Extradiol oxidative cleavage of catechols by ferrous and ferric complexes of 1,4,7-triazacyclononane: Insight into the mechanism of the extradiol catechol dioxygenases

Lin, Gang, Reid, Gillian and Bugg, Timothy D.H. (2001) Extradiol oxidative cleavage of catechols by ferrous and ferric complexes of 1,4,7-triazacyclononane: Insight into the mechanism of the extradiol catechol dioxygenases. Journal of the American Chemical Society, 123 (21), 5030-5039. (doi:10.1021/ja004280u).

Abstract

The major oxygenation product of catechol by dioxygen in the presence of FeCl2 or FeCl3, 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. I-I. J. Chem. Sec. 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. KO2 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.

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Contributors

Author: Gillian Reid

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