The classical repertoire of synthetic organic chemistry is short of methods that allow triple bonds to be transformed into (E)-alkenes with high selectivity in the presence of other reducible sites. Recent advances, most notably in ruthenium-catalyzed trans-hydrogenation, trans-hydrosilylation, trans-hydrogermylation, trans-hydrostannation, and even trans-hydroboration hold the promise of filling this gap. This review illustrates the state-of-the-art in the field by summarizing applications of these emerging methodologies to natural product synthesis. A comparison of ruthenium-catalyzed and radical-induced trans-hydrostannations provides further insights into the application profile of these transformations.
This Account summarizes the state-of-the-art of alkyne trans-addition chemistry by discussing applications of these methodologies to natural product synthesis.
Photosynthetically active chlorophyll molecules are biosynthesized from 5-aminolevulinic acid through protoporphyrin IX. The multistep enzymatic transformations were well investigated, but their pathways and reaction mechanisms have not been fully elucidated in the late stages. The biosynthetic pathways of (bacterio)chlorophylls from protoporphyrin IX are here described from a chemical viewpoint, especially focusing on the reduction processes of the C=C double to C–C single bonds. The regioselective and stereoselective trans-hydrogenation is performed in the enzymatic reductions.
In phototrophs, protoporphyrin IX is enzymatically transformed into (divinyl)chlorophyll-a and bacteriochlorophylls-a/b/g observed in both the photosynthetic antenna and reaction center apparatuses through some trans-hydrogenations of C=C to CH–CH bonds.
We discuss low-molecular-weight compounds, which undergo physical gelation, as gelators. There is a close relationship between crystallization and gelation with respect to the driving forces. Both crystallization and gelation are driven by noncovalent interactions such as hydrogen bonding, van der Waals forces, electrostatic interactions, and π–π interactions. Typical gelators, which include amino acid derivatives, trans-1,2-diaminocyclohexane derivatives, and cyclic(dipeptide)s, are discussed. A concept of “gelation-driving segment” has been proposed. Hydrogelators, functional gelators, and polymer-type gelators are developed by using gelation-driving segments.
Gelators are compounds that can cause physical gelation. The driving forces for the physical gelation are noncovalent bonds. The formation of fibrous aggregates is an important factor responsible for gelation. Aggregation of gelator molecules can be often observed by electron microscopy.
The dispersion of carbon nanotubes is a key issue for maximizing their performance in many applications. Here we describe the design and synthesis of an efficient dispersant for the so-called “super-growth” single-walled carbon nanotubes (SG-SWNTs) to provide a stable and homogeneous dispersion in typical organic solvents simply by sonication. The synthesized dispersant is a copolymer composed of a naphthalene diimide and poly(dimethylsiloxane), and the dispersions were characterized using visible absorption, Raman, AFM, and TGA measurements as well as molecular-mechanics simulations. It was found that the copolymer efficiently dissolves the SG-SWNTs in many solvents including methyl ethyl ketone, methyl isobutyl ketone, and toluene that are widely-used solvents in polymer/resin-related industrial areas.
The previously reported palladium-catalyzed silylation of aryl chlorides with silylsilatranes was theoretically reinvestigated by DFT calculations with a focus on the effect of the transannular Si–N coordination on silyl transfer. The Si–N coordination proved to be not so influential as to accelerate the silyl transfer. However, it should be of benefit to use silylsilatranes as silylating agents because of their easy handling and facile preparation.
A simple and rapid fabrication method of IR transparent poly(dimethylsiloxane) (PDMS) microfluidic devices is presented. Spin coating of PDMS pre-polymer was employed to attain a thin PDMS microchannel layer, allowing for transmission IR microspectroscopy. Key advantages of this approach include simplicity, low fabrication cost, reliability and rapid production time. PDMS microfluidic devices manufactured using this approach were successfully used to demonstrate transmission IR microscopic measurements of on-chip dilution of NaN3 using water. A linear relationship between peak area (centered at ca. 2050 cm−1) of azide and the concentration estimated from the ratio of the flow rates of two inlets was observed in the range from 15 to 150 mM. The kinetics of the reduction of hexacyanoferrate(III) ([Fe(CN)6]3−) by ascorbic acid (AA) was also analyzed. We observed that the peak at ca. 2115 cm−1 (hexacyanoferrate(III)) was decreasing, while the peak at ca. 2040 cm−1 (hexacyanoferrate(II)) was increasing as the reaction was in progress. From the systematic analysis by changing the concentrations of hexacyanoferrate(III) and AA, we evaluated that the reaction order with respect to each substance was 1st order.
Polycyclic, highly-conjugated, and ortho-fused aromatic compound, 9-tetraethyleneglycoxy-11-oxahelicene (TO9H), was synthesized and used to explore the feasibility of fabricating thin films and the photoelectrochemical properties of resultant films. Since TO9H has both the hydrophobic helicene part and the hydrophilic tetraethyleneglycoxy (TEG) chain, it behaved as a water-insoluble amphiphile and hence formed a stable Langmuir monolayer at the air–water interface. Homogeneous films with thickness up to a few hundred angstroms were deposited by using a spin-coating method. Both of the monolayers and the spin-coated films were characterized by several methods. Brewster angle microscopic (BAM) images and atomic force microscopic (AFM) topographic images revealed the homogeneous surface of the TO9H monolayer at the air–water interface and of the deposited monolayer and films on solid substrates, respectively. By using an X-ray reflectivity (XR) technique, the thickness, roughness, and electron density of layers or films were determined. The photocurrent evolution was measured using conventional three-electrode systems with light irradiation from a xenon lamp and obtained results showed that the compound TO9H generated a substantial amount of photocurrent under illumination of light.
Novel metal/periodic mesoporous organosilica nanocomposites were prepared through metal template (Cu, Zn, Ni, and Co) imprinted ligand into periodic mesoporous organosilica (PMO) and their application as a recyclable catalyst in click synthesis of β-hydroxytriazoles described. The structure of the nanocomposite was characterized by X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, N2 adsorption–desorption analysis, scanning electron microscopy (FE-SEM), energy-dispersive spectrometer (EDS), and thermogravimetric analysis (TGA). The results showed that the copper-imprinted periodic mesoporous organosilica nanocomposite (Cu/PMO NC) as a catalyst had good catalytic activity and short reaction times toward the multicomponent reaction for synthesis of β-hydroxytriazoles. Furthermore, the catalyst was regenerated and reused in the reaction for six cycles.
p-tert-Butylcalixarene 1 recognized conformationally flexible aliphatic hydrocarbons (hexane 2, heptane 3, octane 4, nonane 5, decane 6, undecane 7, and dodecane 8) and monoalkyl benzenes (toluene 10, ethylbenzene 11, propylbenzene 12, butylbenzene 13, hexylbenzene 14, and octylbenzne 15) to form host–guest complexes. X-ray diffraction study has revealed that the alkyl chains of 11–15 were selectively recognized by 1, whereas host 1 recognized the aromatic ring of 10. The alkyl chain termini of 3, 4, 6–8, 11–13, and 15 adopted unusual folded conformations in the cavity. M06-2X/6-31G(d,p) level of calculations revealed that the mutual induced-fit shape adjustments between the calixarene cavity and the flexible guests play a key role in maximizing the host–guest interactions directed by the C(sp3)–H/π interactions, leading to the unusual conformations of the alkyl chain termini of the guests and the selective binding of the alkyl chains in the solid state.
We have prepared a novel nickel(II) trinuclear compound [Ni3(tmen)3(µ2-Cl)3(µ3-OMe)(µ3-OH)](BPh4)·0.5MeOH·0.5CH2Cl2 (1; tmen: N,N,N′,N′-tetramethylethylenediamine) with a trigonal bipyramid molecular structure, where the basal plane consists of Ni3(µ2-Cl)3 atoms. Two known compounds [Ni3(tmen)3X4(OH)]X (X = Cl (2), Br (3)) were also investigated. Magnetic study clarified the ground high-spin (S = 3) state, and the intramolecular exchange coupling parameters were determined as 2J/kB = 18.0 ± 0.2, 27.2 ± 0.2, and 19.6 ± 0.2 K for 1, 2, and 3, respectively. Crystallographic analysis clarified the unique geometry of 1. The molecular axes are aligned almost parallel in the crystallographic b axis. Relatively short intermolecular Cl···Cl distances (3.725(1) and 3.847(2) Å) are found among linearly arrayed complex ions. A single-crystal magnetic study gave a zero-field-splitting parameter DS=3/kB = −2.2 ± 0.1 K. An intermolecular exchange coupling parameter was determined as 2j/kB = −0.15 ± 0.01 K from simulation. A pulsed-field magnetization study at 0.5 K for 1 showed a jump around 1.2 T, which is explained in terms of intermolecular interaction. Alternating current magnetic susceptibility measurements revealed the Arrhenius behavior with Ea = 10.1 ± 0.8 K at zero bias field. Therefore, the magnetic properties of 1 can be understood as an antiferromagnetic chain made of potential single-molecule magnets.
A triangular nickel(II) complex ion [Ni3(tmen)3(µ2-Cl)3(µ3-OMe)(µ3-OH)]+ has relatively short intermolecular Cl···Cl distances. The magnetic properties can be understood as an antiferromagnetic chain consisting of potential single-molecule magnets.
The equilibrium constants for the antiparallel dimer formation of ethylene carbonate (EC) in solutions of pure solvents, such as benzene (Bz) and dimethyl carbonate (DMC), were quantitatively determined at room temperature (25 °C) and various concentrations using Raman scattering (RS) experiments. The strong C=O stretching vibrational band of EC observed at approximately 1800 cm−1 split into a monomeric EC signal and signals assigned to antiparallel dimers ((EC)2) in the examined solutions. From the concentration dependence of the C=O stretching vibrational signal intensities, the equilibrium constants (Kd) of the formation of (EC)2 in the solutions were determined assuming a chemical process, 2EC ⇌ (EC)2. Moreover, the fact that the equilibrium constant, Kd, substantially depends on the species in the solvent and on the EC composition strongly manifests that the choice of a solvent effectively governs the antiparallel dimer formation and consequently influences the performance of lithium ion secondary batteries.
A novel, cysteine-containing fluorescent hexapeptide, N-acetyl-Tyr-Cys-Ser-Ser-Cys-Tyr– (YY), targeting Cd(II) ion sensing, was synthesized on various solid supports including two resins, as well as glass and quartz surfaces. The synthesis was based on the Fmoc (9-fluorenylmethoxycarbonyl) and the APTES (3-aminopropyltriethoxysilane) methodologies on the resin and silica supports, respectively. The immobilized ligand, except when coupled to a hydrophobic benzhydrylamine resin, showed a remarkably efficient, pH-dependent Cd(II) capturing ability with a maximum binding capacity around neutral pH. The effect of contact time and metal ion concentration was also studied with a hydrophilic resin supported peptide (YY-NTG). The interaction of YY-NTG with Cd(II) was investigated by pH-potentiometric titrations in aqueous samples containing the resin beads and Cd(II). These studies, together with metal ion capturing experiments under buffer-controlled pH, prove that each immobilized peptide can bind one Cd(II) ion at pH 7.0 in the presence of one equivalent metal ion or metal ion excess. For Cd(II) binding to YY-NTG a notably high, K = 1.3 × 1010 apparent stability constant was determined (1:1 metal-to-ligand ratio, pH 7.0). Analytical results suggest that the concentration of Cd(II) can be measured below 200 nM with this silica-supported peptide. The usefulness of the probe was demonstrated by fluorescence spectroscopy.
2-Hydroxyphenylpyridine and 2-hydroxyphenylbenzoxazole derivatives were synthesized and their photochemical properties were investigated. The results indicated that photocleavage reaction was controlled by intramolecular hydrogen bond.
3-Substituted (Z)-4-hydroxybut-2-en-1-yl acylates were regioselectively obtained in 2–98% yields by acylations of the corresponding 2-substituted (Z)-but-2-ene-1,4-diols with 10 equiv of vinyl acylate in the presence of 50 w/w% of porcine pancreas lipase (PPL) Type II. 2-Substituted (Z)-4-hydroxybut-2-en-1-yl acylates were regioselectively obtained in 4–95% yields by hydrolyses of the corresponding 2-substituted (Z)-but-2-ene-1,4-diyl diacylates in the presence of 100 w/w% of PPL Type II.