SYNTHESIS OF THIOESTERS AND THIOAMIDES USING POTASSIUM THIOCYANATE UNDER MICROWAVE IRRADIATION (2022)

Microwave-assisted synthesis provides more time for chemists to expand their creativeness, develop new processes and new theories. Recent developments in chemical sciences created microwave energy a new proficient way of heating the reaction.1 Microwave energy bids abundant benefits for the execution of synthesis comprising enhanced reaction rates, improved yield and cleaner chemistries. Using this technique, reactions can be performed in minutes, instead of spending hours to days in a conventional manner. The difficulty accompanying effluent disposal is overcome by executing the reactions without the use of solvent under microwave irradiation. Microwave irradiation reaction under solvent-free conditions, mineralsupported catalyzed reactions etc. provides better chemical methods with higher reaction rate, improved yield, good selectivity.2 Chemical reactions that took an hour to days the completion can be performed in minutes. The microwave region is positioned between IR and radio waves in the electromagnetic spectrum. The wavelength of microwave radiation is 1 mm-1 m with the frequencies 0.3-300 GHz. Dielectric heating in microwaves utilizes the capability of various solids and liquids to convert electromagnetic energy into heat to progress the reactions. The microwave technique has been applied in various research technologies like drug release/targeting,3 polymer technology,4 waste treatment,5 ceramics,6 alkane decomposition,7 etc. Consequently, microwave technology entertains as a potential method in green chemistry.8 This technology unlocks new openings as a new heating technique which is very difficult by the regular heating method.9 Organosulfur compounds are versatile intermediates in synthetic chemistry, possess many imperative biological properties.10 The applications and choice of organo-sulfur compounds have augmented enormously as sulfur comprising groups continue to function as an imperative supplementary function in synthetic arrangements.11 Thiocarbonyl comprising compounds are useful synthetic intermediates, that find numerous uses in the preparation of natural products.12 Although various reagents are accessible for the preparation of thioesters or thioamides, there are some drawbacks. Some of the important reagents used for thionation comprises lawessons reagent,13 H2S,14 phosphorous pentasulfide,15 hexamethyldisilathine,16 R3OBF4/NaSH,17 R2PSX,18 (Et2Al)2S,19 bis(tricyclcohexylstannyl) sulfide/BCl3, 20 thiourea,21 etc. Many of these reagents involve high temperatures, prolonged reaction times, or harsh conditions for reaction execution and frequently require problematic column chromatography to separate and remove the unwanted products. Batool A. et al. described a green procedure for the synthesis of thieranes from oxiranes by using ammonium thiocyanate.22 Encouraged by this, and in persistence to our work on the development of biologically active heterocycles,23-26 we thought of using simple and commercially available reagent potassium thiocyanate for the transformation of esters into thioesters by making use of microwave irradiation.