Exploring the reaction pathway involved in the dibenzo-18-crown-6 synthesis from catechol and bis(2-chloroethyl) ether in presence of base

Computational investigations reported in this work have proposed the mechanism involved in the synthesis of dibenzo-18-crown-6 (DB18C6) from catechol and bis(2-chloroethyl) ether in presence of base. The transition states on the reaction path have been identified, and the corresponding barrier heights are estimated. Paths related to the side-product formations, such as benzo-9-crown-3 and 1,2-bis[2-(2-chloroethoxy)ethoxy]-benzene, have been also tracked. The cation-assisted (Na+ and K+ ions) cyclization process (template effect) creates more structurally ordered reactant and transition state species. This facilitates the cyclization step that is an intramolecular S(N)2 process. Additionally, it eliminates possibilities of different side-product formations giving better yield of the crown ether, as reported previously. This cyclization step has a comparatively lower barrier height for Na+ system (by 3 kcal/mol) in gas phase than K+ system; however, it becomes reverse in 1-butanol where the barrier height is almost 2.5 kcal/mol lower during the K(+)DB18C6 formation. Consequently, a faster cyclization has been found in this solvent from the reaction rate studies for the potassium system. This is in-line with the experimental observations reported on the alkali-metal-catalyzed benzo-18-crown-6 ether formation reactions. The dipole moment value of the transition state involved in this step for the larger alkali metal ion system is found to be significantly higher (by 5-8 D) than the reactant species.

Automated summary

Computational investigations in this work elucidated the mechanism of synthesizing dibenzo-18-crown-6 from catechol and bis(2-chloroethyl) ether in the presence of base. The study identified transition states, tracked side-product formation paths, and analyzed the effect of cation-assisted cyclization on reaction rates. Results showed differences in barrier heights between Na+ and K+ systems, with faster cyclization observed in 1-butanol for the potassium system. Additionally, the dipole moment of the larger alkali metal ion system at the transition state was significantly higher than the reactant species.