Homogeneous oxidation of methyl isobutyrate with oxygen catalysed by metal complexes: polyoxometalates versus metalloporphyrins and metallophthalocyanines

Abstract

Two types of homogeneous oxidation of methyl isobutyrate (MIB) with O 2 catalysed by transition metal complexes aiming at methyl α-hydroxyisobutyrate (MHIB) have been studied: (1) low-temperature (30–50 °C, 1 bar O 2) co-oxidation with isobutyraldehyde (IBA); (2) high-temperature (100–140 °C, 10 bar O 2) direct oxidation. As the catalysts, Keggin-type polyoxometalates (PMo 12− n V n O 40; n=1–3) and (PW 11MO 39; M=Ni(II), Co(II), Cu(II), Fe(III), and Ru(III)) as well as metalloporphyrins and metallophthalocyanines of Co(II), Ni(II), Cu(II), Fe(II, III), Mn(III), and Ru(III) have been used. The complexes of Fe(II, III), Co(II), and Ru(III) have been found to be the most active catalysts. The co-oxidation occurs with a fairly good efficiency, allowing up to 0.55 mol MIB to be converted per 1 mol IBA reacted at 50 °C and a molar ratio [MIB]:[IBA]=5:1. About a half of IBA is non-productively consumed to isobutyric acid (IBAcid; up to 95% yield) and acetone. MIB appears to be mainly converted to methyl α-hydroperoxyisobutyrate (MHPIB), the maximum selectivity to MHIB being only 15%. The high-temperature direct oxidation of MIB is found to be more efficient providing 24% selectivity to MHIB at 19% MIB conversion (120 °C). The drawback to this route is the formation of a large amount of IBAcid (39–59% selectivity) and acetone (25–46%). All three types of complexes studied perform quite similarly as the catalysts for these reactions. The important result is that in the low-temperature co-oxidation, the complexes with organic ligands (porphyrins and phthalocyanines) are more active than the corresponding polyoxometalates, whereas the opposite is observed in the high-temperature oxidation. This is in agreement with the views on polyoxometalates as robust, oxidation-resistant analogs of metal complexes with macrocyclic ligands.