Ionic liquids in methyltrioxorhenium catalyzed epoxidation–methanolysis of glycals under homogeneous and heterogeneous conditions

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Abstract

The efficient and high yielding domino epoxidation–methanolysis of glycals has been achieved under environment friendly conditions by oxidation with urea hydrogen peroxide adduct (UHP) and H2O2 in ionic liquids (ILs) catalyzed by methyltrioxorhenium and different heterogeneous methyltrioxorhenium derivatives. The facial diastereoselectivity of the oxidation ranged from satisfactory to excellent ones depending on the substrate and could be optimized by ample screening of catalysts. The oxidations performed with UHP proceeded with a higher degree of diastereoselectivity than those performed with H2O2. High yields of products and conversions of substrates were obtained under mild experimental conditions and by the use of simple work-up procedures.

Graphical abstract

The efficient and high yielding domino epoxidation–methanolysis of glycals has been achieved under environment friendly conditions by oxidation with UHP and H2O2 in ionic liquids (ILs) catalyzed by methyltrioxorhenium and different heterogeneous methyltrioxorhenium derivatives.

Introduction

The use of ionic liquids (ILs) as solvents in organic synthesis has enormously increased over the last decade [1]. In particular, growing attention has been devoted to the use of ILs for catalytic reactions [2]. Indeed, several catalytic reactions display considerable rate acceleration effect when carried out in ILs. Moreover, ILs form biphasic systems with many organic solvents, opening the way to the opportunity of easy isolation and recovery of the homogeneous catalysts. ILs are often considered as green alternative solvents to conventional molecular solvents, due to their negligible volatility that allows easy storage without contamination of the surrounding environment. Nevertheless, their toxicity and biodegradability have not been fully determined yet. Oxidation reactions are among the most important trasformations in organic synthesis, but often use polluting metal oxides and over stoichiometric amounts of oxidants. In spite of notable advances [3], oxidation reactions are mainly carried out in molecular solvents. Only recently, some catalytic oxidations in ILs have been reported, mainly by adapting the traditional molecular solvent methods to ILs [4]. Advances in this regard have been obtained by the use of ILs such as 1-n-butyl-3-methylimidazolium hexafluoro phosphate [BMIM][PF6] [5], 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] [6] and 1-ethyl-3-methylimidazolium bis-triflic amide [EMIM][Tf2N] [7], which are oxygen and water stable compounds. In the last years, increasing attention has been focused on the use of methyltrioxorhenium (CH3ReO3, MTO) [8], in conjunction with environment friendly hydrogen peroxide (H2O2) or the urea hydrogen peroxide adduct (UHP) [9] as oxygen atom donors, due to the excellent catalytic properties showed by this system [10]. These reactions proceed through the formation of the monoperoxo [MeRe(O)2O2] (A) and the bis-peroxo [MeReO(O2)2] (B) η2-rhenium complexes that have been isolated and fully characterized by single-crystal X-ray analysis [11]. Furthermore, MTO showed excellent conversions and selectivities for the epoxidation of olefins in ILs. Under these experimental conditions, the intermediate (B) was more reactive than (A) [12]. Heterogeneous compounds based on the anchorage of MTO on easily available, non-toxic and inexpensive poly(4-vinylpyridine) and polystyrene resins, are also efficient catalysts for the oxidation of hydrocarbons in ILs [13]. Noteworthily, in these transformations the activity of the heterogeneous catalysts was greater than that previously observed in molecular solvents. Moreover, heterogeneous MTO catalysts were easily recycled by filtration and used for successive transformations with similar selectivity and reactivity. We recently reported an efficient and high yielding domino epoxidation–methanolysis of glycals 1 with MTO and UHP in MeOH to afford methyl glycosides 2 under both homogeneous and heterogeneous conditions (Scheme 1) [14]. Similar results have been reported by Quayle and his co-workers for some glucals and galactals under biphasic conditions by the use of catalytic MTO and 30% aqueous H2O2 [15]. The oxidation of glycals is a challenging aim, due to the sensitive nature of the intermediate epoxide. It is therefore essential to perform this reaction in anhydrous and non-nucleophilic solvents. With the aim to develop a greener procedure for the epoxidation–methanolysis of glycals employing methanol not only as the nucleophile of the process but also as the solvent of reaction, we tried to test some ILs as solvent media for this transformation. In the preliminary communication [14a], we also reported our results of a test in [BMIM][BF4] as solvent for a related interesting transformation of glycals into glycosyl phosphates 3 (Scheme 1). Indeed in that case, the use of the IL instead of acetone as solvent avoided undesired ring opening of the intermediate epoxide by water. Albeit we have found later that glycosyl phosphates 3 are obtained in higher yields and better selectivities in molecular solvents with the addition of substoichiometric amount of a nitrogen ligand, such as pyridine or imidazole [16], our preliminary results in ILs were very encouraging.

As a continuation of these studies, we report herein an efficient, stereoselective and environment friendly procedure for the domino epoxidation–methanolysis of a series of structurally diversified glycals to the corresponding methyl glycosides with MTO and heterogeneous MTO systems (IIV, Fig. 1) in two stable ILs, [BMIM][BF4] and [BMIM][PF6], using H2O2 and UHP as primary oxidants. The facial diastereoselectivity of the oxidation ranged from satisfactory to excellent ones depending on the substrate and could be optimized for each glycal employed by ample screening of catalysts.

Section snippets

Experimental

All commercial products were of the highest grade available and were used as such. Glycals 48 were prepared according to literature procedures [17], [18]. NMR spectra were recorded on a Varian Mercury 400 (1H, 400 MHz) or a Bruker (1H, 200 MHz) spectrometer. Chromatographic purifications were performed on columns packed with silica gel, 230–400 mesh, for a flash technique.

Results and discussion

Reactions were performed using catalytic MTO or related catalysts based on the heterogeneisation of MTO on easily available, non-toxic and inexpensive poly(4-vinylpyridine) and poly(4-vinylpyridine N-oxide) 2% [PVP-2/MTO (I)] and 25% [PVP-25/MTO (II) and PVPN-25/MTO (III), respectively] cross-linked with divinylbenzene [19], [20]. Microencapsulated catalyst PS-2/MTO (IV) based on the physical entrapment of MTO on polystyrene 2% cross-linked with divinylbenzene was also used [28], [29]. The

Conclusions

Common room temperature ionic liquids have been found to behave as appropriate media for the MTO catalyzed domino epoxidation–methanolysis of a series of glycals with UHP. Both [BMIM][BF4] and [BMIM][PF6] turned out to work well, with the former one being generally preferable in terms of diastereoselection. Both homogeneous MTO and heterogeneised MTO-based catalysts IIV afforded the corresponding methyl glycosides efficiently, with excellent chemoselectivity and isolated yields. The facial

Acknowledgements

FC and AG thank Istituto Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM, Italy) for financial support (progetto PRISMA 2004) and Ente Cassa di Risparmio di Firenze, Italy for granting a 400-MHz NMR spectrometer. Partial support of a fellowship to GS from INSTM is also gratefully acknowledged.

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