Yoshihisa Takeyama

Cavity‐Directed, Highly Stereoselective [2+2] Photodimerization of Olefins within Self‐Assembled Coordination Cages

Selective encapsulation and isolation of molecules are one of the most attractive features of cagelike molecules.[1] Intermolecular chemical reactions of two or more substrates encapsulated in a molecular cage can be remarkably accelerated and suitably controlled as a result of the dramatically increased concentration and the strictly regulated orientation of the substrates in the cavity.[2] Such systems provide artificial mimics of the sophisticated active site of enzymes.[3] Recently we reported that structurally well-defined coordination cages (1 and 2), which self-assemble from six metal ions and four tridentate ligands, selectively encapsulate large organic molecules at the fixed position of the nanosized cavity.[4, 5] Thus, they are expected to facilitate intermolecular [2 2] photochemical reactions and control the stereoand regiochemistry in stringent geometrical environment. The photodimerization has been studied extensively in some media such as micelles, zeolites, organic hosts (for example, cyclodextrins and cucurbiturils),[6] and crystals.[7] However, a high degree of stereoand regiochemical control is still desired. Here we report remarkably accelerated, highly stereoregulated [2 2] photodimerization of acenaphthylenes (3)[8] and naphthoquinones (4)[9] within the coordination cages (1 and 2) in an aqueous medium that give rise to only syn and head-to-tail isomers. Quantitative formation of a syn dimer of acenaphthylene (3a) within cage 1 was clearly observed in the following experiment: An excess amount of 3a was suspended in a solution of 1 in D2O (2.0 m ) for 10 min at 80 C. Analysis of the D2O solution after filtration of free 3a by 1H NMR spectroscopy showed formation of the encapsulation complex 1 ¥ (3a)2 had occurred (Figure 1a). The signals of 3a were highly upfield-shifted as a result of the efficient encapsulation in the cavity. After the clear solution was irradiated (400 W) for 0.5 h at room temperature, the signals derived from 3a completely disappeared and one set of new signals appeared at 5.84, 5.61, 3.39, and 2.87 (Figure 1b). The signals of 1