Oxidation of catechol with KMnO4 by using crown ethers as phase transfer catalysts

Abstract

The effect of crown ethers as phase transfer catalysts (PTC) was investigated in the liquid-phase oxidation of 3,5-di-tert-butylcatechol (3,5-DtBC) using potassium permanganate. Both the liquid—liquid (aqueous liquid layer including KMnO₄/organic liquid layer including crown ether and 3,5-DtBC) and the solid—liquid (solid KMnO₄/organic liquid layer including crown ether and 3,5-DtBC) systems were studied under mild reaction conditions. The oxidations of 3,5-DtBC in both systems were promoted by using crown ethers as PTCs. In the liquid—liquid system, the influence of organic solvents, acid additives and the type of crown ethers utilized were investigated. The oxidation rate of 3,5-DtBC was increased by using a solvent with a low polarity such as n-hexane and/or by adding an organic or inorganic acid to the system. Crown ethers which have a cavity that conforms to the radius of the K⁺ ion, and were more lipophilic such as dicyclohexano-18-crown-6 (DC18C6) also caused an increase in the oxidation rate. The rate-determining step is thought to be the oxidation reaction step in the organic phase, rather than the phase transfer step of the crown ether—KMnO₄ complex. In the solid—liquid system, the influence of organic solvents and the type of crown ethers was investigated. The rate of 3,5-DtBC oxidation was increased by using a solvent with a high polarity such as chloroform. The complex stability between the crown ether and K⁺ was suggested to be one of the most important factors governing the oxidation rate in the solid—liquid system.