A novel and efficient catalytic epoxidation of monoterpenes by homogeneous and heterogeneous methyltrioxorhenium in ionic liquids
Graphical abstract
A convenient and efficient synthesis of monoterpene epoxides by application of methyltrioxorhenium and heterogeneous poly(4-vinylpyridine)/methyltrioxorhenium and microencapsulated polystyrene/methyltrioxorhenium systems in ionic liquids is described here. Environment friendly, easily available, and low cost urea hydrogen peroxide adduct was used as the primary oxidant. Catalysts were stable systems for at least four recycling oxidations. Experimental results showed that the oxidations performed in (ILs) ionic liquids were more selective and efficient than the ones in molecular solvents.
Introduction
Terpenes are the basis for many perfumes, detergents, flavours, agrochemicals, and therapeutically active substances [1]. Moreover, terpene epoxides are important intermediates in the preparation of commodities and fine chemicals [2]. In the last few years, different studies have been performed to design alternatives to traditional methods for the oxidation of terpenes that usually require stoichiometric amounts of organic peroxyacids such as m-chloroperbenzoic acid. These oxidants produce waste containing a high amount of the corresponding acids, and are not selective for the preparation of acid-sensitive epoxides [3]. Following the recent increasing interest in developing green chemistry procedures, less hazardous oxidative reagents with a low environmental impact should be utilized for industrial purposes [4]. In this context, the use of easily available, low cost and environmentally friendly oxidants, such as hydrogen peroxide (H2O2) [5] and dioxygen, is of particular interest. One of the most useful organometallic complexes for the activation of H2O2 is methyltrioxorhenium (VII) (MeReO3, MTO) [6]. In particular, it has been reported that MTO showed interesting catalytic properties in the epoxidation of olefins with both H2O2 [7], [8], [9] and urea hydrogen peroxide adduct (UHP) [10], [11], a water-free peroxide source, as oxygen atom donors. In the first case, low molecular weight Lewis bases, bearing one or more nitrogen atoms in their structures, were also used as mediators of the oxidation to avoid both the ring opening of the oxiranyl ring to form diols and the rearrangement of the carbon skeleton of the molecule [12]. The formation of two active intermediates, the monoperoxo metal [MeRe(O2)O2] complex (A) and the bis-peroxo metal [MeReO(O2)2] complex (B), has been well established [13]. Theoretical and computational studies have been performed to elucidate the geometrical features of the transition state involved in this oxidation. Specifically, the oxygen atom transfer from complexes (A) and/or (B) to substrate was always described by a concerted process, requiring a butterfly like transition state similar to that previously suggested for cyclic organic peroxides such as dimethyldioxirane [14]. In this context, the epoxidation of monoterpenes with MTO in molecular solvents (CH2Cl2 or EtOH) was described in homogeneous phase using UHP [15] or H2O2 and an excess of pyridine as the Lewis base mediator for the oxidation [16]. Initial efforts to perform the epoxidation of monoterpenes with MTO in alternative solvents, such as fluorinated systems, failed. This failure was probably due to the decomposition of the catalyst [17]. On the other hand, MTO showed excellent values of conversion and selectivity in the epoxidation of simple olefin derivatives in ionic liquids (ILs) with UHP as the primary oxidant. Under these experimental conditions, the bis-peroxo complex (B) was more reactive than the monoperoxo complex (A); the opposite was found in the case of conventional molecular solvents [18]. ILs are attractive substitutes for volatile molecular solvents because they remain liquid at temperatures greater than 300 °C, allowing kinetic control [19]. Moreover, ILs are solvents for a wide range of inorganic, organic and polymeric materials, and they can exhibit Brønsted and Lewis acidity as well as super acidity, enabling many catalytic processes [20]. Finally, several ILs are commercially available since they can be prepared readily and economically. Current advances in oxidation chemistry with ILs utilise 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] [21], 1-ethyl-3-methylimidazolium tetrafluoroborate [22] and 1-ethyl-3-methylimidazolium bis-triflic amide [23] salts, which are oxygen and water stable compounds. During our studies on the chemistry of MTO [24], we have reported that heterogeneous MTO catalysts, based on the heterogenation of MTO on low cost and easily available poly(4-vinylpyridine) 2% I or 25% II cross-linked (with divinylbenzene) [25], can be used for the oxidation of hydrocarbons and pyrrolidine derivatives in ILs with UHP [26]. Similar results were also obtained with polystyrene/MTO catalyst III, produced by microencapsulation of MTO on polystyrene [27]. Catalysts I–III are stable compounds and they retain their catalytic activity for further transformations [28]. Moreover, we have shown that compounds I–III are efficient catalysts for the epoxidation of olefins and monoterpenes in molecular solvents (Fig. 1) [29].
With the aim of developing novel environmental benign procedures for the epoxidation of monoterpenes, we report here the first described epoxidation of monoterpenes in ILs with MTO and MTO heterogeneous catalysts I–III and UHP as the primary oxidant. The monoterpene epoxides were synthesised in high yields and selectivity in the absence of any appreciable side reactions. Experimental results showed that the oxidations performed in ILs were more selective than the ones performed in molecular solvents. This procedure has a double benefit: the use of non-volatile solvents, and recoverable and stable heterogeneous catalysts for the activation of H2O2.
Section snippets
Experimental
All commercial products had the highest grade of purity available and were used as such. UHP, geraniol, nerol, 1(S)-(+)-3-carene, 1(R)-(+)-limonene, [BMIM][BF4] and [BMIM][PF6] were purchased from Sigma–Aldrich Company (Milan, Italy). Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker (200 MHz). Gas chromatography (GC) and GC–mass spectroscopy (MS) analyses of the reaction products were performed on a GCMS-QP5050 Shimadzu apparatus using a SPB column (25 m × 0.30 mm and 0.25 mm film
Results and discussion
The epoxidation of monoterpenes with homogeneous and heterogeneous MTO catalysts and UHP in ILs was investigated using 1(S)-(+)-3-carene 1, 1(R)-(+)-limonene 3, geraniol 6 and nerol 10 as representative cyclic and alicyclic monoterpene derivatives of industrial relevance.
As a general procedure, the monoterpene (1.0 mmol) was added to MTO (5.0%, w/w of the substrate) or heterogeneous MTO catalysts I–III (loading factor of 1.0; the loading factor is defined as mmol of MTO for 1.0 g of support) in
Conclusions
MTO, poly(4-vinylpyridine) 2% I or 25% II cross-linked (with divinylbenzene)/MTO and polystyrene/MTO III systems are efficient and selective catalysts for the conversion of monoterpenes to their corresponding epoxides in ILs using UHP as the oxygen atom donor. Heterogeneous catalysts were stable systems for at least four recycling experiments. With any of the catalysts, the oxidation of allylic monoterpenes 6 and 10 proceeded selectively at the more electron-rich 6,7-double bond in accordance
Acknowledgements
PNR-FIRB, ASI and EU COST CM07035 System Chemistry are acknowledged for their financial support.
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