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Light-driven deracemization enabled by excited-state electron transfer

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Science  18 Oct 2019:
Vol. 366, Issue 6463, pp. 364-369
DOI: 10.1126/science.aay2204

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Charging through the looking glass

Asymmetric catalysis is a commonly applied technique to prepare just one of two mirror-image products in a chemical reaction. But what if you already have the compound you want, stuck in a mixture of left- and right-handed enantiomers? Shin et al. now show that light-induced electron transfer can trigger a favorable succession of proton and hydrogen-atom transfer steps, both of which are susceptible to biasing by catalysts, to preferentially convert a mixture of cyclic urea enantiomers into just one (see the Perspective by Wendlandt).

Science, this issue p. 364; see also p. 304

Abstract

Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C–H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.

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