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Abstract. Nitrophenols can be obtained in high yields via nitrosation-oxidation of phenols by 3-methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl} as a new Brønsted acidic ionic liquid and NaNO^sub 2^ at room temperature. In situ generation of HNO^sub 2^ and a radical-cation mechanism via the nitrous acid catalyzed pathway appear to be applicable to phenol nitration using this reagent system.
Keywords: 3-Methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl}; Brønsted acidic ionic liquid; Nitrosation-oxidation; Nitration; Nitrophenols; Sodium nitrite (NaNO2).
INTRODUCTION
Ambient temperature ionic liquids, especially those based on 1,3-dialkylimidazolium cations, have gained considerable interest as promising alternative green solvents in organic synthesis [1]. These ionic liquids have several interesting properties, such as excellent chemical and thermal stability, non-volatility, noncoordinating natures, safety, good solvating capability, a wide liquid range, and ease of recycling [1]. Furthermore, their hydrophobicity/hydrophilicity can be tuned by appropriate modification of the cations or anions [1]. Therefore, wide applications in catalytic and non-catalytic reactions have been found for room temperature ionic liquids [2-6]. In addition, the synthesis of task-specific ionic liquids which have a functional group in their framework may expand the applications of ionic liquids in organic chemistry [7,8]. Among ionic liquids, acidic ones have been successfully used in many organic transformations [7-10]. In continuation of our previous studies on the applications of acidic reagents and catalysts in organic synthesis [11,12], more recently, we have synthesized ionic liquid 3-methyl-1-sulfonic acid imidazolium chloride {[Msim]Cl} as a new Brønsted acidic reagent from the simple reaction of 1-methylimidazole with chlorosulfonic acid at room temperature (Figure 1) [13]. We believe that this new ionic liquid can be applied as a reagent, as well as catalyst, for different organic transformations. Herein, we found that the acidity of this Brønsted acidic reagent was suitable for the in situ generation of HNO^sub 2^, when used in conjunction with NaNO^sub 2^, for the nitration of phenols.
The nitration of organic compounds is a very active and rewarding area of research, being also the subject of a large body of literature [14]. Moreover, nitro-aromatic compounds have been extensively utilized as chemical feedstock for a wide range of useful materials, such as dyes, pharmaceuticals, perfumes and plastics [15]. The nitration of benzene and toluene is one of the most important routes to substituted aromatics in...