Diverting non-haem iron catalysed aliphatic C–H hydroxylations towards desaturations

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Abstract

Carboxylate-ligated, non-haem iron enzymes demonstrate the capacity for catalysing such remarkable processes as hydroxylations, chlorinations and desaturations of inert, aliphatic C–H bonds. A key to functional diversity is the enzymes' ability to divert fleeting radicals towards different types of functionalization using active site and/or substrate modifications. We report that a non-haem iron hydroxylase catalyst [Fe(PDP)] can also be diverted to catalytic, mixed hydroxylase/desaturase activity with aliphatic C–H bonds. Using a taxane-based radical trap that rearranges under Fe(PDP) oxidation to furnish a nortaxane skeleton, we provide the first direct evidence for a substrate radical using this class of stereoretentive hydroxylation catalysts. Hydroxylation and desaturation proceed by means of a short-lived radical that diverges in a substrate-dependent manner in the presence of carboxylic acids. The novel biomimetic reactivity displayed by this small molecule catalyst is harnessed to diversify natural product derivatives as well as interrogate their biosynthetic pathways.

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Figure 1: Substrate-dependent mixed hydroxylase/desaturase reactivity of non-haem iron catalysts with aliphatic C–H bonds.
Figure 2: Double-oxidation products arise from desaturation products (olefins) in a substrate-dependent manner.
Figure 3: Experiments probing a dehydration pathway in the formation of olefin intermediates.
Figure 4: Experiments probing the existence of a carbon-centred radical.
Figure 5: Rapid diversification of carboxylic acid natural product analogues.

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Acknowledgements

The authors are grateful to Pfizer, Bristol-Myers Squibb and UIUC for financial support. M.A.B. is supported by an Illinois Distinguished Fellowship (2007-2010) and a Harold R. Snyder Fellowship (2010-2011). S.A.R. is supported by the National Science Foundation under the Center for Chemical Innovation in Stereoselective C–H Functionalization (CHE-0943980) and by an Ullyot Graduate Fellowship (2009-2010). J. Guerra and R.M. Williams are thanked for providing the (+)-taxusin used in the radical trap experiments.

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M.A.B. and M.C.W. conceived and designed the experiments outlined in Figs 1 3,4b, and M.A.B. performed these experiments. S.A.R. and M.C.W. conceived and designed the experiments outlined in Figs 4a,5 and S.A.R. performed these experiments. M.A.B and M.C.W co-wrote the paper, with assistance from S.A.R.

Correspondence to M. Christina White.

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Bigi, M., Reed, S. & White, M. Diverting non-haem iron catalysed aliphatic C–H hydroxylations towards desaturations. Nature Chem 3, 216–222 (2011) doi:10.1038/nchem.967

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