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Aliphatic C–C Bond Cleavage of α-Hydroxy Ketones by Non-Heme Iron(II) Complexes: Mechanistic Insight into the Reaction Catalyzed by 2,4′-Dihydroxyacetophenone Dioxygenase

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Inorg. Chem., Article ASAP
DOI: 10.1021/acs.inorgchem.5b01235
Publication Date (Web): November 4, 2015
Copyright © 2015 American Chemical Society
*T.K.P.: e-mail, ictkp@iacs.res.in; fax, +91-33-2473-2805; tel, +91-33-2473-4971.

Synopsis


Iron(II) complexes of a facial N3 and a tripodal N4 donor ligand oxidatively cleave the aliphatic C−C bonds of α-hydroxy ketones in the reaction with dioxygen. An intradiol type mechanism is proposed for the C−C bond cleavage reaction.
http://pubs.acs.org/doi/10.1021/acs.inorgchem.5b01235

Abstract

Abstract Image
2,4′-Dihydroxyacetophenone dioxygenase (DAD) is a bacterial non-heme enzyme that carries out oxygenative aliphatic C–C bond cleavage of 2,4′-dihydroxyacetophenone (an α-hydroxy ketone) with the incorporation of both the oxygen atoms of dioxygen into the cleavage products. The crystal structure of the iron enzyme DAD has recently been determined, but very little is known about the mechanism of the C–C bond cleavage reaction. With the objective of gaining insights into the mechanism of the reaction catalyzed by DAD, six new biomimetic iron(II)-α-hydroxy ketone complexes, [(TpPh2)FeII(PHAP)] (1), [(TpPh2)FeII(HCH)] (2), [(TpPh2)FeII(HBME)] (3), [(TpPh2)FeII(CHPE)] (4), [(6-Me3-TPA)FeII(PHAP)]+ (5), and [(6-Me3-TPA)FeII(HCH)]+ (6) (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine, PHAP-H = 2-phenyl-2-hydroxyacetophenone, HCH-H = 2-hydroxycyclohexanone, HBME-H = 2-hydroxy-1,2-bis(4-methoxyphenyl)ethanone, and CHPE-H = 1-(4-chlorophenyl)-2-hydroxy-2-phenylethanone), have been isolated and characterized. The single-crystal X-ray structure of 2 shows a five-coordinate iron(II) complex with one tridentate facial ligand and a monoanionic bidentate α-hydroxy ketone, resulting in a distorted-square-pyramidal coordination geometry at the iron center. The iron(II) complexes react with dioxygen to oxidatively cleave the aliphatic C–C bonds of the coordinated α-hydroxy ketones to afford 2 equiv of carboxylic acids. Mechanistic studies reveal that the C–C bond cleavage reaction proceeds through an intradiol pathway. Additionally, the coordinated α-hydroxy ketones in all of the complexes, except in complex 4, undergo two-electron oxidation to form the corresponding 1,2-diketones. However, the yields of 1,2-diketones are higher with the iron complexes of the tripodal N4 ligand (6-Me3-TPA) in comparison to the facial N3 ligand (TpPh2). These results strongly support the natural selection of a facial N3 environment at the active site of the iron enzyme DAD.






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