Aromatic alkene radical cations are crucial intermediates in synthesizing complex molecules and cyclic moieties, particularly [2+2] and [4+2] pericyclic products. While single electron oxidants like Ce⁴⁺, Fe³⁺, and hypervalent iodine reagents exist, photocatalysts (PCs) offer a greener alternative by generating highly oxidizing holes under sunlight irradiation. However, homogeneous organic PCs, such as transition metal-coordination complexes and π-conjugated molecules, are limited by the short lifetime of radical cations (on a μs scale). Inorganic semiconductor PCs (isPCs) like TiO₂, CdS, Bi₂MoO₆, and Ag₃PO₄, offer stability, recyclability, and cost-effectiveness, but their efficiency in 1e-oxidative activation of C=C moieties is typically low due to the short lifetime of holes (fs to ns). This study addresses the challenge of extending the lifespan of alkene radical cations for efficient and recyclable photocatalysis in pericyclic reactions. Ag₃PO₄, with its visible-light response, strong oxidizing holes, and weak reducing electrons, is investigated due to its potential to efficiently generate and stabilize radical cations on its surface through electrostatic interactions with the PO₄³⁻ anions.

Extensive research has been dedicated to generating and utilizing aromatic alkene radical cations. Homogeneous organic photocatalysts have been explored, but they suffer from the short lifetimes of the generated radical cations, limiting their efficiency. Inorganic semiconductor photocatalysts offer advantages in terms of stability and recyclability, but their application in 1e-oxidative activation of non-polar C=C bonds is often hampered by the short lifetime of the photogenerated holes and the tendency for the generated radical cations to be quenched by electrons. Among inorganic semiconductor photocatalysts, Ag₃PO₄ shows promise due to its visible-light absorption, production of strong oxidizing holes, and weak reducing electrons. The high charge density of the PO₄³⁻ anions on the Ag₃PO₄ surface allows for strong electrostatic interactions with cationic species, potentially stabilizing radical cations.

The study used a self-synthesized Ag₃PO₄ sample (nanospheres, 230 ± 60 nm diameter) for the model reaction of anethole (1a) to anti-cyclobutane 2a in HFIP under N₂ atmosphere and visible light irradiation (425 nm LED). Control experiments excluded thermocatalytic mechanisms. Various Ag₃PO₄ samples, solvents, and atmospheres were tested. The apparent quantum yield (AQY) was determined, and the catalyst was successfully reused for five cycles with minor efficiency loss and easily regenerated. Other silver salts (AgCl, AgBr, AgI) and commonly used heterogeneous photocatalysts (g-C₃N₄, Bi₂MoO₆, TiO₂) were compared to Ag₃PO₄. The scope of Ag₃PO₄ photocatalysis was explored for various intermolecular and intramolecular [2+2] cycloadditions and Diels-Alder [4+2] cycloadditions, under both LED and natural sunlight irradiation. Laser flash photolysis (LFP) was used to detect transient species and measure the lifetime of the 1a^•+ radical cation. Langmuir adsorption isotherms were used to quantify adsorption of reactants and products on Ag₃PO₄ surfaces. Density functional theory (DFT) simulations provided insights into the adsorption configurations and electronic interactions between 1a/2a and Ag₃PO₄ surfaces. Large-scale synthesis of [2+2] and [4+2] products were also performed under sunlight.

Ag₃PO₄ efficiently catalyzed intramolecular and intermolecular [2+2] and Diels-Alder cycloadditions with high yields and diastereoselectivity. The reaction was successful with a broad scope of substrates, showing tolerance to various substituents and steric hindrances. A high apparent quantum yield (AQY) was observed, particularly in HFIP, which was found to be superior to other solvents. The catalyst was easily recyclable through a simple regeneration process. LFP confirmed the formation of long-lived 1a^•+ radical cations on the Ag₃PO₄ surface, with lifetimes exceeding 2 ms (75 times longer than in homogeneous systems). The long lifetime is attributed to the low reduction power of conduction band electrons in the photoreduced Ag₃PO₄ and the strong electrostatic interaction between the radical cation and the negatively charged Ag₃PO₄ surface. The reaction follows a Langmuir-Hinshelwood mechanism, with a rate-limiting step involving two adsorbed 1a molecules. DFT calculations confirmed the strong adsorption of 1a on the PO₄³⁻-terminated (100) facet of Ag₃PO₄, leading to activation of the CH=CH moiety for oxidation. Large-scale synthesis under sunlight irradiation demonstrated the scalability of the method. The study shows that Ag₃PO₄ outperforms other silver salts and common photocatalysts in the tested reactions.

The findings demonstrate the effectiveness of Ag₃PO₄ as a photocatalyst for pericyclic reactions, achieving high yields and diastereoselectivity across a range of substrates. The remarkably long lifetime of the radical cation intermediate, enabled by the unique properties of Ag₃PO₄, is a key factor in the success of this approach. This contrasts with homogeneous systems and other heterogeneous photocatalysts, which typically suffer from short-lived radical cation intermediates, limiting their overall efficiency. The mechanistic insights gained from LFP and DFT calculations provide a detailed understanding of the interfacial interactions and reaction pathways. The ability to perform large-scale synthesis under sunlight further underscores the practical applicability and sustainability of this method. The work opens avenues for developing new strategies for challenging radical cation/anion-mediated reactions using inorganic semiconductor photocatalysts.

This study successfully demonstrates the high efficiency and versatility of Ag₃PO₄ as a photocatalyst for various pericyclic reactions, including [2+2] and [4+2] cycloadditions. The key to its success is the generation of long-lived radical cation intermediates stabilized on the photocatalyst surface. The methodology is sustainable, scalable, and easily applicable, opening new prospects for industrial fine chemical synthesis. Future research can explore the application of Ag₃PO₄ to other challenging radical-mediated reactions and investigate other inorganic semiconductors for similar applications.

The study primarily focuses on electron-rich aromatic alkenes. The applicability of this method to other substrates with different electronic properties requires further investigation. While the catalyst is recyclable, there is a slight decrease in efficiency after multiple cycles. Optimization of the regeneration process could improve the catalyst's lifespan and maintain its activity over longer periods. The DFT calculations were performed on a simplified model of the Ag₃PO₄ surface; studying a more realistic surface model might offer more detailed insights into the reaction mechanism. The effects of sunlight intensity variations need further consideration for large-scale applications.