Crown Ethers as New Catalysts in the Highly Regioselective Halogenative Cleavage of Epoxides with Elemental Halogen

Abstract

The regioselective ring opening halogenation of some epoxides using elemental iodine and bromine in the presence of a series of new synthetic macrocycle diamides and also dibenzo-18-crown -6, 18-crown-6, and aza-18-crown-6 has been studied. The epoxides were subject to cleavage by elemental halogen (I2 and Br2) in the presence of these catalysts under mild reaction conditions in various aprotic solvents. In this study, reagents and conditions have been discovered with which the individual halohydrins can be synthesized in high yield and with more than 95% regioselectivity. The results can be discussed in terms of a four-step mechanism: (1) formation of a charge-transfer complex between catalyst and halogen, (2) release of halogen nucleophile from the complex, (3) reaction of the active nucleophile at the less sterically-hindered site in the epoxide, and (4) regeneration of catalyst. The major advantages of this method are high regioselectivity, simple regeneration of catalyst and its reuse through several cycles without a decrease in activity, and ease of workup of the reaction.