Influence of hexamethylphosphoric triamide and dimethylformamide admixtures on the mechanism of tris(acetylacetonato)iron(III)-catalyzed ethylbenzene oxidation with molecular oxygen

The influence of electron-donating monodentate ligands L2, such as hexamethylphosphoric triamide (HMPA) and dimethylformamide (DMF), on the mechanism of tris(acetylacetonato)iron(III)-catalyzed oxidation of ethylbenzene with molecular oxygen was kinetically studied. It was found that the admixtures of L2 cause a nonadditive (synergistic) increase in the rate of ethylbenzene oxidation catalyzed by Fe(III)(acac)3, as well as a growth in the α-phenylethyl hydroperoxide selectivity (SPEH) and/or ethylbenzene conversion (C) relative to the Fe(III)(acac)3 catalysis in the absence of L2, changes that are due to the formation of more active complexes, mFe(III)(acac)3 · nL2 and Fe(II)(acac)2 · L2 (L2 is HMPA or DMF) and products of Fe(II)(acac)2 · L2 (L2 is DMF) transformation. The most significant effects of ligands L2 on SPEH and C was revealed in the case of catalysis by the {Fe(III)(acac)3-HMPA} system at 80°C. In the presence of the {Fe(III)(acac)3-DMF} catalyst system, methyl phenyl carbinol (MPC) was produced at a selectivity of SMPC ≅ 58%, a value that exceeds SPEH = 25–30% at the initial steps of ethylbenzene oxidation. The formation mechanism for the principal products—ethylbenzene hyroperoxide, acetophenone, and methyl phenyl carbinol—of ethylbenzene oxidation catalyzed by Fe(III)(acac)3 in the presence of HMPA and DMF was established. The activity of the Fe(II)(acac)2 · L2 complexes formed during ethylbenzene oxidation catalyzed by {Fe(III)(acac)3-L2} (L2 is HMPA or DMF) at the microsteps of chain initiation (O2 activation) and propagation in the presence of the catalyst (Ct) (Ct + RO2· →) was evaluated, and the role of these steps in the mechanism of the iron-catalyzed oxidation of ethylbenzene to α-phenylethyl hydroperoxide was discussed.