Photoinduced electron transfer is an essential step in the conversion of solar energy into chemical energy in photosystems I and II (ref. 1), and is also frequently used by chemists to build complex molecules from simple precursors2. During this process, light absorption generates molecules in excited electronic states that are susceptible to accepting or donating electrons. But although the excited states are straightforward to generate, their short lifetimes makes it challenging to control electron transfer and subsequent product formation—particularly if enantiopure products are desired. Control strategies developed so far use hydrogen bonding, to embed photochemical substrates in chiral environments3 and to render photochemical reactions enantioselective through the use of rigid chiral complexing agents4. To go beyond such stoichiometric chiral information transmission, catalytic turnover is required5. Here we present a catalytic photoinduced electron transfer reaction that proceeds with considerable turnover and high enantioselectivity. By using an electron accepting chiral organocatalyst that enforces a chiral environment on the substrate through hydrogen bonding, we obtain the product in significant enantiomeric excess (up to 70%) and in yields reaching 64%. This performance suggests that photochemical routes to chiral compounds may find use in general asymmetric synthesis.
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This work was supported by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie. We thank O. Ackermann for technical assistance and T. Straßner for discussions.Author Contributions A.B. and F.W. contributed equally to the experimental part of the study. S.G. performed the DFT calculations.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
These Supplementary Notes detail: Overview of the syntheses; synthetic procedures and analytical data; 1H-NMR shift experiments of compound 2 / ent-2; Chiroptical experiments with 2 / ent-2; DFT calculations. (PDF 361 kb)
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Bauer, A., Westkämper, F., Grimme, S. et al. Catalytic enantioselective reactions driven by photoinduced electron transfer. Nature 436, 1139–1140 (2005). https://doi.org/10.1038/nature03955
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