Transition-metal-catalyzed aromatic cross-coupling reactions are highly useful for constructing various organic fine chemicals. The conventional methods, however, usually require the preactivation of aromatic substrates such as stoichiometric halogenation and metalation before the coupling event, which increases the steps leading to target molecules and yields inevitable salts as by-products. In order to cope with these problems, catalytic C–H transformation reactions on aromatic substrates have been one of the most intensive subjects in the area of catalytic organic synthesis. Summarized herein are our efforts for developing direct aromatic and heteroaromatic coupling reactions by using a number of transition metals as catalysts.
Transition-metal-catalyzed direct aromatic coupling reactions as the complementary methods of conventional cross-couplings have recently been significantly developed. Our efforts in the research area are summarized in this account.
Monolithic transparent silica glasses containing LaF3 nanocrystals were prepared by a cosolvent-free sol–gel method based on the pyrolysis of lanthanum trifluoroacetate. The two-step mixing of alkoxide and water, consisting of partial hydrolysis under acidic conditions and subsequent neutralization to catalyze the polycondensation of hydrophobic silica oligomers, makes it possible to shorten the processing time while eliminating the need for cosolvents. The size of the LaF3 nanocrystals decreases with an increase in the volume fraction of LaF3. This suppresses Rayleigh scattering and improves transparency at LaF3-rich compositions. Green and red upconversion photoluminescence (PL) bands, mainly caused by energy-transfer upconversion (ETU), are observed in Er-doped samples under excitation at 980 nm. The PL intensity drastically increases with an increase in the volume fraction of the LaF3 phase as a result of improved crystallinity of LaF3. The quantum yields recorded for the green and red upconversion PL bands were ca. 0.08% and ca. 0.04%, respectively.
Monolithic transparent silica glasses containing LaF3 nanocrystals were prepared by a cosolvent-free sol–gel method based on the pyrolysis of lanthanum trifluoroacetate. In Er-doped samples visible upconversion photoluminescence (PL) caused by energy-transfer upconversion (ETU) is observed under excitation at 980 nm. All of the size of the LaF3 nanocrystals, the Rayleigh scattering intensity, and the nonradiative decay rate decrease with an increase in the volume fraction of LaF3.
We previously developed a new method that combines sputter deposition of Au and the use of a capture medium of poly(ethylene glycol) (PEG) for preparing Au nanoparticles (AuNPs). To further control the size and shape of AuNPs, we improved the method in the present study by adding a low-molecular-weight α-methoxy-ω-thiopoly(oxyethylene) (mPEG-SH) as a nonvolatile stabilizing agent to PEG. The synthesized AuNPs were structurally characterized by transmission electron microscopy, small-angle X-ray scattering, and UV–vis absorption spectroscopy, and the results were compared with our earlier findings of AuNPs prepared in neat PEG. The AuNPs prepared with the addition of mPEG-SH were spherical. The concentration of mPEG-SH played an important role in controlling the size and size distribution of AuNPs, whereas the temperature of the capture medium had a small effect on these characteristics. In addition, heat treatment stability of AuNPs and the effect of elapsed time on AuNP formation were investigated; AuNPs synthesized in PEG solution containing mPEG-SH were much more resistant to the variables than AuNPs synthesized in neat PEG.
Flower- and spiky-ball-like nanoparticles were selectively obtained by the reduction of H[AuCl4] and AgNO3 with ascorbic acid in the presence and absence of poly(vinylpyrrolidone) with sufficient reproducibility. The morphological switch is attributable to the selective passivation of the crystal facets of the initially grown core particles and growing nanostructures on the surfaces. The characteristic optical properties, which are due to the surface nanostructures, were studied via measurement of the extinction spectra and finite-difference time-domain simulations.
We prepared pyridinium-based amphiphilic zwitterions and examined their liquid-crystalline (LC) phase behavior in the presence and absence of lithium salts. Addition of lithium salts induced nanosegregated LC phases such as bicontinuous cubic phase and smectic phase depending on the anion species of the lithium salts. For example, an equimolar mixture of dodecylcarbamoyl-1-(4-sulfobutyl)pyridinium betaine and lithium bis(trifluoromethanesulfonyl)imide (LiTf2N) exhibited a bicontinuous cubic phase between 40 and 100 °C. The exhibition of these nanosegregated phases was attributed to the formation of an ionic liquid-like ion pair composed of the pyridinium cation and the Tf2N anion. The preferential interaction between the pyridinium cation and the Tf2N anion was confirmed with Raman spectroscopic measurement. Moreover, the amphiphilic zwitterions were found to show not only thermotropic LC behavior but also lyotropic LC behavior in the presence of polar solvents such as protic ionic liquids. The results presented here indicate that the combination of amphiphilic zwitterions and suitable additive ions is a potential method to construct nanostructured LC materials showing tuneable phase behavior.
The supramolecular self-assemblies of dibenzofuran (DBF) and 5-dibenzosuberenone (DBS) with α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD) were investigated by absorption, steady-state, and time-resolved fluorescence, Fourier transformation infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), proton nuclear magnetic resonance (1H NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and molecular modeling methods. In all the solvents, the absorption and emission maxima of DBF is red shifted from that of DBS. DBF and DBS show single exponential in water and biexponential in CD solution. In both CD solutions, the change in the emission intensity indicates 1:1 inclusion complex is formed. At higher β-CD concentrations, excimer emission (2:2 stoichiometry, guest2:β-CD2) is detected. Binding constants were determined from the analysis of F0/F versus [CD] plots, resulting in a stronger association for β-CD. In the absorption and fluorescence spectra, the different trend displayed in α-CD seems to be related to a shallow inclusion of both drugs, stabilized by hydrogen bonding between the oxygen atom of the guests and the hydroxy groups of CDs. Self-assembled nanosphere and nanosheet morphology of the inclusion complexes were observed by TEM. The possible modes of theoretical calculations suggest the complex formation is favorable to an enthalpy driven process.
Steady-state coupling between the diffusion and the chemical reaction in an artificial enzyme membrane was investigated on the basis of nonequilibrium thermodynamics. The relaxation length for the chemical affinity of the reaction, which can be considered the reaction layer thickness for the enzyme membrane, was found to be expressed in terms of the maximum velocity and the Michaelis constant for the enzymatic reaction and the diffusion coefficients of the reaction participants. The theoretical expression was applied to the acid phosphatase-immobilized membrane–two aqueous buffered solution system, one of which contains 4-nitrophenyl phosphate as the substrate for the enzymatic reaction. Using the obtained relaxation length, the asymmetric distribution of the chemical affinity of the hydrolysis reaction within the enzyme membrane has been developed in some cases.
Substituent effects on the potential energy surface of XSiSb (X = H, Li, Na, BeH, MgH, BH2, AlH2, CH3, SiH3, NH2, PH2, OH, SH, F, and Cl) were investigated by using B3LYP/Def2-TZVP, B3PW91/Def2-TZVPP, and CCSD(T) methods. The isomers include structures with formal double (Si=SbX) and triple (XSi≡Sb) bonds to silicon–antimony, so a direct comparison of these types of species is possible. Our model calculations indicate that electropositively substituted Si=SbX species are thermodynamically and kinetically more stable than their isomeric XSi≡Sb molecules. Moreover, the theoretical findings suggest that F, OH, NH2, and CH3 substitutions prefer to shift the double bond (Si=SbX) by forming a triple bond (XSi≡Sb).
We previously reported the synthesis of the metal-based fluorescent probes MBFh1 and MBFh2 for hydrogen peroxide (H2O2) imaging, which release the red-emitting fluorophore resorufin, whose response rate is significantly faster than that of other nonmetal-based H2O2 probes, upon reacting with H2O2. In this study, we have synthesized the new fluorescent probe MBFh3 1 and its ester derivatives 2 and 3, in which a mononuclear nonheme iron(III)–monoamido complex and a 3′-O-methyldihydrofluorescein derivative are connected via a propane linker. Probes 2 and 3 are methyl and 1-(ethoxycarbonyloxy)ethyl ester derivatives of 1, respectively. These probes rapidly react with H2O2 to release green-emitting 3′-O-methylfluoresceins. Fluorescent microscopy studies using HeLa and A431 cells showed that intracellular H2O2 can be visualized using probes 1 and 2, but not 3.
Gas-phase acidities of polyfluorinated hydrocarbons have been determined by measuring proton-transfer equilibria and by computing the free energies of deprotonated carbanions and the corresponding neutrals. An excellent linear relationship between acidities and the accumulated inductive effects of fluorine atoms contained in a molecule was observed for the perfluoroalkyl-substituted neopentanes, (Rf1)(Rf2)(Rf3)CCH3, and polyfluorinated bridgehead carbon acids where the contribution of negative hyperconjugation of the Cβ–F bond to the stability of the conjugate anions is absent or negligibly small. On the basis of this relationship, the extent of β-fluorine negative hyperconjugation involved in acidities of polyfluorinated hydrocarbons could be evaluated quantitatively. The negative hyperconjugation was found to be negligibly small in the stable tertiary polyfluorinated carbanions while in the less stable primary and secondary carbanions the contribution of this effect is present certainly, indicating that the negative fluorine hyperconjugation is complementary to the stabilization by the accumulated inductive effect of fluorine atoms. The extent of negative hyperconjugation was found to increase in order of CF3CH2− < C2F5CH2− ≈ C3F7CH2− < i-C3F7CH2−, being qualitatively consistent with the elongation of the Cβ–F bond by deprotonation. The effect of α-fluorine on acidity was found to change complicatedly with the carbanion, e.g., the α-fluorine substitution in CF3CH3 strengthens acidity, no effect in (CF3)2CH2, and strengthens again acidity in (i-C3F7)2CH2. Such varying effect of α-fluorine in the polyfluorinated hydrocarbons would be caused by a subtle balance of effective electronegativity of an anionic center carbon, a varying p–p lone pair repulsion depending on the net negative charge at the formal charge center carbon, and the change in ability of β-F negative hyperconjugation caused by α-F substitution.
Phosphorus tungstic heteropolyacid was synthesized by mixing P2O5 and peroxoisopolytungstic acid (W-IPA) in methanol solution. WO3-based composite films fabricated from phosphorus tungstic heteropolyacid and transparent urethane resin showed reversible photochromic properties. The coloring reaction rate constants of the films with P/W ratios of 0, 0.05, 0.083, and 0.20 were 1.55 × 10−2, 1.29 × 10−2, 1.88 × 10−2, and 1.76 × 10−2 min−1, respectively, while the bleaching half-life periods τ were 21.5, 14.9, 12.6, and 10.2 h, respectively; thus, the bleaching speed of the composite films was accelerated by phosphorus addition.