Catalytic epoxidation with cobalt (II) calix[4]pyrrol and cobalt (II) schiff base complexes

Abstract

The epoxidation of alkenes catalyzed by cobalt(II) calix[4]pyrrole in the presence of 2-ethylbutyraldehyde/O2 was examined. Cyclohexene was converted to cyclohexene oxide in high yield and excellent selectivity. Ligands and solvents are two major influent parameters affecting on the reaction. Cobalt(II) calix[4]pyrrole (68) and cobalt(II) benzimidazole (72) were disclosed to be the most efficient catalyst tested. Various substrates such as (R)- and (S)-limonenes, geraniol, geranyl acetate, 4-vinylcyclohexene, trans-2-hexen-1-ol, 3-octen-1-ol, cis- and trans-stilbenes and cholesterol derivatives were chosen to explore the regio-, chemo- and stereoselectivity of the reaction. The mechanism of the reaction was believed to take place via peroxy acid and free radical process. The condensation of aromatic aldehydes with several diamines was examined. The results clearly indicated that only Schiff bases were formed when using 1,2-ethylenediamine or 1,3-propylenediamine, while an apparent intramolecular cyclization took place with the use of 1,2-phenylenediamine. In the latter case, rigid structure of the diamine should be able to induce the formation of more stable and highly aromatic products. Intriguingly, in the case of 1,2-phenylenediamine, the ratio of the two starting materials and the choice of solvent played significant role in the reaction outcomes.