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Enantioselective photochemistry through Lewis acid–catalyzed triplet energy transfer

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Science  16 Dec 2016:
Vol. 354, Issue 6318, pp. 1391-1395
DOI: 10.1126/science.aai8228

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Asymmetric catalysis by tuning triplets

Triplet excited states manifest a distinct mode of reactivity associated with their unpaired electrons. However, modulating this reactivity to select just one of two mirror-image products, or enantiomers, is difficult. Blum et al. found that Lewis acid coordination lowered the energy of a compound's triplet state, which allowed it to be accessed using an optically excited sensitizer. Because the Lewis acid was essential for triplet formation, it could also direct the reaction pathway toward a single enantiomer.

Science, this issue p. 1391

Abstract

Relatively few catalytic systems are able to control the stereochemistry of electronically excited organic intermediates. Here we report the discovery that a chiral Lewis acid complex can catalyze triplet energy transfer from an electronically excited photosensitizer. We applied this strategy to asymmetric [2 + 2] photocycloadditions of 2′-hydroxychalcones, using tris(bipyridyl) ruthenium(II) as a sensitizer. A variety of electrochemical, computational, and spectroscopic data rule out substrate activation by means of photoinduced electron transfer and instead support a mechanism in which Lewis acid coordination dramatically lowers the triplet energy of the chalcone substrate. We expect that this approach will enable chemists to more broadly apply their detailed understanding of chiral Lewis acid catalysis to stereocontrol in reactions involving electronically excited states.

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