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 Fe Cl2 or FeCl3, 1,4,7-triazacyclononane (TACN), and pyridine in methanol is t he extradiol cleavage product 2-hydroxymuconic semi-aldehyde methyl ester ( Lin, G.; Reid, G.; Bugg, T. D. I-I. J. Chem. Sec. Chem. Commun. 2000, 1119- 1120). Under these conditions, extradiol cleavage of a range of 3- and 4-su bstituted catechols with electron-donating substituents is observed. The re action shows a preference in selectivity and rate for iron(II) rather than iron(III) for the extradiol cleavage, which parallels the selectivity of th e 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 compl exes with dioxygen gave the same product as monitored by UV/vis spectroscop y. KO2 is able to oxidize catechols with both electron-donating and electro n-withdrawing substituents, implying a different mechanism for extradiol. c leavage. Saturation kinetics were observed for catechols, which fit the Mic haelis-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 procee d using monosodium catecholate in the absence of pyridine, but with differe nt product ratios, giving insight into the acid/base chemistry of extradiol cleavage. In particular, extradiol cleavage in the presence of iron(II) sh ows a requirement for a proton donor, implying a role for an acidic group i n the extradiol dioxygenase active site.