Hiroki Takezawa

We synthesized a double-walled knotted cage from a flexible tripodal ligand. The characteristic double-walled structure provided a unique adaptive behavior of the cavity upon inclusion of organic molecules, which was evidenced by NMR and X-ray measurements. The semiflexible host framework, restricted by the knotted topology, enabled kinetic molecular recognition revealing the sequential binding of two different guests from their mixture.

Abstract When substrates are confined in an isolated cavity, they experience circumstances that are distinctly different from those in a bulk solution. Molecular self-assembly has widened the potential of molecular confinement by offering synthetic cavities on the nanometer-scale and allowing chemists to treat molecular aggregates and larger molecules in the cavities. In this account, we introduce the molecular confinement effects of self-assembled cages as a strategy to discover new or hidden p

Polyfluorinated aliphatic compounds were encapsulated by a self-assembled M6L4 coordination host in aqueous media. NMR titration and X-ray crystallographic analyses clearly revealed that the aggregation of the fluorinated moieties of the guests in the host cavity plays a significant role in the binding. Polyfluorinated aromatics did not show such aggregation in the cavity because of their "nonfluorous" nature.

Even flexible linear substrates are conformationally fixed within the hydrophobic confined cavities of enzymes. This enables preorganization of the substrates and their positioning in close proximity to the active center to facilitate stereo- and site-selective reactions. Here, we demonstrate the site-selective electrophilic addition of linear diterpenoids within a self-assembled coordination cage. The reactions proceed through folding of the linear substrates into a U-shaped conformation, which

An overcrowded alkene with an anti-folded conformation was converted to its twisted conformer, accompanied by a dramatic color change from yellow to deep purple, by inclusion in a self-assembled T(d)-symmetric coordination cage. The shape of the caged cavity was suitable and desirable for trapping of the twisted conformer. The twisted conformation was temporarily memorized in the alkene even after guest ejection. Permanent trapping of the twisted conformation was achieved by bromination of the t

Phthalein dyes in the quinone dianion form (pseudo-D3, colored) are transformed into the lactone dianion form (pseudo-T(d), colorless) through encapsulation in a T(d)-symmetric host even under basic conditions (pH ∼10). The compatibility in size and symmetry between the lactone and the cavity is essential to the transformation. Upon addition of a guest that strongly binds to the cavity, the encapsulated phenolphthalein is expelled, the color of the basic solution is regained, and the host-guest

The confined space inside a self-assembled cage enhanced halogen bonding (XB) between iodoperfluorocarbons (XB donors) and NO3(-) anions or H2O molecules (XB acceptors), as confirmed by NMR spectroscopy in solution and by X-ray crystallography in the solid state. The cavity also bound an XB donor-acceptor pair, C6F3I3 and C6H5NMe2, in a selective pairwise fashion.

The ability of a cationic coordination cage to encapsulate molecular guests is enhanced by non-covalent capping of the cage portals with tripodal anions. The capped cage provides new cation binding sites at the portals, which enable accommodation of cationic substrates within the cationic cage. In addition, non-covalent capping allows neutral guests in the cage to be exchanged for cationic ones on demand.

The tetradehydro-Diels-Alder (TDDA) reaction is a useful transformation for the rapid assembly of polycyclic scaffolds from simple linear precursors in a single synthetic step. However, the reaction requires careful substrate design and harsh reaction conditions to overcome the inherent entropic cost for this transformation. Herein we report an efficient site-selective TDDA transformation within a self-assembled Pd₆L₄ cage. Despite the large size, the flexibility of the emp

Desymmetrization of a symmetric skeleton enables late-stage functionalization of molecules. However, reagent-controlled desymmetrization by site-selective reactions of symmetric molecules remains a difficult synthetic strategy. Here, we found that complete confinement of a symmetric molecule within a coordination cage can desymmetrize the guest conformation, making it possible to site-selectively activate or protect the otherwise equivalent reaction sites of calix[4]arene derivatives. Multistep,

The M6L4 cage, self-assembling from six Pd(II) or Pt(II) 90-degree blocks and four triazine-cored triangular ligands, has an effective hydrophobic cavity of about 450 Å3 capable of encapsulating one or more small molecules. Here, from the same components, we successfully constructed an M9L6 cage with an internal volume expanded to 1540 Å3 via the self-assembly of an M8L6 precursor using pillar[5]arene as a template. This cage retains the high molecular recognition ability of the M6L4 cage while

The properties of rare-earth-metal ions in aqueous media often depend on their various hydration modes, which allow them to exist as monomeric or oligomeric species with different coordination numbers. Capturing rare-earth-metal ions in a confined cavity can fix their hydration modes and thus enable their intrinsic properties to be distinguished. However, the isolation of ionic species from bulk aqueous media is not an easy task due to competitive interaction with bulk water. Here, we report the

A 2-biphenylacetylene was fixed into a specific conformation within the confined cavity of a hollow cage, where it underwent a regioselective spirocyclization in the presence of an electrophile. A 5-endo-dig cyclization proceeded selectively in the cage, which stands in sharp contrast to the 6-endo-dig cyclization that normally occurs in common organic media. The folded conformation adopted by the substrate within the cage was examined by ¹ H NMR spectroscopy and X-ray crystallographi

A molecular host with photosensitizing centers provides photo-responsive host-guest properties based on its molecular recognition ability. Here, we construct a self-assembled photoactive Ir(<b>iii</b>) cage-shaped complex that contains anion binding pockets on its rim. The anion recognition ability of the complex enables efficient catalysis of the visible-light-induced <i>E</i>-<i>Z</i> isomerization of an anionic styrene derivative.