Steric and Electronic Control of Regioselectivity in Salophen Metal Complex-Catalyzed Epichlorohydrin Acetolysis

Salophen ligands and their complexes with transition metals Cu(II), Fe(II), Fe(III), and Co(II) were synthesized and characterized by UV-vis, IR, and ¹H NMR spectroscopy. The catalytic activity of the complexes in epoxide synthesis was studied using epichlorohydrin acetolysis as a model reaction. Kinetic studies established that the reaction order with respect to both the acid and the catalyst is consistent with that observed under traditional catalysis (tetraalkylammonium salts, trialkylamines). It has been demonstrated that catalysis by salophen complexes depends on the electronic properties of salophen ligands, namely, electron-donating substituents accelerate the reaction. The products of the reaction catalyzed by Co(II) salophen complexes were identified, and their structures and ratios were characterized by ¹H NMR spectroscopy. Co(II) complexes of substituted salophens provided higher regioselectivity than traditional catalysts, favoring the formation of β-hydroxypropyl esters─key epoxy precursors. Steric parameters of the synthesized complexes were evaluated using SambVca software by calculating the percent buried volume and modeling the steric maps of the ligands. Quantitative structure-selectivity relationships showed that regioselectivity depends on both the steric and electronic properties of the ligand substituents. A mechanistic pathway for this catalysis by salophen metal complexes has been detailed.