engleski

An Eco-Friendly Preparations of Pyridoxal Oxime Quaternary Salts in Deep Eutectic Solvents

Quaternization of pyridine with alkyl halides or related compounds is an example of the Menshutkin reaction, the most common rute to create quaternary salts. The most organic solvents used in conventional synthesis of pyridinium quaternary salts are volatile, hazardous, toxic like anhydrous benzene [1], acetonitrile and acetone [2-3]. Also, a lots of these solvents are considered a hazardous waste when are discarded. Reducing toxicity is one of the most challenging aspects of designing chemical syntheses. Deep eutectic solvents (DESs) have emerged recently as new and green solvents in organic synthesis. After we successfully quaternized pyridine derivatives in DESs in our previous study Bušić et al. [4], we continue to perform syntheses of pyridoxal oxime quaternary salts in eutectic solvents. In this study it was performed quaternization reaction of pyridoxal oxime and substituted phenacyl bromides in 16 choline chloride-based DESs by three synthetic approaches: conventional, microwave and ultrasound (Scheme). The purpose of this work was to investigate which eutectic solvent is most effective for the quaternization reaction. After the performed syntheses, the results show that the conventional method, with a long reaction time, generally gave low product yields in eutectic solvents. In the conventional method, the most effective eutectic solvents were: choline chloride: thiourea, choline chloride: oxalic acid, and choline chloride: levulinic acid. The highest yields of reactions in DESs were obtained using microwave irradiation where the most effective eutectic solvents were: choline chloride: oxalic acid, and choline chloride: levulinic acid. Choline chloride: malic acid, Choline chloride: malonic acid and choline chloride : trans-cinnamic acid gave approximately equal yields. In the ultrasound method the most effective eutectic solvents for the quaternization of pyridoxal oxime with substituted phenacyl bromides were choline chloride: oxalic acid, and choline chloride: levulinic acid and choline chloride : trans- cinnamic acid and reactions were carried outwith yields higher than in the conventional method. The obtained results for quaternization reaction of of pyridoxal oxime show that methods under influence of microwave and ultrasound irradiation were the most effective for quaternization in eutectic solvents.

quaternary salts, deep eutectic solvents, green chemistry

nije evidentirano