Regiospecific Epoxidation of Carvone α,β-Unsaturated Ketone Group

Ind. Eng. Chem. Res. 2009, 48, 10217–10221

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Regiospecific Epoxidation of Carvone r,β-Unsaturated Ketone Group with a Basic Resin Marı´a A. Uguina,* Jose´ A. Delgado, and Jose´ Carretero Department of Chemical Engineering, Faculty of Chemistry, Complutense UniVersity of Madrid, 28040 Madrid, Spain

A basic resin in OH- form (Amberlite IRA-900) has been used to catalyze the regiospecific epoxidation of carvone with H2O2 to obtain 5,6-epoxycarvone, showing high activity and selectivity (>99%). The effects of solvent, temperature, stirring rate, catalyst loading, oxidant/substrate ratio, and oxidant dosage have been analyzed. A second-order kinetic model has been proposed to describe the effect of substrate concentration, catalyst loading, and temperature on reaction rate, and the apparent activation energy of the reaction has been estimated. The catalyst has been tested in five consecutive runs and it has shown to be completely reusable after regeneration of its hydroxyl groups before each use. 1. Introduction Epoxidation of electron-deficient alkenes such as R,βunsaturated ketones is of great interest because R,β-epoxy ketones are important building blocks in organic synthesis for obtaining several functional groups, including (i) β-hydroxy ketones and 1,3-diols groups, which are present in several prostaglandins,1 and (ii) R-hydroxy ketone groups, which are found in some anthracycline antibiotics used in cancer chemotherapy (daunomycin and adriamycin) and also in corticosteroid antiinflamatory drugs.2,3 The epoxidation of these compounds is usually carried out with hydrogen peroxide under strongly alkaline conditions using either bases or salts such as NaOH and KOH or Na2CO3 and K2CO3. However, it is undesirable to use soluble bases, because they are hazardous and lead to a large waste production. The replacement of liquid bases by solid basic catalysts has the advantage of decreasing corrosion and environmental problems, allowing easier separation and recovery of the catalyst. Several heterogeneous basic catalysts have been proposed for the epoxidation of electron-deficient alkenes such as R,βunsaturated ketones, where hydrotalcite is the most studied one.4-7 These materials become active for this system when they are heated at about 673 K to obtain dehydrated and dehydroxylated Mg-Al mixed oxides.8 Reactions are typically carried out in methanol with aqueous hydrogen peroxide,9 in either the presence or absence of nitriles.10 The activity of these catalysts can be improved using a liquid biphasic system formed by an aqueous phase, where the oxidant as well as the catalyst are present, and an organic phase containing the substrate and the reaction products. This system requires a cationic surfactant (DTMA ) n-dodecyltrimethylammonium bromide), which acts as a phase-transfer reagent and promotes the epoxidation.11,12 Catalyst activity can also be increased with the incorporation of tert-butoxide anions in the hydrotalcite interlayers by anion exchange, providing strong basic sites without calcination.13 More recently, a hydroxyapatite [Ca10(PO4)6(OH)2] has been used, after its calcination, in the epoxidation of R,β-unsaturated ketones with hydrogen peroxide as oxidant in different reaction conditions.14 * To whom correspondence should be addressed. Tel: +34 913944113. Fax: +34 913944114. E-mail: [email protected].

Potasium fluoride adsorbed on alumina (KF-Al2O3) has been used as a basic catalyst for the epoxidation of enones, with t-BuOOH as oxidant and acetonitrile-dichloroethane as solvent.15,16 Different anion exchange resins in the hydroxide form, with strong basic character, have been studied as catalysts in several organic reactions, including aldol condensation, Michael reaction, Knoevenagel condensation, or Schiff reaction.17 However, the use of this kind of catalyst in the epoxidation of electrondeficient alkenes has not been reported yet. The employment of a solid basic catalyst offers the advantage that the separation of the catalyst from the reaction mixture is very easy. In this work, the epoxidation of carvone R,β-unsaturated ketone group with hydrogen peroxide using a basic anion exchange resin (Amberlite IRA-900 in OH- form) is studied. The catalyst reuse has also been addressed. 2. Experimental Section Amberlite IRA-900 (a cross-linked styrene-divinylbenzenebased resin in Cl- form, supplied by Fluka) was converted into OH- form putting it in contact with a NaOH solution (0.5 M NaOH). Then, 0.5 L of the solution was passed through a column with 40 g of resin for 2-3 h. Deionized water was passed through the column until the effluent liquid was neutral. The whole process was repeated once, and afterward, the resin was dried at room temperature. In the reuse studies, the resin was filtered once the epoxidation experiment was accomplished and put in contact with 0.5 M NaOH with the liquid/resin weight ratio indicated previously for 2 h at room temperature. After that, it was washed with distilled water up to neutrality and filtered again. This process was repeated once. The concentration of OH- groups within the resin was determined by soaking a known resin volume in a known volume of dilute HCl, followed by the collection and titration of the liquid with a NaOH standard solution, using phenolphthalein as indicator.18 In these experiments, 1 mL of the resin was soaked in 10 mL of 0.1 N HCl. Catalyst textural properties were determined by nitrogen adsorption analysis at 77 K (Micromeritics ASAP 2010) and mercury porosimetry (Thermo Finnigan Pascal 140). The results are summarized in Table 1. In a typical experiment, 0.316 g of the basic resin was added to a mixture of 7 mmol of carvone (carvone/OH- molar ratio

10.1021/ie9010723 CCC: $40.75  2009 American Chemical Society Published on Web 10/30/2009

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Table 1. Textural Properties of Amberlite IRA-900 average particle radius (mm) particle density (kg · m-3particle) particle porosity (m3void · m-3particle) BET surface area (m2 · g-1) average pore radius (nm)

0.23 855 0.292 22 35

) 12.5), 28 mmol of aqueous hydrogen peroxide (33% w/w), and 24 mL of methanol. The epoxidation reaction was carried out in an orbitally stirred Teflon reactor at 50 °C with a stirring rate of 500 rpm. The reaction time was 2 h from the moment at which the catalyst was added to the mixture. The evolution of substrate and reaction products concentration was monitored by taking samples at different times and analyzing them by GC-MS (Agilent Technologies 6890N GC equipped with a J&W DB-23 capillary column and 5973N mass selective detector). Hydrogen peroxide concentration was determined by mixing a sample (previously diluted with water) with TiOSO4 and measuring the resulting yellow peroxo complex concentration by UV-vis spectrophotometry with several modifications with respect to the method reported elsewhere.19 An aliquot of the reaction mixture (0.18 mL) was diluted with deionized water until 10 mL. Then, an aliquot of this mixture (0.25 mL) was added to 0.18 mL of Ti complex solution [7.3 g of TiOSO4 15% in diluted H2SO4 + 32 g of H2SO4 98% + 4 g of (NH4)2SO4 + 75 mL of deionized H2O]. After that, it was diluted with deionized water until 10 mL. The absorption of this yellowcolored solution was measured in a UV-vis spectrophotometer (Shimazdu UV-2401) at 410 nm. Finally, the hydrogen peroxide concentration was determined using a calibration curve obtained with aqueous H2O2 standards. Several parameters were calculated to evaluate the catalyst performance, defined as follows: conversion (%) ) 100 × [1 - (remaining diolefin)/(initial diolefin)]; selectivity (%) ) 100 × (desired product)/∑ products; H2O2 conversion (%) ) 100 × [1 - (remaining peroxide)/(initial peroxide)]; H2O2 efficiency (%) ) 100 × (mmol of epoxides)/ (mmol of H2O2 consumed). 3. Results and Discussion 3.1. Epoxidation of Carvone with NaOH. Epoxidation of carvone with NaOH has been studied in order to compare the activity of homogeneous and heterogeneous hydroxyl anions (fixed to the resin surface). Figure 1 shows the influence of carvone/OH- ratio on the evolution of carvone conversion. All the experiments showed high regioespecificity, giving 5,6epoxycarvone as the main product (selectivity >99%), with a high peroxide efficiency (above 90%). It is observed that the initial reaction rate increases as this ratio decreases. These results are consistent with the mechanism of epoxidation of R,βunsaturated ketones with alkaline hydrogen peroxide,20 where hydroxyl anions are the active species (Figure 2). In this mechanism, one hydroxyl anion reacts with the hydrogen peroxide to form the perhydroxyl anion, and these species attack the substrate to form the R,β-epoxy ketone, regenerating the catalyst. However, the final conversion is less than 100% in all cases, which indicates that the active species are consumed as the reaction advances. This is probably due to secondary reactions between the perhydroxyl anion and the epoxide or between the perhydroxyl and hydroxyl anions and the substrate, as was reported in the epoxidation of phenyl-3-buten-2-one with alkaline hydrogen peroxide.21 Small amounts of byproducts and trace impurities were detected by GC-MS, but they could not be identified. The pH of the reaction mixture was measured before and after reaction, and the results are given in Table 2.

Figure 1. Epoxidation of carvone with different carvone/NaOH molar ratios: (9) 25, (0) 50, (•) 75, (O) 100.

Figure 2. Reaction mechanism proposed for the epoxidation of carvone R,β-unsaturated ketone group. Table 2. Initial and Final pH Values of the Reaction Mixture pH -

carvone/OH

initial

final

25 50 75 100

9.4 9.3 9.1 8.9

7.4 7.1 7.1 7.2

Table 3. Effect of Solvent on Carvone Epoxidation over Amberlite IRA-900 (carvone/OH- molar ratio ) 12.5) product selectivity (%) solvent

conv (%)

methanol ethanol propanol ethyl acetate acetonitrile tetrahydrofurane

31.5 19.2 10.9 1.5 4.6 0.0

5,6-epox >99 >99 >99 57.6 32.2

8,9-epox

39.7 65.7

others

2.7 2.1

H2O2 (%) conv

effic

23 18 17 12 20 7

33 26 13 3 6 0

It is observed that the final pH is practically neutral, indicating that the reaction stops when all the hydroxyl groups have been consumed. An additional experiment was carried out in the same reaction conditions except for a higher OH- concentration (carvone/OH- molar ratio ) 5). Complete carvone conversion and high selectivity to 5,6-epoxycarvone (>99%) were obtained, with a slightly higher content of byproducts than in the rest of the experiments. The final pH was 8.6 in this case, indicating that the neutralization of hydroxyl groups was not complete. In this case, practically all the substrate is converted into the epoxide. 3.2. Effect of Solvent. The effect of different solvents on the epoxidation of carvone with Amberlite IRA 900 (OH- form) has been studied, and the results are shown in Table 3. Alcoholic solvents, particularly methanol, lead to a much higher conversion than the nonalcoholic ones, with 100% selectivity to 5,6epoxycarvone. The highest H2O2 efficiency has been achieved with methanol, indicating that the oxidant decomposition is minimized in this case. Equations 1 and 2 show the reactions where H2O2 is consumed, according to the mechanism proposed

Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009 Table 4. Activity of Different Catalytic Systems in the Epoxidation of Carvone catalyst Amberlite IRA-900 hydrotalcite Al2O3-supported KF a

oxidant

time (h)

yield (%)a

ref

2 24 10

b

this work 6 12, 13

H2O2 H2O2 t-BuOOH

63.7 70 85

Yield of 5,6-epoxycarvone. b Reaction conditions as in Table 6.

stirring rate (rpm) conv (%) H2O2 conv (%) H2O2 effic (%)

100 20.6 23 23

200 24.4 25 24

300 27.8 25 28

400 29.6 24 31

500 31.5 24 33

by Boyer et al.22 for a similar reaction (chalcone epoxidation with H2O2 and Na2CO3). It may be argued that methanol inhibits the decomposition reaction of H2O2 (eq 2), because it solvates OOH- and H2O2 molecules stronger than the other solvents due to its higher dipole moment and smaller size. k1

H2O2+OH- y\z OOH- + H2O2 (fast) k1′

k2

OOH- + H2O2 98 O2 + H2O + OH- (slow)

3.5. Estimation of the Activation Energy of Carvone Epoxidation over Amberlite IRA-900. The activation energy of carvone epoxidation over Amberlite IRA-900 has been estimated from the experimental results obtained at different temperatures. The following second-order kinetic equation for heterogeneous systems has been assumed to describe reaction kinetics.23 nA2 1 dnA ) -kCA2 ) -k 2 W dt V

Table 5. Effect of Stirring Rate on Carvone Epoxidation over Amberlite IRA-900

(1)

(2)

The H2O2 efficiency is much lower with the heterogeneous hydroxyl anions than with the homogeneous ones (30% vs 90%). From this, it is deduced that the internal diffusion resistance in the resin particles is quite important. Thus, the diffusion of carvone is more hindered than that of H2O2 due to its larger size, so that the ratio between the rates of oxidant decomposition and substrate epoxidation is increased. The catalytic activity of the resin in the epoxidation of carvone has been compared with other catalytic systems (Table 4), where it can be observed that the basic resin can give a high yield (63.7%) at a much shorter time. 3.3. Effect of Stirring Rate. The effect of stirring rate has been studied to check the importance of external mass transfer resistance on carvone epoxidation with Amberlite IRA-900 (Table 5). This variable was changed between 100 and 500 rpm. For higher stirring rates, breakage of the resin pearls was noticeable. It is observed that the substrate conversion and H2O2 efficiency continuously increase with stirring rate, indicating that external mass transfer resistance affects the overall reaction rate. 3.4. Effect of Temperature. Figure 3a shows the effect of temperature on the carvone epoxidation over Amberlite IRA900. The initial reaction rate increases with temperature, as expected, but conversion at 2 h only increases from 40 to 50 °C, remaining constant from 50 to 60 °C. These results suggest that the activation energy of carvone epoxidation is lower than that of hydroxyl neutralization. If temperature is increased, the rate of neutralization increases more than the rate of epoxidation, so that the active species are consumed sooner. Temperature had no influence on selectivity. Nevertheless, hydrogen peroxide efficiency is reduced as temperature is increased, due to its higher decomposition into water and oxygen (Figure 3b). A temperature of 50 °C was selected for performing the rest of the experiments. Both conversion and hydrogen peroxide efficiency are high at this temperature, and it is below the limit indicated by the resin manufacturer (60 °C).

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(3)

where nA is the moles of substrate, W is the catalyst weight, V is the liquid volume, k is the kinetic rate constant, CA is the substrate concentration, and t is time. Integrating eq 3, the function nA(t) is obtained 1 1 kW ) 2t nA nAo V

(4)

where nAo is the initial value of nA. Experimental data at 40 and 50 °C could be fitted with eq 4 quite well, as shown in Figure 4. Thus, the proposed model is valid for describing reaction kinetics in these conditions. At 60 °C, the experimental points at times longer than 0.5 h were not considered in the estimation of the kinetic constant, because the deviation between the experimental and theoretical data was too large. In this period, the decrease in reaction rate comes from the consumption of hydroxyl groups, which is not taken into account in the model. Kinetic rate constants derived from the slopes of the straight lines in Figure 4 are 2.41 × 10-8, 3.67 × 10-8, and 6.10 × 10-8 m6 · kg-1 · s-1 · mol-1 at 40, 50, and 60 °C, respectively. These data were fitted adequately with the Arrhenius equation [k ) A exp(-EA/RT)], resulting in an apparent activation energy (EA) of 40.2 kJ · mol-1 and a pre-exponential factor (A) of 0.123 m6 · kg-1 · s-1 · mol-1. It must be indicated that the estimated value of the activation energy includes the effect of external mass transfer resistance, because the stirring rate could not be increased above 500 rpm, as commented in section 3.3. 3.6. Effect of Catalyst Loading. Table 6 shows the effect of increasing catalyst loading (measured as carvone/OH- molar ratio) on carvone epoxidation. It is observed that conversion increases with catalyst loading, while hydrogen peroxide efficiency remains constant. It is important to emphasize that the activity of heterogeneous hydroxyl anions is considerably lower than that of homogeneous ones (9.9 vs 96.5%, with carvone/OH- molar ratio ) 25) due to the importance of internal diffusion resistance in resin particles. 3.7. Effect of Oxidant/Substrate Molar Ratio. Figure 5 shows the effect of the oxidant/substrate molar ratio for two different values of catalyst loading. In both cases, conversion increases with the concentration of oxidant up to a certain limit (conversion about 31 and 63%, with carvone/OH- ) 12.5 and 6.2, respectively). This result can be explained by considering the mechanism proposed by Boyer et al.22 (eqs 1 and 2). Assuming a pseudo-steady-state concentration of active species (perhydroxyl anions), this concentration can be calculated as [OOH-] )

k1[H2O2][OH-] k1′[H2O] + k2[H2O2]

(5)

At low concentrations of H2O2, the concentration of active species and H2O2 are proportional, because the term k2[H2O2]

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Figure 3. Effect of temperature on carvone epoxidation over Amberlite IRA-900.

Figure 4. Plot of 1/nA vs time for carvone epoxidation over Amberlite IRA900 at different temperatures: (0) 60 °C, (9) 50 °C, (O) 40 °C. Table 6. Effect of Catalyst Loading on Carvone Epoxidation over Amberlite IRA-900 carvone/OHconv (%) H2O2 conv (%) H2O2 effic (%)

25 9.9 8 31

12.5 31.5 21 33

8.3 50.7 36 35

6.2 63.7 50 32

in the denominator is small. At higher H2O2 concentrations, this term becomes dominant in the denominator. Thus, the concentration of perhydroxyl anions is no longer dependent on the oxidant concentration, and a plateau is observed. In these conditions, an oxidant excess does not increase the final substrate conversion. 3.8. Effect of Oxidant Dosage. In an additional experiment (with the conditions of a typical experiment), one-fourth of the total oxidant was added every 30 min (reaction time ) 2 h), and the results have been compared with those obtained when all the oxidant was added at the beginning of the experiment. Both experiments showed the same substrate conversion (about 30-31%) and 100% selectivity to 5,6-monoepoxide, while hydrogen peroxide efficiency was increased by 25% when the oxidant was added progressively, due to a lower H2O2 decomposition. These results are in agreement with the hydrogen peroxide decomposition mechanism represented by eqs 1 and 2, in which the rate of decomposition is proportional to H2O2 concentration. 3.9. Heterogeneous Test. Another experiment was performed in order to confirm that the resin acts as a truly heterogeneous catalyst, removing the catalyst by filtration at an intermediate point of the reaction (30 min, total reaction time ) 2 h). The mixture without catalyst was put in the same reaction conditions (temperature and stirring rate), and it was observed that both substrate conversion and H2O2 consumption stopped. From these results, it is deduced that heterogeneous

Figure 5. Effect of H2O2/carvone molar ratio on the catalyst activity on carvone epoxidation with different catalyst loadings (carvone/OH- molar ratio). Table 7. Hydroxyl Group Determination in Successive Runs resin

fresh

first use

second use

conversion (%) OH- groups (mequiv · g-1)

31.5 1.78

5.5 1.23

0.84

epoxidation occurs on the resin surface, and that there is no leaching of the hydroxyl anions into the reaction mixture. 3.10. Reuse of the Catalyst. In a first test, the used resin was washed with methanol and dried at vacuum and room temperature for 10 min. Then, it was reused in the same reaction conditions, showing a notable decrease in activity while the selectivity to carvone monoepoxide was kept constant (Table 7). This behavior is attributed to the loss of hydroxyl groups on the resin surface by secondary reactions, as previously commented in section 3.1. The reduction in the catalyst conversion (82%) was higher than the loss of hydroxyl anions (31%), suggesting that not all the hydroxyl anions on the resin surface are accessible to the substrate because of diffusion limitations. In these experiments, thermal degradation of the resin is unlikely because the reuse experiments were carried out at 50 °C and the maximun operating temperature, according to the resin manufacturer, is 60 °C. Catalyst deactivation of Amberlite IRA-900 resin in the hydroxide form has been observed elsewhere, in the aldol condensation of acetone.18 It was attributed to the accumulation of high molecular weight products within the catalyst pores or to the neutralization of catalytic sites by trace acid impurities. In the subsequent tests, regeneration of the used resin was carried out by anion exchange with a NaOH solution (see Experimental Section). This method was employed in five

Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009

consecutive runs, and the conversion reached in each run was rather similar (32-37%), changing randomly with respect to the average value. These results show that the catalyst activity is recovered with this regeneration method and that the decrease in activity is due to the loss of hydroxyl groups on the resin surface. 4. Conclusions Amberlite IRA-900 in the hydroxide form is an active heterogeneous catalyst for the regiospecific epoxidation of carvone R,β-unsaturated ketone group, resulting in good conversion and high selectivity to 5,6-epoxycarvone (>99%). Catalyst deactivation has been observed, which is attributed to the loss of hydroxyl anions on the resin surface. The reaction rate is controlled by external and internal mass transfer resistance in the conditions studied in this work. This rate increases with temperature as expected, although the rate of active species neutralization also increases. Assuming second-order reaction kinetics, an apparent activation energy of 40.2 kJ · mol-1 has been obtained from the experimental results at different temperatures. Oxidant loading has a positive effect on substrate conversion up to a maximum value, above which it has no effect. This result can be explained by the mechanism proposed in the literature for the formation of active species and oxidant decomposition in the epoxidation of chalcone with Na2CO3.22 Also, the oxidant efficiency is notably increased if it is added progressively to the reaction medium rather than instantaneously, which is consistent with this mechanism. The basic resin is reusable if it is regenerated by anion exchange with a diluted NaOH solution. Acknowledgment Financial support by Ministerio de Educacio´n y Ciencia of Spain (Project PPQ2002-00570) is gratefully acknowledged. Nomenclature A ) pre-exponential factor (m6 · kg-1 · s-1 · mol-1) CA ) substrate concentration (mol · m3) EA ) activation energy (kJ · mol-1) k ) kinetic constant (m6 · kg-1 · s-1 · mol-1) k1, k′1, k2 ) kinetic constants defined in eqs 1 and 2 n ) reaction order nA ) moles of substrate A nAo ) initial moles of substrate A R ) ideal gas law constant (J · K-1 · mol-1) T ) temperature (K) V ) liquid volume (m3) W ) catalyst weight (kg)

Literature Cited (1) Samuelsson, B. From studies of biochemical mechanism to novel biological mediators: Prostaglandin endoperoxides, thromboxanes and leukotrienes. Angew. Chem., Int. Ed. Engl. 1983, 22, 805.

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ReceiVed for reView July 3, 2009 ReVised manuscript receiVed October 5, 2009 Accepted October 17, 2009 IE9010723