Oxidative degradation of monomeric and dimeric phenylpropanoids: reactivity and mechanistic investigation

Abstract

The oxidative degradation of lignin related model compounds catalysed by [N,N′-bis(salicylidene)ethane-1,2-diaminato]cobalt(II), [Co(salen)], have been studied in chloroform. Arylglycerol β-aryl ethers, phenylcoumarans and apocynol showed very high conversion values within 30 minutes of the start of the reaction and their conversion rates were higher than those reported for phenylpropenoidic compounds, methyl (E)-ferulate and methyl (E)-4-hydroxycinnamate. The results of the CW electron paramagnetic resonance (EPR) investigation using the X-band, performed at the reaction temperature (298 K) showed that two phenoxy cobalt radicals ([Co^III(salen)(ROH)(RO˙)] and [Co^III(salen)(RO⁻)(RO˙)]) are involved in the oxidation mechanism of all the phenol compounds ROH. In a frozen solution the [Co^III(salen)(ROH))(RO˙)] radical prevailed and its axial magnetic anisotropy was determined using high frequency, 190 GHz, CW EPR spectra. The X-band EPR monitoring of the phenoxy cobalt radicals during the reaction showed a faster decrease in the amounts of radical in the oxidation of propanoidic phenols compared with that of propenoidic phenols. The lifetime of the radicals formed from substrates having a methoxy group ortho to the aromatic hydroxy was also shorter than from substrates lacking this functionality. Both reactivity and spectroscopic data suggest that the lifetime of the phenoxy cobalt radicals correlates with the conversion rates of the substrates.