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

Area of Science:

• Organic Chemistry
• Photochemistry
• Solid-State Chemistry

Background:

• Constitutional isomers of N-hydroxybenzotriazole (HOBt) derivatives are key in chemical synthesis.
• Understanding solid-state reactivity is crucial for crystal engineering and materials science.
• Acylated HOBt derivatives offer a platform to study isomer-specific solid-state reactions.

Purpose of the Study:

• To investigate the solid-state photocycloaddition reactivity of constitutional isomers of acylated HOBt.
• To elucidate the differences in photochemical behavior between N-acyl and O-acyl HOBt isomers.
• To compare solid-state photochemical reactions with known solution-state behaviors.

Main Methods:

• Synthesis of N- and O-acyl derivatives of HOBt using anthracene-9-carboxylic acid.
• Crystallization and structural analysis of the synthesized isomers.
• Solid-state irradiation experiments using blue light (456 nm) and analysis of reaction products.

Main Results:

• Both isomers crystallized with face-to-face anthracene arrangements, meeting criteria for topochemical [4 + 4] photocycloaddition.
• The N-acyl isomer underwent quantitative [4 + 4] photocycloaddition to form a dimer.
• The O-acyl isomer underwent [4 + 2] cycloaddition with molecular oxygen, yielding anthraquinone, and its dimer showed retro-cycloaddition.

Conclusions:

• Constitutional isomers of acylated HOBt exhibit divergent photochemical reactivity in the solid state.
• Solid-state reactions can yield different products from the same isomers, contrasting with solution-state behavior.
• The study highlights the importance of crystalline arrangement in dictating photochemical outcomes.