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Shape-Selective C–H Oxidation

Recent research from the group of Professor Lawrence Que.

In a VIP paper just published in Angewandte Chemie (DOI: 10.1002/anie.200805342), graduate student Anusree Mukherjee in Larry Que’s group and coworkers describe the first example of shape-selective oxidation of an aliphatic hydrocarbon by an O2-activating iron(II) complex. In Nature, many iron enzymes tap the oxidizing potential of O2 to perform selective substrate oxidations. Oxygenases that require α-keto acid as a cofactor comprise a subset of these enzymes. We have synthesized the complex, [Fe(TpPh2)(O2CC(O)Ph)] (TpPh2 = tris(3,5-diphenylpyrazolyl)borate; see scheme below), to serve as a model for the active site of such enzymes. Indeed we have shown this complex to be able to activate O2 to form a transient FeIV=O oxidant that effects intramolecular hydroxylation of a ligand phenyl ring.[1]

Recently, we have observed that this complex is also able to carry out intermolecular oxidations of added hydrocarbon substrates. In other words, the added substrate can intercept the oxidant generated by O2 activation and be itself oxidized instead of the ligand phenyl ring. Indeed, C-H bonds as strong as those of cyclooctane (96 kcal/mol) are attacked. Most strikingly, when cyclohexene and ethylbenzene are used as substrates in a competitive oxidation experiment, only cyclohexene is oxidized despite the similar strengths of their weakest C-H bonds. We postulate this selectivity to be based on shape, the shape of a cleft formed by two phenyl groups of the TpPh2 ligand that flank the putative FeIV=O oxidant, which favors oblate spheroidal substrates.

Shape selectivity has been very difficult to accomplish in biomimetic chemistry, particularly without a significant effort in synthesis to assemble the target molecule. In our case, [Fe(TpPh2)(O2CC(O)Ph)] is easily synthesized from its components by a one-step procedure. What is amazing is that so simple a complex can achieve shape-selective discrimination of substrates, similar to that often associated with more complex enzyme active sites.

[1] M. P. Mehn, K. Fujisawa, E. L. Hegg, L. Que, Jr., J. Am. Chem. Soc. 2003, 125, 7828.

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