Anion radicals generated by one-electron reduction of oxime derivatives act as iminyl radical equivalents. That is, the intramolecular C–N bond formation of γ,δ-unsaturated or β-aryl oximes is induced by one-electron reduction to give various aza-heterocycles. The catalytic electron transfer processes are developed by using hydroquinones or copper reagents as electron donors. Photo-induced electron transfer is also utilized to transform of γ,δ-unsaturated oximes to dihydropyrroles. Total synthesis of peduncularine was achieved by applying the catalytic radical cyclization of oximes as a key step.
Anion radicals generated by one-electron reduction of oxime derivatives act as iminyl radical equivalents. Catalytic radical cyclization of γ,δ-unsaturated oximes induced by one-electron reduction proceeds to give various aza-heterocycles.
CH arylation of five-membered heteroaromatic compounds such as thiazoles and thiophenes recently developed by the authors is reviewed. The reaction of thiazole with aryl iodides in the presence of a palladium/copper catalyst system and tetrabutylammonium fluoride causes CH arylation at the 2-position of thiazole. The reaction of 2-arylthiazole with aryl iodide by the catalysis of a palladium complex with silver(I) fluoride induces CH arylation at the 5-position. The reactions allow preparation of a variety of 2,5-diarylthiazoles in a facile manner. Spectroscopic, thermal, and electrochemical characteristics of 2,5-diarylthiazole are summarized. Introduction of the 2,5-diarylthiazole moiety, which exhibits photoluminescent and liquid crystalline characteristics, into the side chain of polymethacrylate is also described. CH arylation of thiophene derivatives are described to occur similarly to the case of thiazole. CH homocoupling of thiophene leading to bithiophene, in which the carbon–bromine bond on the thiophene ring is completely intact in the palladium-catalyzed reaction, is also described. Mechanistic studies on the reactions of thiophene are shown with a stoichiometric use of arylpalladium(II) halide complex and thiophene to undergo electrophilic substitution at the thiphene ring toward the palladium atom and the following reductive elimination of the thus obtained aryl(thienyl)palladium complex.
Our recent effort on transition-metal-catalyzed substitution reactions at the CH bond of thiazoles and thiophenes is described. Studies on the spectroscopic, thermal, and electrochemical properties of thiazole and thiophene derivatives directed to functional organic materials are also discussed.
Perchloric acid treatment of sarcophytoxide, a marine cembranoid possessing an epoxide, brought about epoxide–ketone rearrangement affording ketones. When the reaction time was long (22 h), a minor ketone that was antipodal to the ketone obtained in a short-time (10 min) reaction was formed. These puzzling findings, considering that the starting epoxide had three asymmetric carbons, were interpreted by surveying the structures of other ketonic products. The stereochemistry of a major ketone, which had been wrongly assigned, was corrected by extensive analyses of NMR spectra. The correct stereochemistry indicated that the epoxide–ketone rearrangement took a course via a cationic intermediate.
Perchloric acid treatment of sarcophytoxide, a marine cembranoid possessing an epoxide, brought about epoxide–ketone rearrangement affording ketones. The stereochemistry indicated that the epoxide–ketone rearrangement took a course via a cationic intermediate.
In aqueous solution, the interactions of 2,4- and 2,5-diphenyloxazole (2,4- and 2,5-DPO) with β- and γ-cyclodextrin (β- and γ-CD) have been examined by means of absorption and fluorescence spectroscopies. γ-CD forms a 2:1 host–guest inclusion complex with 2,4-DPO, whereas β-CD forms a 1:1 inclusion complex with 2,4-DPO. Equilibrium constants for the formation of these inclusion complexes have been evaluated. Upon the addition of alcohol to 2,4-DPO solution containing γ-CD, the 2:1 γ-CD–2,4-DPO inclusion complex accommodates alcohol to form a 2:2:1 γ-CD–alcohol–2,4-DPO inclusion complex. The equilibrium constant for the formation of the 2:2:1 inclusion complex increases as the alkyl chain of the alcohol is lengthened from 1-pentanol to 1-heptanol. A similar trend in the magnitude of the equilibrium constant has been observed for diols (1,9-nonanediol and 1,10-decanediol). In contrast to 2,4-DPO, 2,5-DPO forms a 1:1 γ-CD–2,5-DPO inclusion complex at a low concentration of 2,5-DPO. At a high concentration of 2,5-DPO, excimer fluorescence of 2,5-DPO has been observed in the presence of γ-CD. From simulations of excimer fluorescence intensity as a function of γ-CD concentration, the excimer fluorescence is ascribed to both a 1:2 γ-CD–2,5-DPO inclusion complex and a 2:2 γ-CD–2,5-DPO inclusion complex.
We investigated the effects of an axial amino ligand on the spectroscopic and electrochemical properties of alkylamino(methoxo)(tetraphenylporphyrinato)antimony(V) bromide 1 and anilino(methoxo)(tetraphenylporphyrinato)antimony(V) bromide 2. Fluorescence measurements of 1 and 2 showed that their fluorescence quantum yields were lower than that of dihydroxo(tetraphenylporphyrinato)antimony(V) bromide (3) due to intramolecular electron transfer (ET) from the axial amino ligand to the excited porphyrin ring. Fluorescence was enhanced by the addition of a proton, which was caused by the protonation of the axial nitrogen atom, thereby preventing ET. From the titration curves of fluorescence quantum yield (Φf) versus proton concentration, we estimated the acid dissociation constants (pKa) for the conjugate acid of the axial amino ligand of 1 to be 4.41–5.03. The reduction potentials (E1⁄2red) of 2 depended strongly on the electronic effect of the p-substituents (X) on the axial aniline group whereas the E1⁄2red of methoxo(p-substituted-phenoxo)(tetraphenylporphyrinato)antimony(V) bromide was little affected by the p-substituents on the phenoxo ligand. These observations were interpreted as reflecting the participation of the axial amino ligand in the LUMO, calculated for 2 by the PM3 method.
Nickel(II) complexes supported by 14-membered pyridine-containing macrocyclic ligands LE (E = NH, S, and O), represented as [NiII(LNH)](ClO4)2 (1a), [NiII(LS)](ClO4)2 (2a), [NiII(LS)](ClO4)(BPh4) (2b), [NiII(LO)(CH3CN)2](ClO4)2 (3a), and [NiII(LO)(CH3CN)2](ClO4)(BPh4) (3b), have been synthesized. X-ray crystal structures of complexes 1a and 2b have square-planar structures in low-spin state, whereas complex 3b exhibits an octahedral high-spin configuration where two solvent molecules occupy the axial positions of the NiN3O1 plane. The absorption spectra of 1a and 2a in coordinating solvents show interconversion of high-/low-spin states, for which the equilibrium constants and thermodynamic parameters (ΔH0 and ΔS0) have been determined. However, in the case of complex 3a in coordinating solvent, the high-spin state dominates at temperatures ranging from −40 to 60 °C. Cyclic voltammetry of the complexes 1a and 2a in CH3CN showed a reversible wave for a NiII/NiI redox couple. The redox potential for 2a (E1⁄2=−1.20 V vs. Ag/AgNO3) is more positive than that for 1a (E1⁄2=−1.48 V). In contrast to these complexes, 3a showed an irreversible redox couple with a significantly large peak separation of 460 mV. This result indicates that the coordination geometries of the nickel complex 3a drastically change between six-coordinate NiII and four-coordinate NiI states.
Heterotrinuclear [(NiIIL)2LnIII(NO3)] complexes (where H3L = 1,1,1-tris[(salicylideneamino)methyl]ethane and Ln = Gd (1), Eu (2), Tb (3), and Dy (4)) were prepared by treating [Ni(H1.5L)]Cl0.5 with Ln(NO3)3·6H2O in the presence of triethylamine. Complexes 1·2CH3OH, 3·2CH3OH, and 4·C2H5OH·0.5H2O crystallized in the triclinic space group, (No. 2), with Z=2, while 2·CH3OH·0.5H2O crystallized in the tetragonal space group, I41⁄a (No. 88), with Z=8. All the complexes had very similar structures. Each complex was a doubly face-sharing trinuclear molecule. The NiII ion is coordinated by the L3− ligand in an N3O3 coordination sphere, and the three phenolate oxygen atoms coordinate to an LnIII ion as bridging atoms. The LnIII ion is eight coordinate with six phenolate oxygen atoms of the two L3− ligands and two oxygen atoms of NO3−. Coordination of the NO3− group entails a bending of the Ni···Ln···Ni angle (ca. 140°). All the complexes involve π–π and CH–π interactions between the neighboring molecules to form a three-dimensional structure. Temperature-dependent magnetic susceptibility and field-dependent magnetization measurements on 1 showed a ferromagnetic interaction between the NiII and GdIII atoms. A ferromagnetic interaction was also suggested for NiII–TbIII (3) and NiII–DyIII (4).
Facile preparations of C-glycosyl β2- and β2,2-amino acids are described. Selective formation of a β-C-glycoside linkage was achieved by the reaction of a 2,3,4,6-tetra-O-acetyl-α-D-gluco/galactopyranosyl bromide (α-acetobromoglucose/galactose) with the carbanion of a cyanoacetate ester. Crystallization selectively afforded one of two diastereomers with respect to the chiral center at the α-carbon of the side chain (C-2), however, this compound was found to epimerize during the following nitrile reduction. Separation of the diastereomers was achieved via the Fmoc derivatives. Diastereomerically pure C-glycosyl β2,2-amino acids were prepared by diastereoselective alkylation of C-glycosylated enolate, followed by nitrile hydrogenation. The present procedure serves as an efficient route to C-glycosylated β-amino acids containing a non-biodegradable linkage.
Addition of co-solvents such as tetrahydrofuran resulted in a great improvement of the enantioselectivity of lipase-catalyzed hydrolysis of butyl 2-(4-substituted phenoxy)propanoates in an aqueous buffer solution. On the other hand, lipase lyophilized from an aqueous solution containing the co-solvents catalyzed highly enantioselective esterification of 2-(4-substituted phenoxy)propionic acids, 2-(4-isobutylphenyl)propionic acid (ibuprofen), and 2-(6-methoxy-2-naphthyl)propionic acid (naproxen) in an organic solvent. An increase in the E value up to two orders of magnitude was observed for some substrates. The origin of the enantioselectivity enhancement caused by the co-solvent addition was mainly attributed to a significant deceleration in the initial reaction rate for the incorrectly binding enantiomer, as compared with that for the correctly binding enantiomer. From the results of FT-IR, CD, and ESR spectra, the co-solvent addition was also found to bring about a partial destruction of the tertiary structure of lipase.
On treatment of allylphenylsilanes with t-BuOK and 18-crown-6 in DMSO, isomerization of the olefinic double bond and subsequent substitution of the phenyl group with a hydroxy group took place smoothly to afford alkenylsilanol derivatives in good yields. The reaction mechanism was investigated using 18O-labeled sulfoxide. We found that a (methylsulfinyl)methyl anion generated from DMSO participated in this reaction.
N-Phosphoramino-protected six-membered cyclic α-aminophosphonates trans-2-[(diethoxyphosphorylamino)alkyl]-4-aryl-5,5-dimethyl-1,3,2λ5-dioxaphosphorinane-2-oxide 4a–4l were synthesized with the help of acetyl chloride. 31P NMR was used to trace the reaction process. A possible reaction mechanism was proposed and the stereochemistry of the title compounds was studied. In order to confirm the structure of 4, products 4f and 4k were recrystallized and determined by X-ray diffraction analysis.
For mass spectroscopy (MS)-based protein structural analyses, effective digestion by proteases is one of the key steps. In this study, we developed a protein digestion method using immobilized enzyme and a microscale vibration unit. Phospholipase A2 was applied as a model substrate to evaluate the new procedure due to its rigid structure and resistance to protease digestion. Digested substrates were analyzed by MS, and cleavage at all expected recognition sites was evaluated. In relation to conventional liquid-phase digestion, the number of matched peptides and sequence coverage improved significantly from 6 to 9, and 41% to 69%, respectively. These results support the efficacy of our novel method in proteomics applications, including protein structural and post-translational modification analyses.
Irradiation of 2,2-diphenyl-2H-1-benzopyran (DPBP) in the presence of a primary or a secondary amine gave an addition product. With a large excess of amine, the apparent quantum yield for product formation reached 0.5. This value corresponds to a minimal quantum yield for formation of DPBP colored ring-opened forms.
Photochromic diarylethene regioisomers having benzothiophene and benzofuran rings were synthesized and their structures were confirmed by X-ray crystallography. The photochromic properties of these diarylethenes were examined in solution as well as in the single-crystalline phase. We found that a diarylethene having 3-methylbenzofuran (FR2) ring show photochromism upon irradiation with UV light. For bis(3-methylbenzofuran) (BFR2) derivatives, the absorption band of the closed-ring isomer was longer than that of the open-ring isomer. The closed-ring isomer of the diarylethene derivatives having one 2-methylbenzofuran (FR3) ring showed a high absorption coefficient (more than 104 dm3 mol−1 cm−1). Although the distance between two reactive carbon atoms was within 0.42 nm, some diarylethene derivatives showed no photochromic reactions in the single-crystalline phase. Diarylethene derivatives having one 2-methylbenzofuran (FR3) ring efficiently showed photochromism in the single-crystalline phase.
The influence of residual chloride ions on the catalytic activity of Co/Al2O3 was investigated for liquid-phase hydrogenation of ketones and aldehydes using Cl−-free and Cl−-containing catalysts. The Cl−-free catalyst showed high activity for hydrogenation of both, whereas the Cl−-containing catalysts showed very low activity for ketone hydrogenation.
The decomposition of organic solvents was divided into three periods: LPSD, LPAPMP, and APMP. We found that the generation behaviors of LPAPMP were different due to the relative permittivity of the solvents, however, LPAPMP generation is effective for decomposing each of the organic solvents.