Color Change of Indole Squaraine between Blue and Yellow with Golden Luster.

Color change of materials by external stimulus has attracted much attention for various applications. We synthesized 1,3-bis(3,3-dimethyl-2-methylene indolenine)squaraine (ISQ), which was a yellow powder with golden luster (ISQ(gold)) and whose color changed to blue (ISQ(blue)) by being ground using mortar and pestle. Scanning electron microscopy indicated that ISQ(gold) consisted of laminated plate-like crystals with a large flat surface, which was attributable to (2 0-2) facets of monoclinic crystals. Measurements of the diffuse reflectance spectra revealed that J- and H-type aggregates of ISQ were formed by increasing the weight percent of ISQ(blue) in the mixture with BaSO4. A thin film, which was prepared by a spin-coating of ethanol with dissolved ISQ(blue) on glass substrates, looked blue on a white paper, whereas it looked yellowish on a black paper. By repeating the coating process, the film thickness increased, and the thick film exhibited yellow with golden luster regardless of the background color. These results indicate that the color depends on whether the transmitted light through ISQ or the reflected light on the ISQ surface mainly contributes to color vision. To neglect the effect of the transmitted light and intensify the reflection on the surface, a pellet with a smooth surface was prepared by pressing ISQ(gold) or ISQ(blue) at 20 kg/cm2. The ISQ(blue) pellet consisted of the laminated sheets and was dark blue-green. However, under light irradiation, the ISQ(blue) pellet exhibited a yellow color with golden luster, whose specular reflection spectrum was similar to that of the ISQ(gold) pellet. Our results indicate that the color of ISQ is tunable by controlling the transmission and the reflection of the light, including the specular and diffuse reflections.

Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene.

4,4'-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4'-biazulene confirmed its axial chirality. The enantiomers of 4,4'-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'-biazulene is comparable to that of 1,1'-binaphthyl.

Total Synthesis of Azulene Derivative, a Blue Pigment Isolated from Lactarius indigo, and Colorant Application of Its Aqueous Dispersion.

Safety concerns in the food industry have increased the demand for natural food colorants. However, the application ranges of natural blue colorants are insufficient because they are scarce in nature, and the currently available natural blue dyes are limited to water-soluble products. In this study, we investigated a fat-soluble azulene derivative isolated from the mushroom Lactarius indigo as a potential candidate for a natural blue colorant. We developed its first total synthesis, where the azulene skeleton was constructed from a pyridine derivative and an ethynyl group was converted into an isopropenyl group using zirconium complexes. Moreover, nanoparticles of the azulene derivative were prepared via reprecipitation method, and their colorant ability was investigated in aqueous solutions. The new candidate food colorant exhibited a deep-blue color in an organic solvent and aqueous dispersion.

Helicene Radicals: Molecules Bearing a Combination of Helical Chirality and Unpaired Electron Spin.

Chiral helicenes that have helical π-conjugated frameworks comprised of ortho-fused aromatic rings have been intensively investigated for several decades owing to their attractive properties and high potential for a variety of applications. In contrast, studies concerning helicene radicals (open-shell helicenes) formed by redox reactions have been severely limited due to inherent high reactivity of organic radicals. However, open-shell helicenes possessing a combination of chirality and unpaired electron spin(s) delocalized on helical π-conjugated orbital(s) are promising for novel synergic magnetic functions. Recently, several kinds of isolable air-stable helicene radicals have been prepared to enable their detailed analysis of the crystal structures and physicochemical properties. This Minireview describes the recent advances and future prospects of organic-based helicene radicals categorized into three groups: 1) ionic radicals, 2) neutral radicals, and 3) diradicals.

Synthesis of 2-Iodoazulenes by the Iododeboronation of Azulen-2-ylboronic Acid Pinacol Esters with Copper(I) Iodide.

Azulen-2-ylboronic acid pinacol ester, prepared by iridium-catalyzed C-H borylation of azulene, efficiently underwent iododeboronation with a stoichiometric amount of copper(I) iodide. This reaction allowed the synthesis of 2-iodoazulene in only two steps starting from azulene. This methodology was successfully applied to analogous azulenes.

Synthesis of Alkylated Pyrimidines via Photoinduced Coupling Using Benzophenone as a Mediator.

The synthesis of alkylated pyrimidines was achieved via benzophenone-mediated photoinduced coupling between saturated heterocycles and sulfonylpyrimidines. The pyrimidine ring was selectively introduced at the nonacidic C(sp3)-H bond proximal to heteroatoms including oxygen, nitrogen, and sulfur. This is a coupling reaction mediated solely by photoexcited benzophenone, an organic molecule, without the aid of any metallic catalysts or reagents.

An Azulene-Fused Tetracene Diimide with a Small HOMO-LUMO Gap.

A newly prepared tetraazulene-fused tetracene diimide (TA-fused TDI) showed absorption in the near-IR region owing to the effective extension of the π-conjugated system as well as a large two-photon absorption cross-section (σ(2) =2140 GM) at 950 nm. Four reversible reduction processes and n-type semiconductivity were also confirmed as attractive electronic properties of this compound.

Aryl Ketone Catalyzed Radical Allylation of C(sp3)-H Bonds under Photoirradiation.

The catalytic introduction of an allyl group at nonacidic C(sp3)-H bonds was achieved under photoirradiation, in which 1,2-bis(phenylsulfonyl)-2-propene acts as an allyl source and 5,7,12,14-pentacenetetrone (PT) works as a C-H bond-cleaving catalyst. A variety of substances, including alkanes, carbamates, ethers, sulfides, and alcohols, were chemoselectively allylated in a single step under neutral conditions. The present transformation is catalyzed solely by an organic molecule, PT, and proceeds smoothly even under visible light irradiation (425 nm) in the case of alkanes as a starting substance.

Alkylation of Nonacidic C(sp3)-H Bonds by Photoinduced Catalytic Michael-Type Radical Addition.

Photoinduced catalytic Michael-type radical addition was achieved via olefin insertion into a nonacidic C(sp3)-H bond, utilizing 2-chloroanthraquinone as a C-H bond-cleaving catalyst and 1,1-bis(phenylsulfonyl)ethylene as an olefinic substrate. The present radical protocol allows carbon chain extension stemming from nonacidic C-H bonds, which complements alkylation at acidic C-H bonds under ionic conditions and installs the active methine site that acts as a versatile synthetic handle for further transformations.

Photo-induced Substitutive Introduction of the Aldoxime Functional Group to Carbon Chains: A Formal Formylation of Non-Acidic C(sp(3) )-H Bonds.

A photo-induced substitutive introduction of an aldoxime functional group to carbon chains was achieved using photo-excited 4-benzoylpyridine as a C(sp(3) )-H bond cleaving agent and arylsulfonyl oxime as an aldoxime precursor. The non-acidic C-H bonds in various substances, including cycloalkanes, ethers, azacycles, and cyclic sulfides, were chemoselectively converted at ambient temperature under neutral conditions. The present transformation is a formal formylation of non-acidic C(sp(3) )-H bonds in a single step.

Photoinduced Oxidation of Secondary Alcohols Using 4-Benzoylpyridine as an Oxidant.

Photoinduced oxidation of secondary alcohols to ketones was achieved by utilizing an equimolar amount of 4-benzoylpyridine as an oxidant. This transformation proceeds at ambient temperature and exhibits high compatibility with polar functionalities including benzoyl, silyl, and methoxymethyl alcohol protecting groups as well as tosyloxy, bromo, sulfonyl, carbamate, ester, and carboxylic acid units. The present oxidation is solely promoted by the action of organic molecules without the aid of metallic reagents.

Activity of antifungal organobismuth(III) compounds derived from alkyl aryl ketones against S. cerevisiae: comparison with a heterocyclic bismuth scaffold consisting of a diphenyl sulfone.

A series of hypervalent organobismuth(III) compounds derived from alkyl aryl ketones [XBi(5-R'C6H3-2-COR)(Ar)] was synthesized to investigate the effect of the compounds' structural features on their antifungal activity against the yeast Saccharomyces cerevisiae. In contrast to bismuth heterocycles [XBi(5-RC6H3-2-SO2C6H4-1'-)] derived from diphenyl sulfones, a systematic quantitative structure-activity relationship study was possible. The activity depended on the Ar group and increased for heavier X atoms, whereas lengthening the alkyl chain (R) or introducing a substituent (R') reduced the activity. IBi(C6H4-2-COCH3)(4-FC6H4) was the most active. Its activity was superior to that of the related acyclic analogues ClBi[C6H4-2-CH2N(CH3)2](Ar) and ClBi(C6H4-2-SO2 tert-Bu)(Ar) and also comparable to that of heterocyclic ClBi(C6H4-2-SO2C6H4-1'-), which was the most active compound in our previous studies. Density function theory calculations suggested that hypervalent bismuthanes undergo nucleophilic addition with a biomolecule at the bismuth atom to give an intermediate ate complex. For higher antifungal activity, adjusting the lipophilicity-hydrophilicity balance, modeling the three-dimensional molecular structure around the bismuth atom, and stabilizing the ate complex appear to be more important than tuning the Lewis acidity at the bismuth atom.

Heterocyclic bismuth carboxylates based on a diphenyl sulfone scaffold: synthesis and antifungal activity against Saccharomyces cerevisiae.

A series of heterocyclic organobismuth(III) carboxylates 4 and 5 [RCO2Bi(C6H4-2-SO2C6H4-1'-)] derived from diphenyl sulfone was synthesized to determine the influence of the carboxylate ligand structure on the lipophilicity and antifungal activity against the yeast Saccharomyces cerevisiae. In contrast to the clear structure-activity relationship between the size of the inhibition zone and the value of ClogP for specific substitution on diphenyl sulfone scaffold 1 [ClBi(5-RC6H3-2-SO2C6H4-1'-)], scaffolds 4 and 5 showed similar inhibition activities irrespective of the ClogP value. This suggests that these molecules function inside the yeast cell by separating into the cationic heterocyclic bismuth scaffold and the anionic carboxylate moiety, and that the bismuth scaffold plays an important role in the inhibition activity.

Unexpected formation of stannolanes and trigonal bipyramidal tin complexes by radical cyclization reaction.

Optically active stannolanes and trigonal bipyramidal pentacoordinated tin complexes were readily prepared by radical cyclization of N-propargylated chiral aza-Morita-Baylis-Hillman adducts induced by Bu(3)SnH. A translocated radical through the cyclization attacked the Bu(3)Sn group in an S(H)2 manner.

Bismuth heterocycles based on a diphenyl sulfone scaffold: synthesis and substituent effect on the antifungal activity against Saccharomyces cerevisiae.

A series of heterocyclic organobismuth(III) compounds 2 [ClBi(5-R-C6H(3)-2-SO2C6H(4)-1'-): R=Me, Ph, MeO, Cl, H, t-Bu, CF3, F, Me2N] was synthesized in order to study the relative importance of structure and specific substitutions in relation to their lipophilicity and antifungal activity against the yeast Saccharomyces cerevisiae. A clear structure-activity relationship between the size of the inhibition zone and the value of ClogP was found for 2. These results suggest that the higher the lipophilicity, the lower the antifungal activity. Thus, 2e (R=H) and 2h (R=F), which had ClogP values of 1.18 and 1.45, respectively, were most active. In contrast, 2b (R=Ph) and 2f (R=t-Bu) had ClogP values of 3.06 and 3.00, respectively, and exhibited no antifungal activity. Compound 6b ClBi[5-(OH)C6H(3)-2-SO(2)-5'-(OH)C6H(3)-1'-] had an estimated ClogP value of 0.81 but exhibited only low activity in spite of its low ClogP value, suggesting that such a considerable decrease in lipophilicity lowers inhibition activity. Bismuth carboxylate 7b derived from p-nitrobenzoic acid and 2e exhibited inhibition activity comparable to those of 2e and 2h despite its higher lipophilicity (ClogP=2.68).

A new efficient route to 2-substituted azulenes based on sulfonyl group directed lithiation.

Directed lithiation of p-tolyl 1-azulenyl sulfone (1) at the 2-position of the azulenyl group was achieved by using lithium 2,2,6,6-tetramethylpiperidide (LTMP). The azulenyllithium thus generated could be efficiently trapped with various electrophiles to form 2-substituted derivatives 2 in moderate to good yields. p-Tolyl 2-trimethylsilyl-1-azulenyl sulfone (2a) was transformed into cyclic sulfone derivative 3a through the directed lithiation in the p-tolyl group and subsequent intramolecular ring closure at the 8-position. 2-(Phenylsulfanyl)-1-azulenyl p-tolyl sulfone (2b) suffered from desulfonylation to form 2-phenylsulfanylazulene (4). The Suzuki coupling reaction of 2-iodo-1-azulenyl p-tolyl sulfone (2d) with arylboronic acids followed by desulfonylation efficiently gave 2-arylazulenes 10.

Two-photon absorption properties of azulenyl compounds having a conjugated ketone backbone.

Two-photon absorption (TPA) properties of newly synthesized conjugated ketone derivatives that include nonalternant azulenyl moieties in the pi-conjugation system, alpha,alpha'-bis(1-azulenylidene)cyclopentanone (1Az), alpha,alpha'-bis(2-azulenylidene)cyclopentanone (2Az), and alpha,alpha'-bis(6-azulenylidene)cyclopentanone (6Az) are reported. TPA spectra of these azulenyl compounds were measured using the open-aperture Z-scan method with a femtosecond laser. The TPA cross section at the peak position (sigma(2)peak) of 1Az was found to be the largest among the three azulenyl compounds, which is almost 7 times larger than that of the alpha,alpha'-bis(1-naphthylidene)cyclopentanone (1Nph), an alternant isomer of 1Az with the same number of pi-electrons. The small detuning energies of the azulenyl compounds compared to those of 1Nph were responsible for the large TPA cross sections. We report that a compound having an azulenyl moiety can be a promising TPA material.

Inhibition of jack bean urease by organobismuth compounds.

Inhibitory activity of organobismuth compounds, triarylbismuthanes 1 and their dihalides 2 and 3, was examined against jack bean urease. Besides triarylbismuth dichlorides 2, triarylbismuth difluorides 3 and bismuthanes 1 exhibited the activity. Of all these compounds, triphenylbismuth difluoride 3a and tris(4-fluorophenyl)bismuth dichloride 2b showed the highest activity. These results indicate that generation of the inhibitory effect is not always governed by the Lewis acidity at the bismuth center. Such a tendency of inhibition by the organobismuth compounds is in good accord with that observed in the antibacterial activity against Helicobacter pylori, suggesting that H. pylori-produced urease may be a therapeutic target by bismuth-based drugs.

Antifungal activity of organobismuth compounds against the yeast Saccharomyces cerevisiae: structure-activity relationship.

Antifungal activity of organobismuth(III) and (V) compounds 1-9 was examined against the yeast, Saccharomyces cerevisiae. A clear structure-activity relationship was observed in these compounds. Thus, triarylbismuth dichlorides 2 [(4-YC6H4)3BiCl2: Y=MeO, F, Cl, CF3, CN, NO2] and halobismuthanes 6 [2-(t)BuSO2C6H4(4-YC6H4)BiX: Y=MeO, Me, H, Cl; X=Cl, Br, I], 7 [Bi(X)(C6H4-2-SO2C6H4-1'-): X=Cl, Br, I], 8 [2-Me2NCH2C6H4(Ph)BiX: X=Cl, Br] and 9 [4-MeC6H4(8-Me2NC10H6-1-)BiCl] showed the growth inhibition effect, while triarylbismuth difluorides 3 [(4-YC6H4)3BiF2] and triarylbismuthanes 1 [(4-YC6H4)3Bi], 4 [2-(t)BuSO2C6H4(4-YC6H4)2Bi] and 5 [4-YC6H4Bi(C6H4-2-SO2C6H4-1'-)] were not active at all irrespective of the nature of the substituents. Generation of the inhibition effect is governed by the facility of nucleophilic reaction at the bismuth center and the Lewis acidic bismuth center is an active site. Of all the bismuth compounds attempted, halobismuthanes 7 derived from diphenyl sulfone exhibited the highest activities. An X-ray crystallographic study of 7a [Bi(Cl)(C6H4-2-SO2C6H4-1'-)] revealed that the bismuth center adopts a seven-coordinated geometry, which is unusual in organobismuth(III) compounds, through the intramolecular and intermolecular coordination between the bismuth and oxygen atoms. The marked inhibition effect of 7 may be attributed to such a highly coordinated geometry, which allows the bismuth center to bind tightly with some biomolecules playing important roles in the growth of S. cerevisiae.

Structure and Properties of 3-(Diethylboryl)pyridines(1).

3-(Diethylboryl)pyridine (1a), a versatile starting material for the preparation of arylpyridines, is notable for its stability under ambient conditions, in spite of little steric hindrance on the boron atom. (1)H and (13)C spectra of 1a indicated that the boryl group does not act as a mere pi-acceptor and that the boron atom is shielded by ca. 50 ppm even when compared with trivalent boron atoms conjugated with the pi-donor. A single-crystal X-ray crystallographic study for 1a revealed formation of a cyclic-tetramer with a void via the intermolecular boron-nitrogen coordination bond. Vapor pressure osmometry in various solvents suggested that 1a comprises the tetramer in these solutions. In order to know the actual structure, synthesis of 3-(2-methoxyethoxy)-5-(diethylboryl)pyridine (1b) and its scrambling experiment with 1a were carried out. Heating at 100 degrees C for 24 h was required to attain the equilibrium of the scrambling of the component molecules in the tetramers. This means that 3-(diethylboryl)pyridines 1a and 1b comprise the rigid cyclic-tetramer in solution at ambient temperature. Compound 1b is stable in aerated tetrahydrofuran containing up to 33 % water.