Self-Assembly of 4-(Diethylboryl)pyridine: Crystal Structures of the Cyclic Pentamer and Hexamer and Their Solvent-Dependent Selective Crystallization.

Two distinct oligomeric structures were obtained by the self-assembly of 4-(diethylboryl)pyridine (1). In the (1)H NMR spectrum of 1 in CDCl3, at least two sets of signals were observed for the pyridyl α- and ß-hydrogen atoms. ESI-MS, VPO, and TLC analysis revealed that 1 assembles mainly into a mixture of cyclic pentamers and hexamers in solution via intermolecular boron-nitrogen coordination bonds. Crystallization of 1 in THF by vapor diffusion of EtOH or in CHCl3 afforded the cyclic hexamer incorporating one THF molecule (16·THF) or 1.5 mol equiv of chloroform molecule (16·CHCl3), respectively. Similarly, a solution of 1 in a mixture of benzene and hexane furnished the cyclic pentamer bearing two benzene molecules (15·C6H6). It seems that the solvent differences affected the crystallization of the two distinct cyclic oligomers of 1, either of which was cocrystallized predominantly with the solvent molecule. Thermogravimetric analysis of the crystals and NMR studies of the solution revealed that the noncovalent interactions between the host and guest are not strong enough to hold the guest molecule in the cavity.

The structure of 3-(diethylborylethynyl)pyridine: a nonplanarly arranged cyclic trimer.

3-(Diethylborylethynyl)pyridines 2 assemble into a cyclic trimer stabilized via intermolecular boron­nitrogen coordination bonds both in solution and in the crystalline state. The outstanding structural features of the methoxy derivative 2b in the crystalline state are that (1) two pyridine rings (P1 and P2) of the cyclic trimer of 2b are almost coplanar, and the third pyridine ring (P3) is largely bent away from P1 and P2, and (2) P3 of the cyclic trimer stacks in a face-to-face fashion with one of the pyridine rings (P3') of an adjacent cyclic trimer. The crystallographic study revealed that the conformation of the cyclic trimer is flexible enough to be affected by the crystal packing.

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.

3-(Dimethylboryl)pyridine: synthesis, structure, and remarkable steric effects in scrambling reactions.

A facile method for the synthesis of 3-(dimethylboryl)pyridine (1a) is described. Compound 1a assembles into a rigid cyclic tetramer stabilized via intermolecular boron-nitrogen coordination bonds both in the crystalline state and in solution. The outstanding structural feature of 1a, as compared with previously reported 3-(diethylboryl)pyridine (2a) (which adopts a cone conformation), is that the tetramer of 1a adopts a 1,2-alternate conformation. To investigate the effect of substituents at the boron atom on the stabilities of the oligomers, scrambling experiments of the component molecules using 1, 2, and 3-(di-n-butylboryl)pyridines 3 were carried out. Although heating at 80-90 degrees C for 20 h was required to attain the equilibrium of the scrambling reactions when the component molecules of the tetramers were 2 or 3, the scrambling in 1 proceeded under relatively mild conditions (60 degrees C, 3 h). This difference in reaction conditions required for 1, as compared to conditions required for 2 or 3, could not be explained solely by the stabilities based on bond lengths or THC. It appears that whereas only an S(N)1-type pathway may be involved in the scrambling of 2 or 3, both S(N)1- and S(N)2-type mechanisms operate simultaneously during scrambling reactions of 1 or an intermediate mechanism between S(N)1 and S(N)2 operates, which was supported by kinetic studies and calculations using model compounds.

Pyridylazulenes: synthesis, color changes, and structure of the colored product.

A facile method for the synthesis of 1- and 2-pyridylazulenes, and of 1,3-dipyridylazulenes, is described. Color and spectral changes of these pyridylazulenes upon the addition of either acid or metal ions were investigated in detail. The color changed from blue to red upon the addition of trifluoroacetic acid or soft metal ions, depending on the substitution patterns of the pyridyl group on the azulene skeleton. The structures of the protonated or coordinated products were examined on the basis of the spectral data. It was found that the protonation or coordination of metal ions occurred on the nitrogen atom of the pyridine ring, but not on the carbon atom of azulene ring. The transition intervals of several pyridylazulenes for use as pH indicators were also determined.

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.