The combination of ascorbic acid 6-palmitate and [Fe3III(μ3-O)]7+ as a catalyst for the oxidation of unsaturated lipids

Recently, iron 2-ethylhexanoate (Fe-eh, 1) in combination with ascorbic acid 6-palmitate (AsA6p) has been reported as a good catalytic system for the oxidation of ethyl linoleate (EL), an unsaturated lipid. In response to the fascinating chemistry of this bio-inspired iron-based catalyst the structure of Fe-eh, and the effect of AsA6p on it, have been studied. Fe-eh was found to be a trinuclear oxido-centered iron(III) cluster of general formula [Fe3III(μ3-O)(eh)6(acetone)3](eh). The Mössbauer spectra of 1 indicate that the trinuclear iron core is not symmetric as the spectra exhibit two equivalent iron(III) sites and one unique iron(III) site. Variable temperature magnetic measurements indicate that the three iron centers are antiferromagnetically coupled.Upon the addition of AsA6p to complex 1 a new species (2), which has not yet been completely identified, is formed. Mössbauer spectroscopy indicates, however, that 2 is a mixed-valence cluster which probably has an iron-core that differs in symmetry from 1. The reduction of 1 to 2 was also monitored in time by electrospray ionization mass spectrometry, revealing that the reduction reaction is kinetically dependent on the molar ratio AsA6p/Fe-eh. Based on these results and other available information, a general mechanism for the oxidation of EL under the effect of the combination of AsA6p/Fe-eh is proposed.

A structural investigation of the ascorbic acid6-palmitate (AsA6p) reaction with iron reveals that mixed-valence trinuclear iron(II)/iron(III) complexes, [Fe3III(O)(2-ethylhexanoate)6-x(1-propoxide)x]+, where x is 0–6, are formed. These complexes are believed to initiate the oxidation of ethyl linoleate, an unsaturated lipid, via a radical-type species that is able to abstract one bis-allylic hydrogen from the unsaturated site of the lipid.Figure optionsDownload as PowerPoint slide