Quantitative Formation of Octa-substituted Cyclobutanes by the [2+2] Photocycloaddition of Stiff-stilbenes.

The creation of new molecules with specific structural motifs is a primary goal in synthetic chemistry. In this context, cyclobutanes (CBs), the highly strained ring systems, are of great interest because of their wide applications from pharmaceutical chemistry to materials science. The [2+2] photocycloaddition is the most straightforward approach for CBs; however, access to fully substituted cyclobutanes presents a significant challenge due to the steric hindrance imposed by eight substituents. Here, we report an efficient synthesis of structurally unique, octa-substituted cyclobutanes from stiff-stilbene precursors, which is enabled by adaptable anion coordination through hydrogen bonding networks. The synthesized spiro-benzocyclopentyl-substituted cyclobutanes are confirmed by single-crystal structures. DFT calculations indicate that these cyclobutanes have high ring-strain energies (~20.1 kcal/mol) and can be completely converted back to stiff-stilbenes. Such a reversible cycloaddition/reversion process offers a promising platform for applications in responsive and energy storage materials.