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.
Lin
G
G
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.
Reid
G
G
Bugg
T D
TD
D002396:Catechols / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006571:Heterocyclic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction
D002396:Catechols; D005290:Ferric Compounds; D005296:Ferrous Compounds; D006571:Heterocyclic Compounds; C075712:1,4,7-triazacyclononane