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Mild Oxidative Deprotection of Aromatic Hydrazones and Semicarbazones with KMnO4 in Ionic Liquid Medium

The selective deprotection of carbonyl groups is often required in organic synthesis.1 Highly crystalline hydrazones, semicarbazones and oximes are very efficient derivatives for the isolation, purification and characterization of aldehydes and ketones.2,3 Considerable interest in the use of selective and cleaner oxidation methods such as heterogeneous oxidation4 and oxidation in ionic liquids have received much attention in recent years.5–11 The advantages of these methods over conventional procedures are that they provide greater selectivity, enhanced reaction rates, cleaner products and manipulative simplicity.12 Although some of these conventional methods have convenient protocols with good to high yields, most suffer from at least one of disadvantages such as use of high temperature and long reaction times, moisture sensitivity, high cost and toxicity of the reagents. Ionic liquids (ILs), which combine good and tunable solubility properties with a negligible vapor pressure and excellent thermal stabilities, have rapidly found a place of choice as valuable substitutes for many volatile solvents.6,11,13,14 Owing to their unique properties, it soon became apparent that ILs are more than simple neoteric solvent. Reactions in ILs exhibit thermodynamic and kinetic behaviors different from those of conventional solvents.15,16 Often this lead to improved process performance such as better selectivity and/or conversion; furthermore, ionic liquids lead to enhanced stability of organometallic reagents9,10,17,18 and biocatalysts,3,11 allow for product recovery as well as the possibility of recycling of homogeneous catalysts. In continuation of our studies on the application of ionic liquid media,19,20 we found that dialkylimidazolium halides such as butylmethylimidazolium bromide ([Bmim]Br) has many advantages over conventional solvents.21–26 Herein we report an efficient method for the oxidative cleavage of hydrazones and semicarbazones with potassium permanganate in the presence of [Bmim]Br (BMIM Br) under mild and solvent-free conditions.