Influence of ligand substitution on molybdenum catalysts with tridentate Schiff base ligands for the organic solvent-free oxidation of limonene using aqueous TBHP as oxidant

The oxidation of limonene by aqueous TBHP has been analyzed in the presence of molybdenum complexes [MoO2L](2) as catalysts with five different tridentate ligands L in the absence of organic solvents (greener reaction conditions). The ligands are based on a common salicylidene amino(thio)phenolate, SA(T)P, backbone with differences in the coordination sphere (ONO for L = SAP vs. ONS for L = SATP) or in the salicyl moiety functionalization by OH groups for the ONO ligands. The process gives a regioselective endocyclic epoxidation to a kinetically controlled 1:1 mixture of the cis-LimO and trans-LimO epoxides and/or the isomeric diols alpha x-LimD and eq-LimD by the subsequent ring opening in the presence of water, with a product distribution that depends on the ligand, reaction time and temperature. In combination with control experiments of the cis/trans-LimO ring opening, the investigations demonstrate the catalytic action of the metal complexes in both the epoxidation and the ring opening steps, with the cis-LimO stereospecifically producing the alpha x-LimD product and the less reactive trans-LimO leading to a 4:3 mixture of alpha x-LimD and eq-LimD. The ONS system [MoO2(SATP)](2) exhibits the highest catalytic activity in both steps. (c) 2017 Elsevier B.V. All rights reserved.