Takaaki Harada

Specific complexation of disaccharides with diphenyl-3,3′-diboronic acid that can be detected by circular dichroism

Abstract For the development of new receptor molecules that can recognize sugar molecules, we synthesized diphenyl-3,3′-diboronic acid (2). The distance between the two boronic acids in 2 is designed so that it can selectively form cyclic 1:1 complexes with disaccharides. It was shown that 2 forms 1:1 complexes with several disaccharides and gives the characteristic exciton coupling in CD spectroscopy owing to immobilization of the two phenyl rings. Thus, the absolute configuration was successfully predicted from the sign of the exciton coupling.

Cone - "Partial-cone" Isomerism in Tetramethoxycalix[4]arenes. Influence of Solvent Polarity and Metal Ions

Abstract It was found that the “cone” - “partial-cone” equilibrium in 25,26,27,28-tetramethoxycalix[4]arene derivatives is sensitively affected by the solvent polarity: the cone% increases with the increase in the solvent polarity. The novel finding was rationalized in terms of the difference in the dipole moment: cone conformers with four dipole moments orientating into the same direction are more polar than partial-cone conformers with one inverted phenyl unit and therefore are more stabilized in polar solvents. Alkali metal cations such as Li + and Na + strongly interact with four methoxy oxygens, resulting in the shift of the equilibrium to cone.

Computational studies of calix[4]arene homologs : influence of 5,11,17,23- and 25,26,27,28-substituents on the relative stability of four conformers

Abstract The effects of 5,11,17,23- and 25,26,27,28-substitutents on the relative stability of four conformers of calix[4]arene are discussed on the basis of a computational method using molecular mechanics (MM3) calculations. It is shown that a substituent in the lower rim site has a decisive role in the determination of the stability order among four conformers of calix[4]arene, while a substituent in the upper rim site only slightly affects the energy differences. It is also shown that in [1 4 ]metacyclophanes with no substituent in the lower rim the structure of 1,2-alternate conformers is very different from the typical 1,2-alternate structure common to calix[4]arenes.

Relative stabilities of teramethoxycalix[4]arenes: combined NMR spectroscopy and molecular mechanics studies

Relative stabilities of four conformers and thermodynamic parameters for interconversion among four conformers of 25,26,27,28-tetramethoxycalix[4]arene (1a) and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene (1b) have been determined by 1H NMR spectroscopy. The relative stability of 1a is in the order partial cone (most stable) > cone > 1,2-alternate and 1,3-alternate (undetected) and that for 1b is in the order partial cone (most stable) > cone > 1,2-alternate > 1,3-alternate (least stable). These orders are reproduced well by MM3, in contrast with MM2. In particular, the energy differences between partial cones and cones computed by MM3 (0.27 kcal mol–1 for 1a and 1.50 kcal mol–1 for 1b) show good agreement with those determined by 1H NMR spectroscopy (0.32 ± 0.13 kcal mol–1 for 1a and 1.2 ± 0.3 kcal mol–1 for 1b). Both the computational and the spectroscopic results suggest that the basic skeletons for cones, 1,2-alternates and 1,3-alternates are relatively rigid (sharp potential energy surfaces) whereas that for partial cones is more or less flexible (flattened potential energy surface). Thus, introduction of the tert-butyl groups into the para-positions destabilizes cones and 1,2-alternates because it is difficult to reduce the increased steric crowding by the conformational change. In 1,3-alternates four phenol units are parallel, so that introduced tert-butyl groups would increase the steric crowding to a lesser extent. The basic skeleton for partial cones changes significantly upon introduction of tert-butyl groups, indicating that the increased steric crowding is relaxed by the conformational change. The finding clearly explains why partial cones frequently appear as the most stable conformer.

Combined NMR spectroscopy and molecular mechanics studies of OH-depleted calix[4]arenes: On the influence of OH groups on the relative stability of calix[4]arene conformers

Abstract The effects of OH groups on the relative stabilities and the structures of four conformers of OH-depleted calix[4]arenes are discussed on the basis of a computational method using molecular mechanics (MM3(92)) calculations. The results are compared with various NMR spectroscopic experiments. From these studies, it is found that the calix[4]arene framework with no OH group in the lower rim (tetra-OH-depleted calix[4]arenes: 4a and 4b ) favors a cone conformation with C 4 symmetry and the changes in the relative stabilities in calix[4]arenes with OH-groups are due to the formation of stable hydrogen bonds and/or the relaxation of the steric crowding around OH-substituents in a lower rim site. NMR studies suggest that mono-OH-depleted calix[4]arene 1b forms intra- and intermolecular hydrogen bonds among phenolic OH groups, while di- and tri-OH-depleted calix[4]arenes ( 2b and 3b ) scarcely form it. They also show that the ring inversion barriers of calix[4]arenes increase with the number of OH groups in 25, 26, 27 or 28-position.

Fluorescence ‘reading-out’ of the molecular-recognition process

The binding of guest molecules sensitively influences the fluorescent behaviour of barbiturate-incorporated fluorescent pyrenes, making it possible to ‘read-out’ the molecular-recognition process by a fluorescence spectroscopic technique.

Regioselective intramolecular-bridging of calix[8]arenes: Unexpected isolation of a doubly-bridged unimolecular capsule-like compound

Abstract We report new methods for the synthesis of intramolecularly-bridged calix[8]arenes by bis(bromomethyl) reagents. We could isolated not only singly-bridged compounds but also a doubly-bridged compound. The doubly-bridged compound has a unique capsule-like structure, the cavity of which consists of eight phenolic oxygens.

Specific complexation of saccharides with dimeric phenylboronic acid that can be detected by circular dichroism

Abstract For the development of new receptor molecules that can recognize sugar molecules, we synthesized 2,2′-dimethoxydiphenylmethane-5,5′-diboronic acid (1) and diphenyl-3,3′-diboronic acid (2). It was shown that in the presence of 1, some mono- and disaccharides result in a CD band at 275 nm. It was also shown that 2 forms 1:1 complexes with several disaccharides and gives the characteristic exciton coupling in CD spectroscopy owing to immobilization of the two phenyl rings. The results indicate that the CD spectroscopic method using 1 or 2 as receptor molecules serves as a new sensory system for sugar molecules, and the absolute configuration of disaccharides was successfully predicted from the sign of the exciton coupling of the CD spectrum by using 2.

Molecular design of calixarene-based ion-selective electrodes

In order to obtain insights into relationships between the calix[4]arene structure and the ion selectivity in the electrode system, 20 ionophoric calix[4]arenes were synthesized and their ion selectivity (with Na+ as a standard) estimated. Among these ionophoric calix[4]arenes, 25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]-p-t-octylcalix[4]arene afforded the highest logKNA,Mpot value (−3.1) in the presence of 2-fluorophenyl-2′-nitrophenylether (10) as the best of 13 plasticizers. This is the first example in which the Na+/K+ selectivity exceeds a factor of 103 in the electrode system based on the neutral carrier. The high Na+ selectivity is attributed to modification of the upper rim which ostensibly has no relation with the component of the cavity. This paper demonstrates the potential relationships between the unique structure of the calix[4]arene-based ligands and selectivity performance for the design of ion-selective electrodes.

Binuclear metal complexes. LII [1]. Synthesis spectral and magnetic properties of binuclear copper(II) complexes of amides derived from 3-amino-1-propanol and various amino acids, and crystal structure of copper(II) complex of α-aminosobutyric acid derivative

Abstract Binuclear copper(II) complexes with amides derived from 3-amino-1-propanol and various amino acids were prepared and characterized by elemental analysis, infrared and electronic spectra and magnetic susceptibilities (78–300 K.). They exhibit a band around 28 × 103 cm−1 characteristic of alkoxo-bridged structure and show a very strong antiferromagnetic interaction. The crystal structure of one of them, [Cu2{NH2C(CH3)2 CONCH2CH2CH2O}2], was determined by the single-crystal X-ray diffraction method. Crystals are monoclinic, space group P21/n, a = 16.661(16), b = 10.256(4), c = 5.333(1) A; β = 98.69(2)°. The structure was solved by the heavy atom method and refined by the block-diagonal least-squares method to an R factor of 0.038. The crystal structure consists of alkoxo-bridged binuclear units. The coordination geometry about the copper ion is square-planar. The structure detail is in harmony with the magnetic and spectral properties.