Shinnosuke Horiuchi

Naphthalene Diels−Alder in a Self-Assembled Molecular Flask

Despite its inertness toward pericyclic reactions under common conditions, naphthalenes readily undergo Diels-Alder reactions when coencapsulated with a suitable dienophile within the cavity of a self-assembled host. Localization of the reactant pair significantly reduces the entropic cost of the reaction, and preorganization within the host cavity controls both the regio- and stereoselectivity of the reaction: electronically disfavored exo adducts were obtained, and with substituted naphthalenes, the reaction takes place on the less electron-rich, unsubstituted ring. Our findings highlight the fact that judicious tuning of substrate size and shape within molecular flasks can unveil new and unusual reactivities for otherwise unreactive molecules.

Noncovalent Trapping and Stabilization of Dinuclear Ruthenium Complexes within a Coordination Cage

A dinuclear ruthenium complex, [(η(5)-indenyl)Ru(CO)(2)](2), was noncovalently enclathrated within a self-assembled coordination cage. In the cavity, rapid cis-trans isomerization and ligand exchange between the terminal and bridging carbonyls were suppressed, and only the carbonyl-bridged cis configuration was observed by X-ray crystallographic analysis.

Diels–Alder Reactions of Inert Aromatic Compounds within a Self-Assembled Coordination Cage

A self-assembled coordination cage serves as a nanometer-sized molecular flask to promote the Diels-Alder reactions of aromatic hydrocarbons with N-cyclohexylmaleimide. The coordination cage accelerated the Diels-Alder reaction of anthracene at the electronically unfavorable, terminal benzene ring to give a compact, cavity-restrained syn-adduct. Activation-parameter measurements for the reactions revealed considerable reduction in the entropy cost, and preorganization of the substrates is a dominant factor in the enhanced reactivity. Owing to this entropy-cost reduction, otherwise-unreactive aromatic compounds, such as naphthalenes or triphenylene, also underwent Diels-Alder reactions in a regio- and stereocontrolled fashion. In the naphthalene Diels-Alder reaction, X-ray crystallographic analysis of the guest-inclusion complex clarified the reinforced orientation and proximity of the substrate pairs before the reaction. A perylene Diels-Alder adduct was stabilized inside the cage and protected from aerial oxidation.

Remote chiral transfer into [2+2] and [2+4] cycloadditions within self-assembled molecular flasks

A self-assembled chiral coordination cage was prepared from triangular triazine-panel ligands and Pd(II) complexes with chiral diamine auxiliaries. The chiral environment of the cage is induced by the structural deformation of the triazine panels ascribed to the steric bulk of the substituents on the chiral auxiliaries. The chiral cage can accommodate a pair of two hydrophobic molecules to form a specific diastereomeric ternary complex. We succeeded in conducting unusual [2+2] and [2+4] asymmetric cycloadditions from the identical ternary complex including a maleimide derivative and aromatic compounds with 6–50% enantiomeric excess (ee). It is remarkable that the remote chirality on the auxiliaries is efficiently transmitted to the chiral orientation of achiral ligands, which define a chiral cavity, to induce up to 50% ee. The present strategy is widely applicable to cavity-directed asymmetric reactions and maintaining inherent properties of the cage.

Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing Ammonia

The fundamental biogeochemical cycle of nitrogen includes cytochrome c nitrite reductase, which catalyzes the reduction of nitrite ions to ammonium with eight protons and six electrons (NO2- + 8H+ + 6e- → NH4+ + 2H2O). This reaction has motivated researchers to explore the reduction of nitrite. Although a number of electrochemical reductions of NO2- have been reported, the synthetic nitrite reduction reaction remains limited. To the best of our knowledge, formation of ammonia has not been reported. We report a three-step nitrite reduction cycle on a dinuclear ruthenium platform {(TpRu)2(μ-pz)} (Tp = HB(pyrazol-1-yl)3), producing ammonia. The cycle comprises conversion of a nitrito ligand to a NO ligand using 2H+ and e-, subsequent reduction of the NO ligand to a nitrido and a H2O ligand by consumption of 2H+ and 5e-, and recovery of the parent nitrito ligand. Moreover, release of ammonia was detected.