Radical cation cycloadditions have long been known as unique reactions and yet are intensively studied chemical transformations in the field of modern synthetic organic chemistry. The key to developing new reactions of this class is the generation and control of highly reactive radical cation intermediates. Herein, we report TiO2 photochemical [2+2] and [4+2] cycloadditions using aryl vinyl ethers as radical cation precursors. The constructions of various four‐ and six‐membered rings are made possible by these radical cation cycloadditions. Although detailed mechanistic aspects remain unclear, 2,3‐dimethyl‐1,3‐butadiene (3) is found to be more radicalophilic than 2‐ethyl‐1‐butene (2) and effectively traps the radical cation of the aryl vinyl ethers to construct six‐membered rings.

Both intermolecular and intramolecular electron transfers can be the key in the determination of synthetic outcomes of photochemical and electrochemical reactions. Herein, we report dispersed TiO2 nanoparticles in combination with methoxybenzene to be unique heterogeneous photocatalysts for facilitating the formation of novel cyclobutanes. Although the mechanistic details are as yet unclear, the results described herein imply that methoxybenzene is adsorbed onto the TiO2 surface, coming in close proximity to the forming cyclobutane radical cation, to realize a pseudo-intramolecular electron transfer between the species.