Kazuhisa Hiratani

A new synthetic method for rotaxanes via tandem claisen rearrangement, diesterification, and aminolysis

A novel methodology to make rotaxanes via covalent bond formation has been developed. Rotaxanes composed of crownophanes having two phenolic hydroxy groups as a molecular rotor and an axle having diamide moieties were synthesized in moderate yields via three step processes: tandem Claisen rearrangement, intramolecular diesterification, and aminolysis.

The architecture of dinuclear Ni and Cu complexes: twisted and parallel forms controlled by the self-assembly of Schiff base ligands

Novel dinuclear complexes were synthesized through the self-assembly of macrocyclic Schiff base ligands and either nickel(II) or copper(II) ions. X-Ray structural analysis revealed that the complexes had either a double helix (twisted) structure in which each atom had a distorted square-planar coordination or a non-helical (parallel) structure in which the metals had octahedral coordination. The helical complexes were rather unusual in that their helicity originated not in the coordination centre, but mainly in the linker moieties in the ligand. Several factors influencing the formation of the helical structure are discussed.

Spontaneous macrocyclization via recombination of a Schiff-base linkage

Abstract A macrocyclic compound that has two salenH 2 moieties was synthesized via recombination of an acyclic Schiff base in quite high yield, and was characterized by X-ray crystallography. The recombination occurred in a slow diffusion system of chloroform–methanol, where the starting material may spontaneously form a molecular assembly suitable for the subsequent reaction. This assumption was supported by the crystal structure of an analogue of the acyclic Schiff base.

[3]Rotaxane synthesized via covalent bond formation can recognize cations forming a sandwich structure

A novel [3]rotaxane composed of two 25-membered crownophanes and one axle molecule having two anthryl end groups was successfully synthesized via covalent bond formation followed by aminolysis, and can incorporate caesium ion into the space between the two macrocycles as a 1 : 1 sandwich-type complex, whereas it makes a 1 : 2 complex with lithium ion.

Efficient synthesis of novel macrocyclic tetraamide compounds: a unique reaction process involving both self-assembling and folding of intermediates

A macrocycle having two isobutenyl and four amide moieties was successfully formed via two kinds of acyclic intermediates. These key intermediates possess the ability to self-organize due to intra- or intermolecular hydrogen-bonding interactions. In both intermediates, preorganized structures were favorably subject to nucleophilic attack at the carbonyl group by a terminal amino group, and preorganization made it possible to form the macrocycle under mild conditions.

Synthesis of Novel Bis(benzoxazole) Derivatives by Tandem Claisen Rearrangement and Their Fluorescence Behavior

Novel bis(benzoxazole) derivatives were easily synthesized from isobutenyl bis(amide−ether)s by tandem Claisen rearrangement and subsequent intramolecular cyclization of the amide−phenol intermediates. The yields of the bis(benzoxazole)s depended on whether the reaction was carried out with or without solvent, as well as on the substituents on the aryl group and the carbonyl group. The solvent effect was dramatic. No significant difference in the overall reaction rate constant with variation of the substituents on the carbonyl group was observed, but the nature of the aryl group on the ether had a large effect on the tandem Claisen rearrangement. The corresponding amide−phenol derivatives were confirmed as intermediates in this rearrangement. The fluorescence quantum yields of the obtained bis(benzoxazole)s were high, but the yields were lower than those of the corresponding monobenzoxazoles. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

A novel synthesis of chiral rotaxanes via covalent bond formation.

Chiral rotaxanes composed of the asymmetric crownophane incorporating two hydroxy groups as a rotor moiety and the asymmetric axis were effectively synthesized via covalent bond formation, i.e. tandem Claisen rearrangement, esterification, and aminolysis.

The second stable conformation of the methoxy groups of o-dimethoxybenzene: stabilization of perpendicular conformation by CH–O interaction

The conformational preference of the methoxy groups of o-dimethoxybenzene was analyzed by MP2/6-311G** level ab initio calculations. The two methoxy groups are coplanar in the most stable conformation. One methoxy group is nearly perpendicular in the second stable conformation. The calculated energy difference between the two conformations is only 0.16 kcal mol−1, which indicates that the second methoxy group at the ortho position stabilizes the perpendicular conformation. The calculated structure suggests that CH–O interaction stabilizes the perpendicular conformation. Calculated charge distributions indicate that electrostatic interaction between the two methoxy groups increases the relative stability of the perpendicular conformation.

Self-assembly of double stranded dinuclear titanium(IV)–Schiff base complexes and formation of intramolecular μ-oxo bridges

The reaction of titanium isopropoxide with a Schiff base ligand containing an isobutenyl linker leads to double stranded dinuclear titanium(IV)-Schiff base complexes through self-assembly with concomitant formation of intramolecular mu-oxo bridges upon hydrolysis.

A Macrocyclic Effect on the Reduction of p-Quinones

Abstract A macrocyclic compound that has four phenolic and four aromatic amino groups is synthesized. This compound reacts with p-quinones to give regiospecific oxidation products. During the reaction, the quinone is reduced to its hydroquinone form. The rate constant for the macrocycle is significantly large compared to its acyclic analogues. Analysis of the rate constants for several p-quinones indicates that the rate determining step involves an electron transfer process. Cyclic voltammetry measurement suggests that the complexation of the macrocycle with the quinone is related to lowering of the activation barrier of electron transfer. The role of the phenolic groups in the complexation is discussed. Based on the experimental results, the macrocyclic effect is evaluated to be 27 kJ mol−1 as the difference in Gibbs energy change necessary for the complexation.