Coordination-driven self-assembly of anthraquinone-based metal-organic cages for photocatalytic selective [2 + 2] cycloaddition.

Abstract

Visible-light-promoted [2 + 2] cycloaddition provides a straightforward and efficient way to produce cyclobutanes, which are the core skeleton in commercial pharmaceuticals and fine chemicals. However, the control of the conformation to produce syn-head-to-head (syn-HH) cyclobutanes remains a grand challenge. In this work, we report the design and synthesis of anthraquinone-based metal-organic cages (MOCs) for the [2 + 2] photocycloaddition of chalcones to generate syn-HH cyclobutanes. Guided by the coordination-driven self-assembly strategy, one D2 and three D4h symmetric MOCs are constructed from anthraquinone-derived dicarboxylate linkers and 4-tert-butylsulfonylcalixarene capped tetrametallic clusters. The porous cages feature large hydrophobic cavities and photoactive anthraquinone units and are demonstrated to be efficient and recyclable photocatalysts for [2 + 2] cycloaddition of chalcones. The syn-HH diastereomers are obtained with up to 13 : 1 diastereomeric ratio (dr). The cage catalysts provide a well-defined confined space to accommodate the substrates, thus leading to enhanced selectivity relative to the free anthraquinone catalyst.