Mechanistic insights into photochromic behavior of a ruthenium(II)-pterin complex

The pterin‐coordinated ruthenium complex, [Ru^II(dmdmp)(tpa)]⁺ ( 1 ) (Hdmdmp=N,N‐dimethyl‐6,7‐dimethylpterin, tpa=tris(2‐pyridylmethyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex ( 2 ) was fully characterized to reveal an apparent 180° pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the Ru^II center to give an intermediate complex, [Ru(dmdmp)(tpa)(acetone)]²⁺ ( I ), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (?) of this photoreaction was determined to be 0.87 %. The subsequent thermal process from intermediate I affords an isomeric complex 2 , as a result of the rotation of the dmdmp^2? ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first‐order kinetics, and the rate constant at 298 K was determined to be 5.83×10^?5 s^?1. The activation parameters were determined to be ΔH^≠=81.8 kJ mol^?1 and ΔS^≠=?49.8 J mol^?1 K^?1. The negative ΔS^≠ value indicates that this reaction involves a seven‐coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2 , without going back to starting complex 1 , and thus the reaction proceeds with 100 % conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1 . The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first‐order process in acetonitrile were determined to be ΔH^≠=59.2 kJ mol^?1 and ΔS^≠=?147.4 kJ mol^?1 K^?1. The largely negative ΔS^≠ value suggests the involvement of a seven‐coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru^II center is a determinant factor in the photoisomerization of the Ru^II–pterin complex.     

Enantioselective organocatalytic reductive amination of aliphatic ketones by benzothiazoline as hydrogen donor

The chiral phosphoric acid-catalyzed enantioselective reductive amination of aliphatic ketones with aromatic amines was successfully achieved by the use of benzothiazoline as the hydrogen donor. Corresponding chiral aliphatic amines were obtained with excellent enantioselectivities.     

Concise route to 3-arylisoquinoline skeleton by Lewis acid catalyzed C(sp3)-H bond functionalization and its application to formal synthesis of (±)-tetrahydropalmatine

An expeditious route to furnish an isoquinoline skeleton via hydride shift mediated C-H bond functionalization was developed. In this process, an unusual [1,5]-H shift without the assistance of the adjacent heteroatom took place to produce tetrahydroisoquinoline derivatives in good to excellent chemical yields. The formal synthesis of (±)-tetrahydropalmatine was achieved by exploiting this new transformation.     

Chiral phosphoric acid catalyzed enantioselective transfer deuteration of ketimines by use of benzothiazoline as a deuterium donor: synthesis of optically active deuterated amines

By use of 2-deuterated benzothiazoline as a deuterium donor in combination with a chiral phosphoric acid, the transfer deuteration of ketimine and α-iminoester took place smoothly to give α-deuterated amines in high yields with excellent enantioselectivities. The remarkable kinetic isotope effect suggests that carbon-deuterium bond cleavage is the rate-determining step.     

Chiral Brønsted acid-catalyzed hydrophosphonylation of imines—DFT study on the effect of substituents of phosphoric acid

The enantioselective hydrophosphonylation reaction of diisopropyl phosphite with aldimine furnished α-amino phosphonates with high enantioselectivities by means of a chiral phosphoric acid. DFT calculation of the effect of 3,3′-substituents of the phosphoric acid revealed the reason for the high enantioselectivities.     

Irreversible dynamics of the phase boundary in U(Ru0.96Rh0.04)2Si2 and implications for ordering

We report measurements and analysis of the specific heat and magnetocaloric effect-induced temperature changes at the phase boundary into the single magnetic field-induced phase (phase II) of U(Ru0.96Rh0.04)2Si2, which yield irreversible properties similar to those at the valence transition of Yb(1-x)Y(x)InCu4. To explain these similarities, we propose a bootstrap mechanism by which lattice parameter changes caused by an electric quadrupolar order parameter within phase II become coupled to the 5f-electron hybridization, giving rise to a valence change at the transition.     

Molecular evolution of toxin genes in Elapidae snakes

The venom of the sea krait, Laticauda semifasciata, consists primarily of two toxic proteins, phospholipase A₂ (PLA₂) and a three-finger-structure toxin. We have cloned both toxic protein genes, including the upstream region. PLA₂ genes contain three types of inserted sequences: an AG-rich region, a chicken repeat 1-like long interspersed nucleotide element sequence and an intron II 3′ side repeat sequence. The molecular divergence of L. semifasciata PLA₂ genes was defined on the basis of the inserted sequences and their sequence homology. The length of intron I in the three-finger-structure toxin genes differs from species to species. The alignment analysis of intron I of the three-finger-structure toxin genes revealed that the intron I sequence of the ancestral gene comprised ten genetic regions. A hypothetical evolutionary process for the three-finger-structure toxin genes has also been developed.     

Effect of ZnO plasma plume dynamics on laser ablation

The dynamic behaviors and optical properties of a ZnO plasma plume produced by pulsed laser ablation using a Nd:YAG laser (wavelength: 532 nm, pulse width: 3 ns) were studied by fast photography using a commercial gated charge coupled device (CCD) camera linked with a delay circuit and by optical emission spectroscopy at various ambient oxygen pressures. Fast photography was conducted with a resolving power of 0.25 μs and the expansion behaviors of the laser ablation plume were observed. Plasma plume expansion velocity decreased with oxygen partial pressure. The flow of the plasma plume in the early stage of expansion of up to 3 ms agreed well with the drag model.     

Neutron scattering studies on heavy-fermion superconductors

We review recent neutron scattering studies on uranium-based heavy-fermion superconductors. The coupling between magnetic and superconducting order parameters was observed in UPt₃, UPd₂Al₃, URu₂Si₂, and UNi₂Al₃. In UPd₂Al₃, the superconducting gap appears in the spin excitation spectra. These results are indicative of the strong interplay between magnetism and superconductivity. We also report the unusual behaviors of the weak antiferromagnetic ordering, the long-range magnetic correlation in UPt₃ at ultra-low temperatures, and the pressure-induced magnetic transition from the weak (0.02μ_B/U) to a high moment state (0.4μ_B/U) at 1.5 GPa in URu₂Si₂.     

Fluorescent lanthanide chelates for biological systems

Certain lanthanide chelate complexes are known to emit strong fluorescence with very distinct physical properties that are different from those of organic fluorescent compounds: the fluorescence of lanthanide complexes is long-lived with the half decay-time of several hundreds microseconds to 2 ms. The complexes are excited by UV light and emit fluorescence in the visible region. The emission profile is very sharp and the wavelength is specific to each metal, for instance, Eu³⁺ complexes emit at 615 nm and Tb³⁺ at 545 nm regardless of the ligand. These properties show that the complexes can be excellent fluorescence labels for proteins and DNAs and, when time-resolved fluorometry is employed, provide highly sensitive detection methods in biotechnology. Among many labels we have developed, BHHCT-Eu³⁺ and BPTA-Tb³⁺ are suitable for immunoassay, DNA hybridization assay, and DNA chip technology. Homogeneous DNA hybridization assay systems using fluorescence resonance energy transfer and fluorescence intercalators will be introduced.