Research ArticleChemistry

Light-driven, heterogeneous organocatalysts for C–C bond formation toward valuable perfluoroalkylated intermediates

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Science Advances  11 Nov 2020:
Vol. 6, no. 46, eabc9923
DOI: 10.1126/sciadv.abc9923
  • Fig. 1 Sketch of synthesis and structure of the different CN materials.

    Graphical sketch of the synthesis of the various CN structures, with the associated photograph of the as-obtained powdery material. Standard thermal conditions applied to melamine lead to synthesis of g-CN, where the morphology is typically described by melem units connected in-plane; oxidative treatment presumably introduces small amounts of oxygenated functional groups on the surface (ox-CN), while reductive treatment partly removes N atoms, creating planar vacancies (red-CN); higher-temperature thermal treatment under inert atmosphere generates partially amorphous structure by misalignment of CN planar domains (am-CN). Note that the graphic rendering of the structure is only an idealized depiction used for the benefit of discussion. Real CN structures are much more complex. Photo credit: Francesco Longobardo, University of Trieste, Italy.

  • Fig. 2 High-resolution microscopy of the four samples.

    (A) Representative HRTEM image of am-CN. Inset: FFT of a selected area showing the pattern of a typical amorphous material. (B) Representative HRTEM image of red-CN. Inset: High magnification of a selected area, showing the crystal lattice fringes, where a 0.32-nm interlayer spacing is measured. (C) Representative HRTEM of g-CN with the FFT (inset) showing spots assigned to the expected 0.32-nm interlayer spacing and (D) high magnification of a selected area of (C) with the FFT showing the intralayer XRD pattern with a 0.68-nm spacing. (E) Representative HRTEM image of ox-CN with the inset showing the lattice fringes with the 0.32-nm interlayer spacing and (F) EDX elemental mapping of ox-CN: carbon (red), nitrogen (green), and oxygen (blue).

  • Fig. 3 Physical characterization.

    Near-infrared (NIR) Raman spectra (A) and XRD diffractogram (B) of the four materials. Crystallite sizes reported in the table of (B) were calculated by applying the Scherrer equation to the (002) reflection. (C) High-resolution XPS spectra in the C1s binding energy (B.E.) range. a.u., arbitrary units. (D) DRS spectra and the corresponding band gap (inset).

  • Fig. 4 NMR investigation and proposed mechanism.

    (A) T2 CPMG spectra of perfluorohexyl iodide in red-CN. The CF2I resonance is at approximately −69 parts per million (ppm), that of the CF3 at approximately −92 ppm, and the (CF2)4 peaks in the range of −120 to −145 ppm. (B) Corresponding T2 CPMG decay plots for the NMR signals of CF2I, (CF2)4 and CF3 fitted using a single CPMG exponential decay. (C) 19F NMR T1/T2 ratio of the different moieties of perfluorohexyl iodide in the various CN-based photocatalytic materials used in this work. (D) Proposed reaction mechanism that drives the photocatalytic perfluorobutylation of 1a. The perfluorinated substrate binds the catalyst surface via iodine halogen bond. After photoexcitation and charge separation in the semiconducting catalyst, excited electrons are injected from the catalyst to the substrate, forming the radical Ia. The radical then attacks the aromatic molecule, leading to the cascade reaction generating the final product. Photo credit: Francesco Longobardo, University of Trieste, Italy.

  • Fig. 5 Evaluation of the scope of the photochemical reaction.

    Survey of the organic compounds and perfluoroalkyl iodides that can participate in the photocatalytic process. Conditions: Reactions are conducted in Schlenk tubes in DMF (0.25 M) on 0.1 mmol scale of 1, 0.6 mmol of perfluoroalkyl iodide 2, 0.1 mmol of K2CO3, and 0.27% (w/v) of am-CN, degassed by four freeze-pump-thaw cycles and irradiated for 4 to 24 hours by a blue light-emitting diode (LED) strip (450 nm). [a] Outdoor experiment and the used setup (from 9:00 to 13:00 of 5 August 2019, in Trieste; see fig. S10). Photo credit: Giacomo Filippini, University of Trieste, Italy.

  • Table 1 Catalytic tests on the model reaction.

    Optimization studies and control experiments. Reactions were performed on 0.1 mmol scale. TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl. Photo credit: Giacomo Filippini, University of Trieste, Italy.

    Embedded Image

    *Yield determined by 1H-NMR spectroscopy using 1,1,2-trichloroethene as the internal standard.

    †Isolated yield.

    Supplementary Materials

    • Supplementary Materials

      Light-driven, heterogeneous organocatalysts for C–C bond formation toward valuable perfluoroalkylated intermediates

      Giacomo Filippini, Francesco Longobardo, Luke Forster, Alejandro Criado, Graziano Di Carmine, Lucia Nasi, Carmine D’Agostino, Michele Melchionna, Paolo Fornasiero, Maurizio Prato

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      • Supplementary Text
      • Scheme S1
      • Figs. S1 to S12
      • Tables S1 to S9
      • References

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