Reaction in well-designed solids allows yielding products with high selectivity and unique compounds that cannot be obtained in solution. However, the precise tuning of the arrangement of reactants in solids for the versatile application of solid-phase reactions remains a challenging subject. Here, a [2 + 2] photocyclization reaction at different positions of the carbon− carbon bonds is described in which the spatial arrangement of 4-styrylpyridines (4-spy) is changed by guest molecules in a flexible metal−organic framework. The 4spy molecules undergo photodimerization between two carbon−carbon double bonds in the guest-free framework, whereas a reaction between olefinic and aromatic carbon− carbon bonds or the absence of reaction takes place in the solvent-incorporated form. This reactivity, which can be termed as "dynamic topochemistry" contributes to enforce the applicability of solid-phase reactions in synthetic chemistry.

Topochemical reactions provide selective products based on the molecular position; however, they generally require molecules to be placed in strictly limited orientations and distances, making them less versatile. In this study, we found that by confining trans-4-styrylpyridine (4spy) as a reactive substrate in a flexible metal−organic framework (MOF) nanospace, [2+2] cycloadducts can be selectively obtained, even when the distance between two C�C bonds of 4-spy in the crystal is 5.9 Å, which is much larger than the conventionally observed upper limit (4.2 Å). Such an unusual cyclization reaction is suggested to occur due to the transient proximity of the 4-spy due to the "swing" motion in the nanospace. The MOF nanospace, with its high degree of molecular structural freedom, can be applied to different platforms that do not require the fine constraints of reactive distances for solid-phase reactions.