peri‐Xanthenoxanthene (PXX): a Versatile Organic Photocatalyst in Organic Synthesis

Abstract

Recent years have witnessed a continuous development of photocatalysts to satisfy the growing demand of photophysical and redox properties in photoredox catalysis, with complex structures or alternative strategies devised to access highly reducing or oxidising systems. We report herein the use of peri‐xanthenoxanthene (PXX), a simple and inexpensive dye, as an efficient photocatalyst. Its highly reducing excited state allows activation of a wide range of substrates, thus triggering useful radical reactions. Benchmark transformations such as the addition of organic radicals, generated by photoreduction of organic halides, to radical traps are initially demonstrated. More complex dual catalytic manifolds are also shown to be accessible: the β‐arylation of cyclic ketones is successful when using a secondary amine as organocatalyst, while cross‐coupling reactions of aryl halides with amines and thiols are obtained when using a Ni co‐catalyst. Application to the efficient two‐step synthesis of the expensive fluoro‐tetrahydro‐1H‐pyrido[4,3‐b]indole, a crucial synthetic intermediate for the investigational drug setipiprant, has been also demonstrated.

KuQuinone as a Highly Stable and Reusable Organic Photocatalyst in Selective Oxidation of Thioethers to Sulfoxides

“…Importantly, no catalyst deactivation was observed in 20 runs, meaning that the system could be further recycled and recharged, reaching TONs higher than 4000. Therefore, the remarkable robustness of KuQ definitely emerged, and such feature is actually rarely reported for organic photocatalysts, since they often undergo deactivation due to self-oxidation , and/or degradation . Recycle and recharge experiments have also been performed under air atmosphere, still confirming the high photocatalytic performance of KuQ (Figure S2–S3).…”

“…Despite the wide substrate scope and the reported good yields, such methods still present relevant drawbacks, such as the formation of overoxidation products and long reaction times (5–40 h) (Figure ). In addition, recycle and recharge experiments are rarely reported for organic photocatalysts, likely because of catalyst self-oxidation , and/or degradation . Nevertheless, quinoid-based heterogeneous catalysts demonstrated peculiar stability in the photooxidation of thioethers, being simply recovered by filtration and reused for more than 20 cycles, without significant loss of activity .…”

Customising excitation properties of polycyclic aromatic hydrocarbons by rational positional heteroatom doping: theperi-xanthenoxanthene (PXX) case

“…Polycyclic aromatic hydrocarbons (PAHs) are attracting great interest in the recent years for preparing organic semiconductors that, absorbing in the UV-vis spectral range of solar irradiation, generate excited states of interest for lightharvesting applications 1 and photocatalysis. [2][3][4][5] Capitalising on the wealth of organic synthetic methodologies, chemists have tailored the photophysical properties of PAHs by: (i) changing the size and edge of the aromatic scaffold; (ii) varying the molecular planarity; (iii) changing the resonance energy of the constituting monomeric units; (iv) varying the peripheral functionalisation of the edges through the insertion of either electron-donating or -withdrawing substituents; (v) replacing C(sp 2 ) atoms by isostructural analogues, i.e., heteroatom doping. [6][7][8][9] Amid all approaches, the doping route can effectively tailor the photophysical properties of a PAH without the need to add peripheral groups while maintaining the structural topology.…”

“…Owing to the possibilities of introducing the heteroatom in different ratios doping heteroatom and carbons), positions, and symmetries, numerous congurations are possible even for small PAHs. While the general inuence on the energy level of the frontier molecular orbitals of replacing a C(sp 2 ) atom with a more electronegative heteroatom such as N is well established (i.e., lower-energy rigid shi of the HOMO and LUMO levels), [10][11][12][13][14][15][16][17][18][19][20][21] it is more difficult to predict the inuence of tailoring heteroatom position on optoelectronic properties. 22,23 Herein, we raise the question of whether it is possible to rationally customise optical bandgap properties of a PAH by a specic heteroatom doping pattern and if this can be conceived a priori.…”

Transforming Non-innocent Phenalenyl to a Potent Photoreductant: Captivating Reductive Functionalization of Aryl Halides through Visible-Light-Induced Electron Transfer Processes

“…Although various closed-shell organic molecules have been utilized in PET, excited-state potentials beyond −2.0 V vs SCE remain difficult to achieve (Figure a) . However, there are only few reports of a PET process which is capable to work beyond this redox window to reduce abundant halide feedstocks by involving open-shell organic radicals as excited-state donors (Figure b). − Pioneer work by König et al . utilized a consecutive PET (ConPET) with perylene diimide ( PDI ) system to harness the energy of multiple photons of visible light in a single catalytic cycle to achieve photocatalytic reduction .…”