Photoredox Annulation of Polycyclic Aromatic Hydrocarbons

Abstract

The rise of interest in using polycyclic aromatic hydrocarbons (PAHs) and molecular graphenoids in optoelectronics has recently stimulated the growth of modern synthetic methodologies giving access to intramolecular aryl–aryl couplings. Here, we show that a radical-based annulation protocol allows expansion of the planarization approaches to prepare functionalized molecular graphenoids. The enabler of this reaction is peri-xanthenoxanthene, the photocatalyst which undergoes photoinduced single electron transfer with an ortho-oligoarylenyl precursor bearing electron-withdrawing and nucleofuge groups. Dissociative electron transfer enables the formation of persistent aryl radical intermediates, the latter undergoing intramolecular C–C bond formation, allowing the planarization reaction to occur. The reaction conditions are mild and compatible with various electron-withdrawing and -donating substituents on the aryl rings as well as heterocycles and PAHs. The method could be applied to induce double annulation reactions, allowing the synthesis of π-extended scaffolds with different edge peripheries.

Parameters Influencing the Photochemical Cyclodehydrochlorination (CDHC) Reaction

“…[41] Thioxanthen-9-one undergoes rapid intersystem crossing exhibiting triplet quantum yields close to unity and shows a strong absorption band between 350 and 400 nm, meaning that UVA (365 nm) can be used for the irradiation. [42,43] The reaction was first performed in THF with a substrate concentration of 5 mM (Entry 1, Table 4). The conversion was incomplete after 21 hours.…”