Mechanism of Isopropylbenzene Hydroperoxide Decomposition in the Presence of Crown Ether Metal Complexes

Kinetic regularities of isopropylbenzene hydroperoxide decomposition in the presence of homogeneous catalysts (CaCl2, SrCl2, BaCl2) with dibenzo-18-crown-6 ether were studied. The catalytic decomposition of isopropylbenzene hydroperoxide (IPBHP) in chlorobenzene was studied by the ampoule method in a nitrogen atmosphere at [IPBHP]0=0.1÷2.5 mol/l, [Cat]0 = (0.5÷7)×10-3 mol/l in the temperature range 90-130°C. The hydroperoxide concentration was monitored iodometrically. Complexes of dibenzo-18-crown-6 ether (DBC) with calcium, strontium, and barium chlorides were synthesized by the interaction of alcohol solutions of metal chlorides and a solution of DBC in acetone. After removing the solvent, the metal complexes were white, finely crystalline substances. The molecular weight of the МCl2∙DBС complexes was (found/calculated), g/mol: Ca - 482.98/471.38, Sr - 527.55/519.13, Ba - 571.61/568.64. The electrical conductivity of catalyst solutions in chlorobenzene was studied by conductometry. Quantum chemical calculations, based on the results of which the possibility of the formation of the hydroperoxide-MCl2·DBC intermediate was assessed, were carried out using the Priroda and Gaussian 16, Revision B.01 software packages using the density functional theory method. To optimize the structures of intermediates and molecules involved in their formation and to calculate equilibrium vibration frequencies, the generalized gradient functional of Perdew-Burke-Ernzerhoff (PBE) was used. The solvation effect was taken into account within the framework of the self-consistent reactive field (SCRF) model. It has been established that DBC crown ether metal complexes accelerate the decomposition of isopropylbenzene hydroperoxide. The catalytic activity of the catalysts under study decreases in the order of Sr > Ca > Ba. The conductometry method was used to establish the presence of conductivity in solutions of the studied catalysts in chlorobenzene, which serves as evidence of the decomposition of metal complexes in the reaction system into ions and indicates in favor of the cationic mechanism of the reaction between isopropylbenzene hydroperoxide and catalysts. It has been established experimentally and theoretically that the formation of intermediate adducts HPIPB - M2+∙DBC (M2+∙= Ca2+, Sr2+, Ba2+) is possible and energetically favorable. The solvent (chlorobenzene) practically does not affect the course of the main stages of isopropylbenzene hydroperoxide decomposition.