Takeshi Namikoshi

Top-Down Preparation of Self-Supporting Supramolecular Polymeric Membranes Using Highly Selective Photocyclic Aromatization of Cis-Cisoid Helical Poly(phenylacetylene)s in the Membrane State

A novel, highly selective photocyclic aromatization (SCAT) of π-conjugated polymers from phenylacetylene having two hydroxyl groups to exclusively yield a 1,3,5-trisubstituted benzene derivative was developed, and its success was confirmed by (1)H NMR, GPC, and TOF-MS. The SCAT reaction has many unique characteristics. (1) It is a quantitative reaction: it gave only the corresponding cyclic trimer, i.e., a 1,3,5-trisubstituted benzene derivative, quantitatively (100%). No byproducts were produced under the best conditions. (2) It is an intramolecular reaction: it occurred between three adjacent monomer units in one macromolecule. (3) It is a stereospecific and topochemical or template reaction: the reactivity strongly depended on the configuration and conformation of the starting polymer substrates. (4) It is a photoreaction: high selectivity (100%) was observed only by the use of visible light irradiation, not by heating. (5) It is a solid-state reaction: high selectivity (100%) was observed only in the solid state, not in solution. In addition, (6) the resulting cyclic trimers could form a self-supporting membrane, despite their low molecular weights. This new approach resulted in a new class of supramolecular polymers consisting of a 1,3,5-trisubstituted benzene derivative, numbers of which were linearly linked by hydrogen bonds and stacked benzene derivatives. Since SCAT has such high selectivities and is useful for the preparation of a self-supporting supramolecular polymer membrane, many applications can be expected.

Helix-Sense-Selective Polymerization of Achiral Phenylacetylenes and Unique Properties of the Resulting Cis-cisoidal Polymers

ABSTRACT One of the authors discovered helix-sense-selective polymerization (HSSP) of an achiral substituted phenylacetylene which has two hydroxymethyl groups and a relatively hydrophobic substituent by using a chiral catalytic system in 2003. The one-handed helicity is stable and static in nonpolar solvents because it is maintained by intramolecular hydrogen bonds. Since the resulting polymer has extremely tight helicity in its main chain, it shows many interesting and unusual properties including highly selective photocyclic aromatization (SCAT). In this review, the discovery and development of the HSSP and SCAT reactions, and the properties and application of the resulting polymers prepared by HSSP and the resulting supramolecular polymers prepared by SCAT will be examined.

Site-Selective Aliphatic C−H Silylation of 2-Alkyloxazolines Catalyzed by Ruthenium Complexes

The Ru‐catalyzed dehydrogenative silylation of 2‐alkyloxazolines with 1,1,1,3,5,5,5‐heptamethyltrisiloxane took place site‐selectively at methyl C(sp3)−H bonds located γ to the nitrogen atom of the oxazolyl groups. Pyridine and pyrazole rings could also be used as a directing group. A catalytic mechanism based upon successive σ‐bond metathesis is proposed.

Ruthenium‐Catalyzed Ortho‐Selective Aromatic CH Borylation of 2‐Arylpyridines with Pinacolborane

The ruthenium‐catalyzed dehydrogenative borylation of 2‐arylpyridines with pinacolborane took place at ortho‐positions of the benzene ring. Density functional theory calculations and kinetic isotope effect experiments suggest that the catalytic cycle should involve oxidative addition of the CH bond, the rate‐determining σ‐bond metathesis of pinacolborane with the ruthenium hydride complex, and reductive elimination of the CB bond.

Synthesis of Stable and Soluble One-Handed Helical Homopoly(substituted acetylene)s without the Coexistence of Any Other Chiral Moieties via Two-Step Polymer Reactions in Membrane State: Molecular Design of the Starting Monomer

A soluble and stable one-handed helical poly(substituted phenylacetylene) without the coexistence of any other chiral moieties was successfully synthesized by asymmetric-induced polymerization of a chiral monomer followed by two-step polymer reactions in membrane state: (1) removing the chiral groups (desubstitution); and (2) introduction of achiral long alkyl groups at the same position as the desubstitution to enhance the solubility of the resulting one-handed helical polymer (resubstitution). The starting chiral monomer should have four characteristic substituents: (i) a chiral group bonded to an easily hydrolyzed spacer group; (ii) two hydroxyl groups; (iii) a long rigid hydrophobic spacer between the chiral group and the polymerizing group; (iv) a long achiral group near the chiral group. As spacer group a carbonate ester was selected. The two hydroxyl groups formed intramolecular hydrogen bonds stabilizing a one-handed helical structure in solution before and after the two-step polymer reactions in membrane state. The rigid long hydrophobic spacer, a phenylethynylphenyl group, enhanced the solubility of the starting polymer, and realized effective chiral induction from the chiral side groups to the main chain in the asymmetric-induced polymerization. The long alkyl group near the chiral group avoided shrinkage of the membrane and kept the reactivity of resubstitution in membrane state after removing the chiral groups. The g value (g = ([θ]/3,300)/ε) for the CD signal assigned to the main chain in the obtained final polymer was almost the same as that of the starting polymer in spite of the absence of any other chiral moieties. Moreover, since the one-handed helical structure was maintained by the intramolecular hydrogen bonds in a solution, direct observation of the one-handed helicity of the final homopolymer has been realized in CD for the solution for the first time.

Preparation of aliphatic–aromatic polyimide particles by polycondensation of diethyl hexafluoroisopropylidenediphthalate and diaminooctane in ethylene glycol

Polycondensation of nylon salt-type monomer composed of diethyl hexafluoroisopropylidenediphthalate and diaminooctane was performed at 130°C in ethylene glycol to form confetti-shaped particles. Before the polycondensation, the salt monomer solution was homogeneous. As the polycondensation proceeded, the solution became turbid and polyimide particles grew to around 7 μm for 8 h. After the polycondensation, small particles precipitated and got deposited onto large particles to form confetti-shaped particles. Rod-shaped particles were also formed by the polycondensation at low monomer concentration in a long reaction time. They were found to be crystalline in form.

Preparation of aliphatic polypyromellitimide particles by polycondensation of nylon-salt-type monomers derived from aliphatic diamines with diethyl pyromellitate in ethylene glycol

Polycondensations of nylon-salt-type monomers composed with diethyl pyromellitate and aliphatic diamines with various methylene lengths were performed at 130 °C in ethylene glycol to afford flower-like particles. Before the polycondensation, the salt monomer solution was homogenous. As the polycondensation proceeded, the solution became turbid and polyimide particles grew to around 15 µm in 8 h. Further polycondensation broke the particles into small pieces. The polyimide particles showed the distinctive odd-even effect of the methylene chain of diamine in both inherent viscosity and crystallinity. The polyimide particles using diamine with even-methylene length had higher crystallinity and lower inherent viscosity than those using diamine with odd-methylene length.