Photochemical Radical Cation Cycloadditions of Aryl Vinyl Ethers

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.