A promising electro-oxidation of methyl-substituted aromatic compounds to aldehydes in aqueous imidazole ionic liquid solutions
Introduction
Aromatic aldehydes are important intermediates due, among other things, to their widespread applications in the synthesis of pharmaceuticals, food flavoring agents, cosmetics and spices. Many strategies [1], [2], [3], [4], [5], [6] exist for their preparation including the catalytic oxidation of aromatic compounds. Most of these approaches for the oxidation of alkyl-substituted aromatics present limitations owing to the low selectivity and yield toward aldehyde formation, mostly less than 65% at temperatures over 300 °C. Thus, it is worthwhile to develop an alternative method for the efficient synthesis of aromatic aldehydes starting from readily available alkyl-substituted aromatic ring systems.
Electro-oxidation of methyl-substituted aromatics to the corresponding aldehyde has been studied in a variety of differing solvents and electrolytes [7], [8], [9], [10], [11], [12], [13], [14]. Indirect electro-oxidation using Ce3+/Ce4+ or Cr2+/Cr4+ play a major role in the processes that have been explored, with the highest GC yield reported to be 85% [7], [9], [12], [13], [14]. Due to the low solubility of methyl-substituted aromatics in the traditional inorganic salt electrolyte reaction media, organic solvents such as methanol, acetonitrile or acetone can be added. However, this diminishes the “greenness” of the resulting procedures since the solvents need to be removed and either recycled or disposed of in a responsible manner [10], [13]. In the process of electro-oxidation, the over-oxidation of aldehydes to acids [9] or polymerization has been reported [10], [15]. Clearly a solvent should be chosen in order to effectively solubilize the organic substrate and allow it to be easily separated from the products.
Room temperature ionic liquids (RTILs) have both ionic and organic characteristics [16], and can therefore be used both as the solvent and as the electrolyte. Due to their excellent electrochemical properties, ILs have been used in batteries [17], dye solar cells [18], and fuel cells [19], as well as the electrochemical deposition of metals [20], and electrochemical synthesis [21], [22], [23]. Direct electro-oxidation is sometimes operationally simpler than indirect electro-oxidation [11], [24], and there are some reports concerning about the direct electro-oxidation of CH bonds to form aldehydes [8], [10], [11], but to our knowledge no report describes the process in a semi-aqueous IL reaction medium. In keeping with our continuing efforts to utilize direct electro-oxidation in organic synthesis, we now report the direct electro-oxidative synthesis of aromatic aldehydes in aqueous imidazole IL solutions, thereby providing a new “green method” for the production of aromatic aldehydes.
Section snippets
Experimental
1-Butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) were synthesized using literature procedures [25], [26], [27]. Other ILs, such as 1-butyl-3-methylimidazolium hydrogen sulfate (BMIM-HSO4), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM-O3SCF3), 1-butyl-3-methylimidazolium tosylate (BMIM-O3SC6H4CH3) and 1-butyl-3-methylimidazolium acetate (BMIM-OOCCH3) were analytical grade, and were purchased and used without
Electrochemical properties of the ILs
In an electrochemical system, good conductivity of the electrolytic solution is needed in order to complete the electrical circuit. It is, of course, well known that the viscosity of ILs limits the movement of ions thereby leading to poor conductivity. We have elected to add water to the IL because of its conductivity and also because it can serve as an oxygen source during the electro-oxidation procedure. The conductivity of different concentrations of several ILs in an aqueous solution is
Conclusions
A series of aqueous solutions of ILs were shown to be suitable electrolytes for the electro-oxidation of methyl-substituted aromatic compounds. The electro-oxidation process was irreversible, and was sensitive to the pH of the electrolyte. In-situ FTIR analysis of the oxidation process further confirmed that the main product corresponded to an aromatic aldehyde. The further oxidation of the aldehyde was much inhibited, and polymerization did not occur. Recycling experiments demonstrated
Acknowledgements
The authors thank the National Natural Science Foundation of China (21076192), the 973 Project from the Ministry of Science and Technology of China (2012CB722604) and the Innovation Team of the Key Science and Technology of Zhejiang province (2009R50002).
References (35)
- et al.
Appl. Catal. A: Gen.
(1999) - et al.
J. Mol. Catal. A: Chem.
(2000) - et al.
Catal. Today
(2003) - et al.
Catal. Today
(2004) - et al.
J. Mol. Catal. A: Chem.
(2013) - et al.
Catal. Today
(1995) - et al.
Catal. Today
(1999) - et al.
J. Electroanal. Chem.
(2001) - et al.
Hydrometallurgy
(2010) - et al.
J. Mol. Liq.
(2010)