Isomer-dependent reactivity in the solid state: topochemical [4 + 4] <i>vs.</i> [4 + 2] cycloaddition reactions

This study investigates the reactivity of constitutional isomers of acylated <i>N</i>-hydroxybenzotriazole (HOBt) in solid-state photocycloaddition reactions. To understand the difference in solid-state reactivity of the isomers, we have synthesized two isomeric <i>N</i>- and <i>O</i>-acyl derivatives of HOBt <i>via</i> acylation with anthracene-9-carboxylic acid. Both <i>O</i>-anthr-9-oyl oxy-benzotriazole 1 and its <i>N</i>-acyl isomer 2 crystallized with the face-to-face arrangement of their anthracene units, with a separation of 3.8 Å between them, satisfying Schmidt's criteria, suggesting that they can undergo a topochemical [4 + 4] photocycloaddition reaction. Upon irradiation using blue light (456 nm), the isomer 2 underwent a quantitative [4 + 4] cycloaddition reaction yielding the dimer D2. However, isomer 1 did not undergo [4 + 4] cycloaddition; instead, it underwent a photochemical [4 + 2] cycloaddition with molecular oxygen, eventually yielding anthraquinone (AQ). The dimer D2 underwent a retro-cycloaddition reaction to revert back to the isomer 2. While isomers give the same product in solution-state reactions, our study establishes that isomers can react differently in the solid state and can give different products.