Light-driven [2+2] cycloaddition is the most direct strategy to build tetrasubstituted cyclobutanes, core components of many lead compounds for drug development. Significant advances in the chemoselectivity and enantioselectivity of [2+2] photocycloadditions have been made, but exceptional and tunable diastereoselectivity and regioselectivity (head-to-head versus head-to-tail adducts) is required for the synthesis of bioactive molecules. Here we show that colloidal quantum dots serve as visible-light chromophores, photocatalysts and reusable scaffolds for homo- and hetero-intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, including aryl-conjugated alkenes, with up to 98% switchable regioselectivity and 98% diastereoselectivity for the previously minor syn-cyclobutane products. Transient absorption spectroscopy confirms that our system demonstrates catalysis triggered by triplet–triplet energy transfer from the quantum dot. The precisely controlled triplet energy levels of the quantum dot photocatalysts facilitate efficient and selective heterocoupling, a major challenge in direct cyclobutane synthesis.
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Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC 1900373 (8). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. All the other data supporting the findings of this study are available within the article (Figs. 1–4 and Table 1) and its Supplementary Information (Supplementary Tables 1–8, Supplementary Scheme 1, Supplementary Figures 1–4 and Supplementary Sections L and M) or from the corresponding author upon reasonable request.
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We thank R. Thomson and A. Lee for helpful discussions and T.D. Harris, D. Zee and A. Thorarinsdottir for use of their glove box. Research primarily supported by the Air Force Office of Scientific Research (grant 9550-17-1-0271) (synthesis, photocatalysis and analytical chemistry) and by the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award no. DE-SC0000989 (calculations). C.R.R. thanks the International Institute for Nanotechnology at Northwestern University for a fellowship. This work made use of the IMSERC at Northwestern University, which has received support from the NIH (1S10OD012016-01/1S10RR019071-01A1), Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois and the International Institute for Nanotechnology (IIN).
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Experimental details, materials and methods, details of the QD catalyst recycling experiment, control studies, description of the transient absorption spectroscopy studies, a kinetic model for the competing formation of syn- and anti-photocycloaddition products, triplet energies of the substrates, cyclic voltammetry, band-edge energies of the CdSe QDs, X-ray crystallographic data, HPLC chromatograms and NMR spectra.
Crystallographic Information File for compound 8, CCDC 1900373.
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Jiang, Y., Wang, C., Rogers, C.R. et al. Regio- and diastereoselective intermolecular [2+2] cycloadditions photocatalysed by quantum dots. Nat. Chem. 11, 1034–1040 (2019). https://doi.org/10.1038/s41557-019-0344-4
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