Selective dimerization of 2-cyclopenten-1-one, which proceeds through the [2+2]-cycloaddition mechanism, was successfully carried out under soft UV irradiation with a reagent trapped within the pores of a metal–organic framework. Such an approach allowed us to achieve very high (up to 98%) selectivity in the formation of the head-to-tail anti-dimer, among other possible diastereomeric dimers, while a similar selectivity rarely exceeds 60% using other known approaches [Photochem. Photobiol. Sci., 2002, 1, 991] The metal–organic framework used, [Eu₂(DMF)₄(ttdc)₃] (DMF = N,N-dimethylformamide; H₂ttdc – trans-thienothiophene-2,5-dicarboxylic acid), containing regular nano-sized pores, acted as a matrix—a “reaction vessel”—which adsorbed the reagent molecules. The highly ordered arrangement of 2-cyclopenten-1-one molecules in the MOF pores, facilitated by the host–guest hydrogen bonds, predetermined the geometry of its dimer at the molecular level and subsequently the extremely high selectivity of its formation, as confirmed by ¹H NMR spectroscopy. The stability and geometric rigidity of the porous coordination network structure exclude the formation of other possible product isomers in the reaction. The crystal structure o the inclusion compound of the target product with the MOF was determined by single-crystal X-ray diffraction analysis, providing unambiguous confirmation of the reaction occurring distinctively within the voids of the coordination framework and additional insights into the reaction selectivity in terms of the host–guest interactions. Similar results were obtained for 2-methyl-2-cyclopenten-1-one, while for the 3-methylated derivative, its diastereoselective [2+2] dimerization was found to be impeded due to the different guest orientation in the MOF pores, which did not facilitate a specific product-like pre-organization of the reagent molecules [Chem. Commun., 2023, 59, 9380].

This work was supported by the Russian Science Foundation, Project № 19-73-20087.