The Oxidation Of Arylaldehydes In EtOH Under Metal-free Conditions

The oxidation of aldehydes is an important transformation, which can be used for the synthesis of carboxylic acids. Generally, the oxidation of aldehydes requires the use of stoichiometric metal oxidants, expensive transition metal catalysts. However, the difficulty for the removal of the metals from products hinders the application of such a useful reaction. In recent years, ionic liquids have aroused increasing interest due to their unique physical and chemical properties, and have been applied in a lot of research fields. The ionic liquids have shown to possess promising catalytic properties in the oxidation of aldehydes. However, these transformations described in literature are usually carried out in a variety of environmentally unacceptable solvents, such as DMF, DMSO and so on. Thus, it is necessary to develop an environmentally friendly reaction system of oxidation of aldehydes.In the present paper, we report the synthesis and characterization of a series of poly(ethylene glycol)(PEG) functionalized imidazolium salts and their application in the oxidation of arylaldehydes. We further developed a catalyst-free reaction system for the oxidation of arylaldehydes to acids in EtOH. The main contents are as follows:1. A series of poly(ethylene glycol)(PEG) functionalized imidazolium salts with the different length of PEG moiety and different functional groups have been designed and synthesized via a two-step procedure. The first step is a reaction of polyethylene glycol with methyl sulfonyl chloride to form sulfonate esters, and then followed a nucleophilic substitution between the sulfonate esters and substituted imidazoles to generate the desired imidazolium salts. All the salts were characterized by means of NMR spectroscopy and high-resolution mass spectrometry.2. A fast and efficient protocol for the PEG-functionalized imidazolium salt-catalyzed carboxylation of arylaldehydes has been developed, providing the carboxylic acids in good to excellent yields. The reaction is carried out in the presence of NaOH in combination with environmentally friendly ethanol/water as the solvent. Under the optimized conditions, arylaldehydes including heterocyclic aromatic aldehydes could be easily converted to the corresponding carboxylic acids. The less active substrates could also provide the corresponding acids in good yield by raising the temperature to60℃. 3. A green and catalyst-free protocol for the oxidation of arylaldehydes in EtOH was developed under a0.8MPa oxygen atmosphere. In the absence of a catalyst, the reaction system shows that arylaldehydes with an electron-withdrawing group and heterocyclic aromatic aldehydes could be converted to the corresponding carboxylic acids. Under the optimized conditions, the oxidation of4-nitrobenzaldehyde could be finished within2h in85%isolated yield of4-nitrobenzoic acid and5%isolated yield of4-nitrobenzyl alcohol.