A DFT study on the reaction pathway involved in the metal‐ion‐templated synthesis of benzo‐21‐crown‐7(B21C7) from catechol and hexaethylene glycol ditosylate in the presence of base

This present work explores the mechanism of the alkali metal‐ion (Na+ and K+) aided benzo‐21‐crown‐7(B21C7) formation from catechol and hexaethylene glycol di‐p‐toluenesulfonate in the presence of base [Correction added on 29 December 2022, after first online publication: benzo‐21‐crown‐7(f) has been corrected to benzo‐21‐crown‐7(B21C7).]. The density functional theory (DFT)‐based studies (in gas‐phase and in acetonitrile) have identified two SN2 steps along the reaction path, where each step is characterized by an energy barrier due to the presence of a transition state (TS). The metal‐ion template effect eases the binding between the catechol oxygen and the carbon atom holding the tosyl leaving group. This metal‐ion framework facilitates the cyclization process, which is responsible for the better yield of the crown ether. In case of K+‐coordinated system in solution, the TS dipole moment value for this step is 10 Debye higher than the corresponding reactant, whereas for Na+‐coordinated system, this difference is much smaller (~7.5 Debye). A comparison of the barrier heights of this intramolecular SN2 step in acetonitrile shows a higher value for Na+ ion system (30 kcal/mol) than that of K+‐coordinated system (22 kcal/mol). Expectedly, a higher rate constant for the cyclization has been found from the kinetics study of the latter, which is in‐line with the reported experimental results. Additionally, we have also explored the consequences of the involvement of a second metal ion in the process, a possibility, which was predicted earlier for large ring‐sized crown ethers. Mechanism of the alkali metal‐ion (K+ and Na+) template synthesis of benzo‐21‐crown‐7 (B21C7) from catechol and hexaethylene glycol di‐p‐toluenesulfonate in the presence of base has been investigated through DFT studies. Both one‐metal‐ion model and two‐metal‐ion models in the cyclization steps have been explored.