Preparation, characterization and use of 3-methyl-1-sulfonic acid imidazolium hydrogen sulfate as an eco-benign, efficient and reusable ionic liquid catalyst for the chemoselective trimethylsilyl protection of hydroxyl groups

Abstract

New and novel ionic liquid (3-methyl-1-sulfonic acid imidazolium hydrogen sulfate) is a recyclable and eco-benign catalyst for the chemoselective trimethylsilyl protection of hydroxyl groups under solvent-free conditions to afford trimethylsilanes in excellent yields (92–100%) and in very short reaction times (1–8 min). The catalyst was characterized by FT-IR, ¹H NMR and ¹³C NMR studies. All the products were extensively characterized by ¹H NMR, IR, GC–MS and melting point analyses. A mechanism for the catalytic activity is proposed. The catalyst can be recovered and reused without loss of activity. The work-up of the reaction consists of a simple separation, followed by concentration of the crude product and purification.

Introduction

Ionic liquids (based imidazolium or other organic cations) have received considerable interest as eco-friendly solvents, catalysts and reagents in green synthesis because of their unique properties, such as low volatility, nonflammability, high thermal stability, negligible vapor pressure and ability to dissolve a wide range of materials [1], [2], [3], [4], [5], [6], [7], [8], [9]. Among them, Brønsted acidic ionic liquids have designed to replace solid acids and traditional mineral liquid acids like sulfuric acid and hydrochloric acid in chemical procedures [10], [11], [12], [13], [14]. As mentioned imidazolium salts having a Brønsted acidic group are of importance, and they have been successfully utilized as catalyst in organic synthesis. These subjects encouraged us to synthesize functionalized imidazolium salt, with Brønsted acidic property, including ionic liquid 3-methyl-1-sulfonic acid imidazolium hydrogen sulfate {[Msim]HSO₄}. We wish to use it as catalyst for different organic transformations.

Transformation of hydroxyl groups to their trimethylsilyl ethers is of value from different views [15]. This conversion enhances solubility of the compounds in non-polar solvents, increases thermal stability, and used extensively to increase volatility of the compounds for gas chromatography and mass spectrometry as well. Moreover, protection of hydroxyl groups and their transformation to the corresponding silyl ethers is of vital importance in the total synthesis of complex organic molecules. For these vast applications of silyl ethers presentation of new methods, using new catalysts is of demand from industries and academia. Generally, the formation of silylethers was carried out by the treatment of alcohols with silylchlorides or silyl triflates under the influence of basic conditions [16]. However, some of these methods frequently suffered from drawbacks such as lack of reactivity and the difficulty in removing amine salts derived from the reaction of produced acids and bases during the course of the reaction. Hexamethyldisilazane (HMDS) is a cheap and commercially available reagent that can be used for the preparation of trimethylsilyl ethers from hydroxyl compounds. O-Silylation of alcohols using HMDS is an attractive alternative, since the only by-product of the reaction is ammonia which is easily removed from the reaction mixture. However, its main drawback is its poor silylating power, which needs forceful conditions and long reaction times [17]. A variety of catalysts have been reported for the activation of HMDS; of them trichloroisocyanuric acid (TCCA) [18], zirconium sulfophenyl phosphonate [19], ZnCl₂ [20], Envirocat EPZGO [21], tungstophosphoric acid [22], K-10 montmorillonite [23], iodine [24], lithium perchlorate [25], cupric sulfate pentahydrate [26], H-β zeolite [27], MgBr₂ [28], lithium perchlorate supported on silica gel [29], sulfonic acid-functionalized silica [30], magnesium triflate [31], InBr₃ [32], zirconium triflate [33], ZrCl₄ [34], NBS [35], iron(III) trifluoroacetate [36], silica supported perchloric acid [37], Fe₃O₄ [38], poly(N-bromobenzene-1,3-disulfonamide) [39], barbutric acid [40], Al(H₂PO₄)₃ [41], tetrabutylammonium phtalimide-N-oxyl (TBAPINO) [42], TiCl₃(OTf) [43], Nafion^® SAC-13 [44] and Bu₄NBr [45] are ones. Although the silylation ability of HMDS has been promoted in the presence of these catalysts, yet some of the presented methods suffer from long reaction times, low yields, and drastic reaction conditions and sometimes, a tedious workup is required. In addition, in most of the reported methods, selectivity of the protocols is either poor or is not reported properly. Consequently, a new procedure that addresses these drawbacks is desirable.