Yukie Mori

Syntheses and Vibrational Circular Dichroism Spectra of the Complete Series of [Ru((―)- or (+)-tfac)n(acac)3-n] (n = 0 ∼ 3, tfac = 3-Trifluoroacetylcamphorato and acac = Acetylacetonato)

Twenty four kinds of Ru(III) complexes expressed by the formula of Delta- or Lambda-[Ru((-)- or (+)-tfac)(n)(acac)(3-n)] (n = 1, 2, and 3, (-)- or (+)-tfac = (-)- or (+)-3-trifluoroacetylcamphorato and acac = acetylacetonato) were prepared in a pure diastereomeric form. The separation of these diastereomers was accomplished chromatographically by rational use of two antipodal chiral columns. The separated complexes were identified by means of mass spectra, (1)H NMR, electronic circular dichroism, and partly X-ray diffraction analyses. When a pair of Delta- and Lambda-[Ru(acac)(3)] (n = 0) was added to this group, they constituted the complete series of mixed ligand complexes including ligand chirality under the condition of no mixing of chiral ligands. The vibrational circular dichroism (VCD) spectra of their CDCl(3) solutions were recorded in the wavenumber region of 1000 approximately 1800 cm(-1). The results provided a benchmark for systematically examining the effects of the stereochemical properties on VCD spectra such as the degree of ligand substitution, DeltaLambda configurations, geometrical isomerism, and ligand chirality. As a result, the geometrical isomers of trans- or cis-[Ru((-)- or (+)-tfac)(2)(acac)] and mer- or fac-[Ru((-)- or (+)-tfac)(3)] were clearly differentiated by their VCD signals, which was hardly possible from their IR spectra alone.

MAGNETIC FIELD EFFECTS ON THE PHOTOINDUCED ELECTRON TRANSFER OF 10-METHYLPHENOTHIAZINE WITH 4-(4-CYANOBENZOYLOXY) TEMPO IN FLUID SOLUTIONS

Abstract Magnetic field effects on both the quenching of the triplet excited state of 10-methylphenothiazine ( S =1) with 4-(4-cyanobenzoyloxy)TEMPO ( S =1/2) and the dynamic behavior of generated radical ion pairs ( S =1/2 and 3/2) were directly observed by a nanosecond laser photolysis technique. In 2-propanol, with increasing field from 0 T to 2 T, the quenching rate was decreased by (11.8±1.2)% and the yield of the escaped cation radical was increased by (90±8)%.

Modeling of tRNA‐assisted mechanism of Arg activation based on a structure of Arg‐tRNA synthetase, tRNA, and an ATP analog (ANP)

The ATP–pyrophosphate exchange reaction catalyzed by Arg‐tRNA, Gln‐tRNA and Glu‐tRNA synthetases requires the assistance of the cognate tRNA. tRNA also assists Arg‐tRNA synthetase in catalyzing the pyrophosphorolysis of synthetic Arg‐AMP at low pH. The mechanism by which the 3′‐end A76, and in particular its hydroxyl group, of the cognate tRNA is involved with the exchange reaction catalyzed by those enzymes has yet to be established. We determined a crystal structure of a complex of Arg‐tRNA synthetase from Pyrococcus horikoshii, tRNAArgCCU and an ATP analog with Rfactor = 0.213 (Rfree = 0.253) at 2.0 Å resolution. On the basis of newly obtained structural information about the position of ATP bound on the enzyme, we constructed a structural model for a mechanism in which the formation of a hydrogen bond between the 2′‐OH group of A76 of tRNA and the carboxyl group of Arg induces both formation of Arg‐AMP (Arg + ATP → Arg‐AMP + pyrophosphate) and pyrophosphorolysis of Arg‐AMP (Arg‐AMP + pyrophosphate → Arg + ATP) at low pH. Furthermore, we obtained a structural model of the molecular mechanism for the Arg‐tRNA synthetase‐catalyzed deacylation of Arg‐tRNA (Arg‐tRNA + AMP → Arg‐AMP + tRNA at high pH), in which the deacylation of aminoacyl‐tRNA bound on Arg‐tRNA synthetase and Glu‐tRNA synthetase is catalyzed by a quite similar mechanism, whereby the proton‐donating group (–NH–C+(NH2)2 or –COOH) of Arg and Glu assists the aminoacyl transfer from the 2′‐OH group of tRNA to the phosphate group of AMP at high pH.

Magnetic-field effects on reactions of triradicals generated by photolysis of benzophenone–diphenylmethane–nitroxide trifunctional compounds

Abstract Magnetic-field effects on the decay kinetics of chain-linked triradicals have been investigated in acetonitrile at 293 K. The decay rate of the triradical linked by an 8-methylene chain was 1×10 7 s −1 under zero field and decreased with increasing field to attain a constant value of 2×10 6 s −1 at 2–10 T. These values were one order larger than those of the corresponding ketyl-diphenylmethyl biradical.

Structures and Photochemical Reactions of 1-(9-Anthryl)-2-aroylethylenes: Competing cis-trans Isomerization and Skeletal Rearrangement

These compounds rapidly undergo photochemical cis-trans isomerization in solution, while irradiation of cis-(I) in the solid state almost exclusively afforded an intramolecular [4+2] cyclization product, suggesting that the isomerization would be inhibited by crystal lattice constraints. cis-(IV) also underwent [4+2] cyclization in the solid state, while in the case of cis-(III) only cis-trans isomerization was observed both in the solid state and in solution

Contribution of Cation-π Interactions in Iminium Catalysis

Ab initio calculations were carried out for a benzyl-substituted iminium cation derived from (E)-crotonaldehyde and a chiral imidazolidinone that was developed as an organocatalyst by MacMillan et al. At the MP2 level of theory it is predicted that the phenyl group is close to the iminium moiety in the most stable conformer, suggesting that the cation-π interaction contributes to the stabilization of this conformer. Energy decomposition analyses on model systems indicate that the electrostatic and polarization terms make significant contribution to the attractive interactions between the benzene ring and the iminium cation.

Structural aspects of the mechanical and thermal dissociation of the central bond in 2,2′-bis(2,3,4-triarylchromenyl)s

2,2′-Bis(2,3,4-triphenylchromenyl)(1a) has been shown to undergo reversible homolytic cleavage of the C(2)–C(2′) bond to give green-coloured chromenyl radicals when subjected to mechanical force or heating in the solid state. 2,4-Bis-(p-chlorophenyl)(1b), 2,4-bis-(p-tolyl)(1c), 2,4-bis-(p-methoxyphenyl)(1d), and 2,4-bis-(p-bromophenyl)(1e) derivatives have been synthesized, and substituent effects on the radical dissociation investigated. Upon pressing 1b exhibits the highest degree of dissociation, while thermal dissociation is facilitated by electron-releasing methyl (1c) and methoxy (1d) groups. Dissociation enthalpies (ΔH) for 1a–e in dichloromethane solutions are evaluated to be 12–15 kcal mol–1. The small values of ΔH are considered to be mainly due to overcrowding around the C(2)–C(2′) bond. MMP2 calculation predicts that meso-1a is more stable than the (±)-isomer and the lowest-energy conformation of meso-1a is a centrosymmetric anti form. In the optimized geometry of meso-1a C(2)–C(2′) the bond length is calculated to be 1.584 A, which is rather longer than a normal Csp3–Csp2 bond. AM1 calculations give similar results and suggest that through-bond interactions between the lone-pairs on the oxygen atoms and/or 2- and 2′-aryl groups are not significant compared with the steric repulsion.

Effects of counterion and solvent on proton location and proton transfer dynamics of N-H···N hydrogen bond of monoprotonated 1,8-bis(dimethylamino)naphthalene.

The proton location and proton transfer (PT) dynamics of a hydrogen bond are under the influence of the static and dynamical properties of the solvent and counterions. In the present study, the N-H distances were determined for salts of 1,8-bis(dimethylamino)naphthalene, DMANH(+)X(-) (X(-) = BPh4(-), ClO4(-), and Cl(-)), in acetonitrile (AN) solution, and DMANH(+)Br(-) in water by observing the (15)N spin-lattice relaxation caused by the (15)N-(1)H magnetic dipolar coupling under assumption that the PT time was shorter than the NH reorientation time (∼10(-11) s). The obtained N-H distances decreased in the following order: DMANH(+)BPh4(-) > DMANH(+)ClO4(-) > DMANH(+)Br(-)/H2O > DMANH(+)Cl(-), indicating that interactions with the environment affect the PT potentials. To understand the results at the molecular level, Car-Parrinello molecular dynamics simulations were performed for DMANH(+), DMANH(+) in water, and DMANH(+)-Cl(-) ion-pair in AN. The results of simulation suggest that (1) the N-H distance decreases in the presence of a solvent and counterion; (2) the PT time is probably ∼10(-12) s, which confirms the above assumption used for the NMR relaxation data analyses; and (3) fluctuation of the interactions with the solvent or counterion has a significant role in PT. Quantum nuclear effects on the hydrogen bond were also examined.

Photochemical reactions of anthracene–naphthalene bichromophoric systems linked by a three-carbon chain

Photochemical reactions of bichromophoric compounds having 9-anthryl and 1-naphthyl groups linked by a three-carbon chain have been investigated in solution and in the solid state. Direct photolysis of 1a–c,e–g in degassed benzene gave both cyclomers 2a–c,e–g and head-to-tail anthracene dimers 3a–c,e–g through intra- and inter-molecular [4 + 4] cycloaddition, respectively, while 1-acetoxy derivative 1d gave only cyclomer 2d. The reactivity for the cyclomerisation of 1a–g was lower than that of the bisanthracene system. The conjugated carbonyl group in 1e and 1f significantly retarded the reactions. On biacetyl sensitisation neither cyclomerisation nor dimerisation occurred, but alkene 1e underwent cis–trans isomerisation. In the solid state, (E)-1e gave dimer 3e in 22% yield, while the other compounds were photostable or gave the corresponding dimer in a lower yield. X-Ray structure analysis showed that in the crystal of (E)-1e adjacent anthracene rings related by an inversion centre were stacked with 3.45 A separation, which was favourable for head-to-tail dimerisation.

Thermo-analytical and X-ray diffractometric observations of a plastically crystalline phase in molecular mixed-ligand complex

Abstract Thermal properties and structures of molecular complexes [M(hfac) 2 ( tmen )] (M = Ni, Cu) were examined by a differential thermal analysis and an X-ray diffractometry above room temperature. The Ni- and Cu-complex crystals were found to melt through two, II-to-I and I-to-L, phase transitions at 373 K and 403 K respectively, and through a I-to-L phase transition at 410 K, respectively. The phase I of the Ni-complex was concluded to be in the plastically crystalline state and in the face-centered cubic system. It was noted that the difference between the phase relations of the two complexes should originate from the different coordination bonds as correlated with the electronic configurations of the central metal ions.