Extradiol Oxidative Cleavage of Catechols by Ferrous and Ferric Complexes of 1,4,7-Triazacyclononane: Insight into the Mechanism of the Extradiol Catechol Dioxygenases

The major oxygenation product of catechol by dioxygen in the presence of FeCl(2) or FeCl(3), 1,4,7-triazacyclononane (TACN), and pyridine in methanol is the extradiol cleavage product 2-hydroxymuconic semi-aldehyde methyl ester (Lin, G.; Reid, G.; Bugg, T. D. H. J. Chem. Soc. Chem. Commun. 2000, 1119--1120). Under these conditions, extradiol cleavage of a range of 3- and 4-substituted catechols with electron-donating substituents is observed. The reaction shows a preference in selectivity and rate for iron(II) rather than iron(III) for the extradiol cleavage, which parallels the selectivity of the extradiol dioxygenase family. The reaction also shows a high selectivity for the macrocyclic ligand, TACN, over a range of other nitrogen- and oxygen-containing macrocycles. Reaction of anaerobically prepared iron-TACN complexes with dioxygen gave the same product as monitored by UV/vis spectroscopy. KO(2) is able to oxidize catechols with both electron-donating and electron-withdrawing substituents, implying a different mechanism for extradiol cleavage. Saturation kinetics were observed for catechols, which fit the Michaelis--Menten equation to give k(cat)(app) = 4.8 x 10(-3) s(-1) for 3-(2',3'-dihydroxyphenyl)propionic acid. The reaction was also found to proceed using monosodium catecholate in the absence of pyridine, but with different product ratios, giving insight into the acid/base chemistry of extradiol cleavage. In particular, extradiol cleavage in the presence of iron(II) shows a requirement for a proton donor, implying a role for an acidic group in the extradiol dioxygenase active site.