Radical cation Diels–Alder reactions of arylidene cycloalkanes

Ag3PO4 enables the generation of long-lived radical cations for visible light-driven [2 + 2] and [4 + 2] pericyclic reactions

Photocatalytic redox reactions are important for synthesizing fine chemicals from olefins, but the limited lifetime of radical cation intermediates severely restricts semiconductor photocatalysis efficiency. Here, we report that Ag3PO4 can efficiently catalyze intramolecular and intermolecular [2 + 2] and Diels-Alder cycloadditions under visible-light irradiation. The approach is additive-free, catalyst-recyclable. Mechanistic studies indicate that visible-light irradiation on Ag3PO4 generates holes with high oxidation power, which oxidize aromatic alkene adsorbates into radical cations. In photoreduced Ag3PO4, the conduction band electron (eCB-) has low reduction power due to the delocalization among the Ag+-lattices, while the particle surfaces have a strong electrostatic interaction with the radical cations, which considerably stabilize the radical cations against recombination with eCB-. The radical cation on the particle's surfaces has a lifetime of more than 2 ms, 75 times longer than homogeneous systems. Our findings highlight the effectiveness of inorganic semiconductors for challenging radical cation-mediated synthesis driven by sunlight.

Arene C-H Amination with N-Heteroarenes by Catalytic DDQ Photocatalysis

Arene C-H aminations using catalytic amounts of a 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) photocatalyst are described. Benzene, which has an oxidation potential of 2.48 V (vs SCE), was functionalized by pyrazoles, triazoles, tetrazoles, purines, and tert-butoxycarbonyl amine. Arenes underwent amination via a combination of ultraviolet (UV) light and a DDQ photocatalyst without a typical co-oxidant. Although the mechanism remains an open question, DDQH₂, which is generated from DDQ after oxidation, is reactivated to DDQ under UV light irradiation conditions, possibly with the assistance of adventitious O₂ and/or a solvent as the terminal oxidant(s) in this system.