Catalytic epoxidation of cyclooctene using molybdenum(VI) compounds and urea-hydrogen peroxide in the ionic liquid [bmim]PF6

Catalysis Communications 8 (2007) 987–990 www.elsevier.com/locate/catcom

Catalytic epoxidation of cyclooctene using molybdenum(VI) compounds and urea-hydrogen peroxide in the ionic liquid [bmim]PF6 Matthew Herbert, Agustı´n Galindo, Francisco Montilla

*

Departamento de Quı´mica Inorga´nica, Facultad de Quı´mica, Universidad de Sevilla, Aptdo 553, 41071 Sevilla, Spain Received 29 June 2006; received in revised form 5 October 2006; accepted 5 October 2006 Available online 17 October 2006

Abstract Commercially available molybdenum(VI) compounds are shown to be good catalysts in the epoxidation of cyclooctene with the ureahydrogen peroxide adduct (UHP) in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6. The cyclooctene oxide was isolated by simple extraction with organic solvent and the ionic liquid-catalyst mixture recycled and reused.  2006 Elsevier B.V. All rights reserved. Keywords: Epoxidation; Homogeneous catalysis; Molybdenum(VI); Ionic liquids

1. Introduction Epoxidation of alkenes is an important synthetic transformation, allowing the conversion of simple substrates into valuable precursors in the synthesis of fine chemicals [1]. Catalysis of these reactions by transition metal complexes is therefore an important area of research. Oxygen aside, hydrogen peroxide is the most economical and environmentally benign of the potential oxidants that might be used in epoxidations [2–5]. A number of transition metal complexes have been shown to be useful for the homogenous catalytic epoxidation of olefins. Rhenium catalysts give excellent results in the epoxidation of olefins, particularly methyltrioxorhenium(VII) (MTO) [6,7]. However the economic and environmental cost of using this rare metal makes cheaper, commoner catalysts desirable. Although organomolybdenum oxides make less effective catalysts in olefin epoxidations using hydrogen peroxide than organorhenium oxides [8,9], beginning with the early work of Mimoun and co-workers [10,11], at the present day significant contributions to literature knowledge regarding *

Corresponding author. Tel.: +34 954 557160; fax: +34 954 557153. E-mail address: [email protected] (F. Montilla).

1566-7367/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.10.008

molybdenum catalysed olefin epoxidations have been made by several research groups [12,13]. Oxo-molybdenum epoxidations in literature normally employ organic peroxides, such as tert-butyl hydroperoxide, as oxidant [14] with the use of hydrogen peroxide generally proving too difficult. Efficient molybdenum catalysed epoxidation using hydrogen peroxide as oxidant has been also recently described [15]. During the last few years interest in room temperature ionic liquids (RTIL’s) as alternatives to commonly used organic solvents has grown markedly. RTIL’s can solubilise many inorganic compounds, whilst being immiscible with many common extraction solvents meaning that a catalyst will often be immobilised in the ionic liquid whilst products are separated by extraction, allowing the catalyst to be recycled [16]. Use of RTIL’s as solvents in alkene epoxidations has been studied and several examples are known [17–23]. Several dioxo-compounds of molybdenum have been reported to homogeneously catalyse olefin epoxidation using tert-butyl hydroperoxide as oxidant [24,25] and, recently, a DFT mechanistic study of this reaction has appeared [26]. Continuing our group’s study of oxo-compounds [27,28], we began a study of alkene epoxidations using

988

M. Herbert et al. / Catalysis Communications 8 (2007) 987–990

urea-hydrogen peroxide adduct (UHP) as oxidant in order to carry out reactions without the presence of water and hence limit hydrolysis side-reactions [18,29]. In this communication we present our preliminary results in the epoxidation of cyclooctene using the ionic liquid (IL) 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6, as reaction solvent, urea-hydrogen peroxide adduct (UHP) as oxidant and several commercially available molybdenum(VI) compounds as catalysts. The catalyst-IL solution proved to be recyclable and, after extraction of the cyclooctene oxide, was reused with further catalytic cycles showing little loss of efficiency. As far as we know, no precedent exists for the epoxidation of olefins using the Mo(VI)-UHP oxidation system in [bmim]PF6.

2.4. Recovery of the [Mo]/[bmim]PF6 solution mixture

2. Experimental

After the extraction of the cyclooctene oxide, the ionic solvent-Mo catalyst mixture was dissolved in dichloromethane (10 mL) giving a cloudy suspension. The white solid was removed by filtration and the dichloromethane then evaporated under reduced pressure yielding the ionic liquid as a yellow solution containing the remaining molybdenum catalyst. The clean ionic liquid was recovered by removing remaining Mo-catalyst from [bmim]PF6 as follows. The ionic liquid-Mo catalyst mixture was dissolved in an equal volume of dichloromethane and this solution then washed twice with an equal volume of 1 M NaOH(aq) and twice with an equal volume of distilled water. The solution was dried with MgSO4, filtered and the dichloromethane evaporated under reduced pressure to yield clean, molybdenum free ionic liquid.

2.1. Materials

3. Results and discussion

All preparations and other operations were carried out under dry aerobic conditions. Solvents were dried, using standard procedures. Cyclooctene, UHP, MoO3 and ammonium molybdate were purchased from Aldrich and they were used as supplied. Compound Mo(O)(O2)2(bipy) was prepared as previously reported [30]. Infrared spectra were recorded on Perkin–Elmer Model 883 spectrophotometer. 1H and 13C NMR spectra were run on Bruker AMX-300 spectrometer.

3.1. Epoxidation of cyclooctene with UHP using a Mo(VI) catalyst in [bmim]PF6

2.2. Synthesis of 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6 The ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6, was prepared as previously reported [31]. The resulting ionic liquid was purified by dissolving it in dichloromethane and filtering through silica gel. After removing under vacuum the volatiles and warming up to 60 C for one hour, [bmim]PF6 was obtained as a very slightly yellow, clear liquid (57 g, 80% relative to 0.25 mol of 1-methylimidazole as starting material). 2.3. Representative experimental procedure for the Mo(VI) catalysed reactions A mixture of cyclooctene (0.36 g, 3.25 mmol), UHP (0.46 g, 4.88 mmol), and MoO3 (11.5 mg, 0.08 mmol) in [bmim]PF6 (5 mL) was prepared in atmospheric conditions, sealed and heated at 60 C with stirring for 18 h. The reaction mixture was then extracted with diethyl ether (6 · 8 mL). The ether extracts were dried with MgSO4 and the solvent removed under reduced pressure in an ice bath (to reduce any loss of volatiles). Cyclooctene oxide was obtained as a slightly yellow gum (0.36 mg, 88%).

We selected the epoxidation of cyclooctene as test reaction for the study of the activity of a number of easily available molybdenum(VI) compounds, such as MoO3 and ammonium dimolybdate, (NH4)2Mo2O7. Several solvents and oxidants were investigated (see Table 1) under the following reaction conditions: micro-reactor (5 mL of solvent) with a ratio 1:1.5:0.025 of [cyclooctene:oxidant:catalyst] maintained at 333 K. After 18 h, the reaction mixture was analysed. The best medium/oxidant combination was the [bmim]PF6, with UHP (Scheme 1). Epoxide conversion was found to be strongly dependent on the solvent. Using conventional solvents, such as H2O, methanol or Cl2CH2 (entries 6–11), observed conversions were only very low, due probably to the low solubility of some of the reagents in the solvent (i.e., UHP is insoluble in most organic solTable 1 Epoxidation of cyclooctene using Mo(VI) catalyst in several solventsa Entry

Solvent

Precatalyst

Oxidant

Conversion (%)b

1 2 3 4 5 6 7 8 9 10 11

[bmim]PF6 [bmim]PF6 [bmim]PF6 [bmim]PF6 [bmim]PF6 MeOH MeOH H2O H2O H2O Cl2CH2

– (NH4)2Mo2O7 MoO3 MoO3 Mo(O)(O2)2(bipy) (NH4)2Mo2O7 MoO3 (NH4)2Mo2O7 MoO3 MoO3 (NH4)2Mo2O7

UHP UHP UHP H2O2c UHP UHP UHP UHP UHP H2O2 UHP

<5 >90 >90 0 >90 0 1 0 <5 <5 0

a

Reaction conditions: [ciclooctene:UHP:catalyst, 1:1.5:0.025; Vsolvent, 5 mL; T, 333 K; t, 18 h. b Conversion to cyclooctene oxide was calculated by NMR analysis. c Cyclooctane-1,2-diol is observed.

M. Herbert et al. / Catalysis Communications 8 (2007) 987–990

989 O

O

O

N

MoO3 Mo2O72-

N

O

O [O]

O

[O]

O

Scheme 1.

M O

O

O

(b) O O

+ O

O

O O

O

M

vents, such as ether or Cl2CH2). Conversely, in [bmim]PF6, almost complete reaction was seen after approximately 18 h using both (NH4)2Mo2O7 (entry 2) and MoO3 (entry 3) as catalytic precursors. The high yield of the reaction appears to result from the high solubility of all reagents (i.e. cyclooctene, UHP and Mo precursor) in the ionic solvent. In the absence of the Mo catalyst conversions to epoxide were only insignificant (entry 1). Cyclooctane-1,2-diol was obtained as product when aqueous hydrogen peroxide (30%) was used as oxidant (entry 4) due to ring opening of the sensitive epoxide in the presence of large amounts of water. Therefore, the use of the water-free hydrogen peroxide source, UHP, is crucial for successful epoxidation. The best result obtained for the epoxidation of cyclooctene is entry 3 where the reaction in [bmim]PF6 using UHP gave excellent conversion when catalysed by the cheap and commercially available MoO3. Recently, MoO3 has been reported as a good catalyst in the same reaction, but using tert-butyl hydroperoxide as oxidant in toluene [32]. In the epoxidation of cyclooctene, this system requires a longer reaction time at a higher temperature than for the reaction described by Bhattacharyya and co-workers [15], which is an excellent system for epoxidation. Our results are comparable to other Mo catalysed reactions in RTIL’s that have been previously reported [24,25]. However, although further work is needed in order to generalise the applications of this system, the advantages of the cheap, reusable and readily commercially available catalyst are notable. We assume that the accepted mechanism for olefin epoxidations catalysed by oxo-molybdenum(VI) complexes in conventional solvents is also operative in our process [33,34]. There are two plausible pathways for this reaction (see Scheme 2): (a) coordination of the olefin to the oxygen atom of the g2-peroxo ligand to form a spirocyclic transition state; (b) coordination of the olefin to the Lewis-acidic

O

M

Mo(O)(O2)2(bipy) O

PF6-

O

O

(a)

[bmim]PF6

[bmim]PF6 :

O M

"Mo(VI)"/UHP

O

O

M O

O

Scheme 2.

molybdenum center, followed by insertion into the Mo–O bond. In both of them, an oxobis(peroxo) Mo-species, formed by the reaction of the oxo-molybdenum precursor with the oxidant, is proposed as the real catalyst of the oxidation. The characteristic yellow colour of the oxo-peroxo Mo(VI) species, observed in the catalytic reaction mixture, and the fact that the Mo-complex Mo(O)(O2)2(bipy) [30] affords complete conversion in the same way as Mo2 O27 or MoO3 (entry 5) support this hypotheses. 3.2. Recovery and reuse of the catalyst-IL mixture for the oxidation of cyclooctene One of the main reasons for using an ionic liquid as reaction medium was to study the possibility that it might be recycled and reused along with the dissolved molybdenum catalyst. Recovery of the products after epoxidation in RTIL’s has been reported previously [35]. The main difficulty encountered in recycling the ionic liquid [bmim]PF6, used in our system, is the elimination of the reaction residues (the reaction generates urea, which was insoluble in the IL). After extraction of the epoxide product with diethyl ether, dissolving the ionic liquid in dichloromethane allows the removal, by filtration, of the un-reacted UHP and urea which are both insoluble in the chlorinated solvent. Having removed the dichloromethane under reduced pressure the [Mo]/[bmim]PF6 solution is ready to be used again. When MoO3 is used as catalyst for the oxidation of cyclooctene to cyclooctene oxide, we found that recycling of the ionic liquid is viable, but after four consecutive runs a decrease in isolated yields was observed (Table 2), indicating leaching of the Mo catalyst. The leaching was

Table 2 Reuse of the IL-catalyst mixture for the oxidation of cyclooctene to cyclooctene oxidea Catalyst

MoO3

Run Conversion (%)b

1 >90

a b

Mo2 O27 2 >90

3 75

4 30

5 –

1 >90

Reaction conditions: [cyclooctene:UHP:cat], 1:1.5:0.025; VRTIL, 5 mL; T, 333 K; t, 18 h. Conversion to cyclooctene oxide was calculated by NMR analysis.

MeReO3 2 50

3 10

4 5

5 –

1 >90

2 –

990

M. Herbert et al. / Catalysis Communications 8 (2007) 987–990

even more marked when dimolybdate (NH4)2Mo2O7 was used as catalyst. To compare the effectiveness of the recovery process, we analysed the same system substituting MTO as the epoxidation catalyst [18]. The Re-catalyst/IL system cannot be recovered (Table 2), due probably to decomposition of the Re catalyst during the recycling procedure. Thus, although recovery of the MoO3/[bmim]PF6 mixture was not excellent, allowing only three effective cycles, it was at least possible, in contrast to the equivalent system with MTO, a well-established epoxidation catalyst. Additionally, the IL can be reused in further epoxidation reactions by simply re-charging fresh Mo catalyst (see Section 2.4.). 4. Conclusions The selective and efficient epoxidation of cyclooctene to the corresponding oxide using a UHP-MoO3 catalyst system in [bmim]PF6 has been demonstrated. Although the results presented here are limited to cyclooctene, the following characteristics of this system are notable: the mild reaction conditions, the recyclable solvent–catalyst mixture, the simple and green oxidant and the cheapness and commercial availability of the catalyst. We are currently undertaking further studies extending this system to other olefin substrates and ionic liquid solvents. Acknowledgements This research was supported by the European Commission (Marie Curie Training Network MRTN-CT-2004504005, SuperGreenChem Network), the Spanish Ministerio de Educacio´n y Ciencia (Research Project CTQ2004-84/ PPQ, FEDER supported) and Junta de Andalucı´a. References [1] R.A. Sheldon, J.K. Kochi, Metal-Catalysed Oxidations of Organic Compounds, Academic Press, New York, 1981. [2] B.S. Lane, K. Burguess, Chem. Rev. 103 (2003) 2457. [3] G. Grigoropoulos, J.H. Clark, J.A. Elings, Green Chem. 5 (2003) 1. [4] G. Fioroni, F. Fringuelli, F. Pizzo, L. Vaccaro, Green Chem. 5 (2003) 1. [5] M.C.-Y. Tang, K.-Y. Wong, T.H. Chan, Chem. Commun. (2005) 1345. [6] G.S. Owens, J. Arias, M.M. Abu-Omar, Catal. Today 55 (2000) 317.

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