 1.5 equiv. of H2O2 (30% aqueous solution) as the oxidant, only a 20% yield of 3-phenylglycidol was observed, with a selectivity of 24%. This is in accordance with previous studies on the use of HOOH as an oxidant in vanadium-catalyzed epoxidation reactions . TBHP has the advantage that it can be applied in non-aqueous solutions, thereby favoring electrophilic oxidation. Different ratios of TBHP to alcohol were applied ranging from 1 to 2.5 (Table 5). An increase of the TBHP concentration from 1 to 1.5 equiv. resulted in an increase in both the chemical yield and selectivity of 3-phenylglycidol production. However, increasing the TBHP concentration to higher amounts is not effective and slightly decreases both the chemical yield and selectivity to 3-phenylglycidol. Probably, the product starts to decompose at higher amounts of the oxidant. In an attempt to decrease the product decomposition, the oxidant was added to the reaction solution slowly. However, no difference in overall yield and selectivity was observed. Therefore, it is advisable to use 1.5 equiv. of TBHP as the oxidant relative to cinnamyl alcohol, in order to achieve optimum epoxide yield.
|Run||Catalyst||Catalyst load (mol%)||Yield (%)||Selectivity to 3-phenylglycidol (%)|
|Run||Oxidant||Molar ratio oxidant:reactant||Yield (%)||Selectivity to 3-phenylglycidol (%)|
In this contribution, a systematic investigation of the epoxidation reaction of cinnamyl alcohol to 3-phenylglycidol in different ILs has been carried out. The effects of three different ILs as solvents, i.e., (i) [bmim][Tf2N], (ii) [bmim] [TfO] and (iii) [bmim][NO3], on the yield and selectivity were determined. The results clearly indicate that the use of hydrophobic [bmim][Tf2N] is required to reach reasonable selectivities for 3-phenylglycidol. In that sense, [bmim][Tf2N] is the ideal substitute for toluene, the conventional organic solvent for this reaction, which shows identical product profiles, both in terms of selectivity as well as in reactivity. In contrast, the use of [bmim][NO3] as a hydrophilic solvent includes a homolytic reaction pathway and leads to mainly alcohol oxidation, with 78% selectivity to cinnamaldehyde as the product.
Optimization of the conditions revealed that product stability is leading in this. Therefore, low temperatures and low TBHP loadings are required. Vanadium is therefore a very active catalyst that leads to fast epoxidation. The optimum conditions for the epoxidation reaction of cinnamyl alcohol to 3-phenylglycidol were obtained using [bmim][Tf2N] as the solvent with 3 mol% VO(acac)2 as the catalyst and 1.5 equiv. of TBHP as the oxidant at 25°C.
Apart from VO(acac)2, VO(Salophen)(TfO) is an excellent source of vanadium, which is much more stable in solution. In this case 1 mol% of vanadium is sufficient to reach the maximum conversion and selectivity of 62% and 63%, respectively. VO(Salophen)(TfO) has been shown to be an excellent sulfide catalyst with tunable properties. Therefore, sulfoxide production under miscibility switch conditions can be considered as a possibility as well.
Financial support by Agenschap Nl, EOS project “Duurzame manier om reacties en scheidingen te combineren met behulp van ionische vloeistoffen en superkritisch kooldioxide” is gratefully acknowledged.
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