Ryo Amemiya

Chiral recognition in noncovalent bonding interactions between helicenes: right-handed helix favors right-handed helix over left-handed helix

Our studies of helicenes are summarized in regard to chiral recognition phenomenon in noncovalent bonding interactions. The interactions between helical molecules show a tendency for pairs of the same configuration of the helicenes to form more stable complexes than pairs of enantiomeric helicenes. The observations are made in charge transfer complexation, crystallization, homocoupling reaction, layer structure formation, self-aggregation, and double helix formation. The interactions between a helicene and a right-handed helical polymer, double strand DNA, are also described.

Synthesis and structure of built-up organic macromolecules containing helicene.

Built-up macromolecules are acyclic molecules with molecular weights of several thousand daltons, which are synthesized by connecting small molecular units using stepwise methods. The chemical study of built-up macromolecules reveals some noteworthy properties that are different from those of conventional biological and synthetic macromolecules. A characteristic feature of built-up organic macromolecules is that their structures and properties are discontinuous at a certain molecular weight. For such macromolecules, variation in the small molecular units and the formation of cyclic structures substantially affect the structure and properties. The built-up organic macromolecules obtained by connecting helicenes with amide, acetylene, and amine groups are discussed in this paper. Some chiral built-up macromolecules are linked by covalent bonds, and the effects of linking on the structure are compared.

Hetero-Double-Helix Formation by an Ethynylhelicene Oligomer Possessing Perfluorooctyl Side Chains

Monomeric to pentameric (P)-ethynylhelicene oligomers possessing perfluorooctyl side chains were synthesized. The circular dichroism (CD) and vapor pressure osomometry (VPO) studies indicated the formation of a helix-dimer for the (P)-pentamer, for example, in trifluoromethylbenzene at 5 degrees C at concentrations above 2 x 10(-6) M. Compared with a (P)-pentamer possessing decyloxycarbonyl side chains, the perfluorooctyl (P)-pentamer exhibited lower solubilities in organic solvents, formed a thermodynamically more stable helix-dimer, and exhibited a mirror image CD spectrum. The perfluorooctyl (P)-pentamer formed a hetero-helix dimer with a decyloxycarbonyl (M)-pentamer but not with a (P)-pentamer. It indicated higher stability of the hetero-helix dimer over the homo-helix dimers.

Reversible Double-Helix–Random-Coil Transition Process of Bis{hexa(ethynylhelicene)}s

Two compounds with two hexa(ethynylhelicene) parts connected by a flexible hexadecamethylene and a rigid butadiyne linker were synthesized. The 1H NMR spectroscopic and CD analyses and vapor-pressure osmometry (VPO) of these two compounds revealed intramolecular double-helix formation. Upon heating a 5-microM solution in toluene, the double-helix structure unfolded to form a random coil, and on cooling it folded again into a double helix. The thermodynamic stabilities of both structures were dependent on temperature, and the structural change in both compounds is due to the large enthalpies and entropies under equilibrium. The rate constants of their unfolding were obtained by assuming a pseudo-first-order reaction; the compound with a rigid linker unfolded slower than that with a flexible linker. The former has a larger activation energy, and its double-helix and random-coil conformers were separated by chromatography. The rate of folding was also faster for the flexible-linker compound with larger activation energy. The rate constants for the folding of both compounds slightly decreased with increasing temperature, which was ascribed to the presence of exothermic pre-equilibrium and rate-determining steps. The folding was markedly accelerated with increasing random-coil concentration, which suggests the involvement of self-catalysis. A mechanism of folding was proposed. The involvement of different mechanisms of folding and unfolding was suggested by the kinetic studies, and it was confirmed by the presence of hysteresis in the melting profiles. The difference in linker structure also affected the thermal-switching profiles of the double-helix-random-coil structural changes.

Two-Component Gel Formation by Pseudoenantiomeric Ethynylhelicene Oligomers

Substantial interest has been expressed in the field of lowmolecular-weight gelators, which contain self-assembled structures of small molecules held together by noncovalent interactions. Along with the single low-molecular-weight gel systems, two-component gelators have been developed, in which an individual component forms an isotropic solution, and a gel is formed only in the presence of both components. Fibrous aggregates are formed from two components associated by metal–ligand coordination, salt formation, or hydrogen bonding. The two-component gel system confers tunability and controllability, because the gel properties can be varied by changing the components. However, systematic understanding of the relationship between gelation ability or gel properties and the structure of each component is still preliminary, and it is desirable to develop general and versatile methods for two-component gel formation as well as methods to control gel properties. With regard to chirality, which is generally critical for gelation, racemates are poor gelators because they tend to crystallize. Therfore, very few examples are known of racemic mixtures which form gels more effectively than single enantiomers. Herein, we reveal two-component organogel formation using chiral oligomers, each containing a different number of repeating 1,12-dimethylbenzo[c]phenanthrene units, namely helicene, which are referred to as pseudoenantiomeric oligomers in this study. This is a novel and general methodology for two-component gel formation, and various gel systems can be obtained by changing the combinations of the oligomers. In addition, the gel formation is thought to proceed with nonplanar p–p interactions between helicenes, a driving force which is different from that of conventional gels. This study was initiated after the observation that racemic oligomers formed gels, but optically pure oligomers did not (Figure 1). When toluene solutions of (M)-3 (2 mm) and (P)-3 (2 mm), ethynylhelicene trimers containing (M)and (P)helicene units, respectively, were mixed in a 1:1 ratio at 25 8C, a turbid gel was formed. The gel turned into a clear yellow solution when heated to 110 8C, and a gel was formed after cooling to room temperature. No solution of (M)-3 and (P)-3 formed again after cooling to room temperature. This thermoreversible gel formation was observed at a concentration of greater than or equal to 1 mm for each component.

Side Chain Effect on the Double Helix Formation of Ethynylhelicene Oligomers

Three series of ethynylhelicene oligomers with different side chains were synthesized: (P)-bD-n (n = 2-6) with branched alkyloxycarbonyl side chains; (P)-S-n (n = 2-7) with decylsulfanyl side chains; and (P)-DF-n (n = 4, 6, 8, 10) with alternating decyloxycarbonyl and perfluorooctyl side chains. The double helix formation of these side chain derivatives was compared to that of (P)-D-n with decyloxycarbonyl side chains. CD, UV-vis, and vapor pressure osmometry (VPO) studies showed that (P)-bD-n formed double helices as well as (P)-D-n. CD studies in trifluoromethylbenzene at different temperatures and concentrations indicated that the stability of the aggregate of (P)-bD-6 was similar to that of (P)-D-6. Bulkiness of side chains had little effect on aggregation, which indicated that π-π interactions of the aromatic moiety were essential for double helix formation. (P)-S-n were random coils in all solvents examined except in trifluoromethylbenzene. Whereas (P)-D-7 formed a double helix at 1 × 10(-3) M in toluene, (P)-S-7 was a random coil. This result indicated that the double helix forming ability of (P)-S-n was substantially lower than that of (P)-D-n. Based on the previous observation that (P)-F-n formed a more stable double helix than (P)-D-n, the order of stability may be summarized as follows: (P)-F-n > (P)-D-n and (P)-bD-n >(P)-S-n. The lower stability of (P)-S-n compared to that of (P)-F-n was ascribed to the softness and/or the electron-rich nature at the m-phenylene moiety. (P)-DF-n did not form a stable double helix. It was speculated that a regular alternating arrangement of soft/hard or electron-rich/deficient moieties is important for stable double helix formation. Side chains of ethynylhelicene oligomers can play significant roles in determining the stability of double helices.

SnCl4-Promoted Ethenylation Reaction of Hydroxylated Heteroarenes

Reaction of hydroxylated heteroarenes and acetylene in the presence of SnCl 4 and Bu 3 N (or Et 3 N) gives the corresponding ethenylated arenes. The reaction takes place at the neighboring position of the hydroxy group, andis applicable to quinolines, an isoquinoline, pyridines, and N-trifluoromethanesulfonylated indoles provided that the optimized conditions for the ethenylation and workup are employed.

Pendant-Type Helicene Oligomers with p-Phenylene Ethynylene Main Chains: Synthesis, Reversible Formation of Ladderlike Bimolecular Aggregates, and Control of Intramolecular and Intermolecular Aggregation

Pendant-type (P)-helicene oligomers with p-phenylene ethynylene main chains up to a tetramer were synthesized by a building block method. The (P)-tetramer reversibly formed a ladderlike bimolecular aggregate upon cooling and disaggregated upon heating in (trifluoromethyl)benzene. Two bis(tetramer)s, in which two (P)-tetramers were connected by hexadecamethylene linkers, were also synthesized. The head-to-tail bis(tetramer) formed an intramolecular aggregate, and the head-to-head bis(tetramer) formed an intermolecular aggregate in toluene. The results suggest the antiparallel aggregation structure of the pendant-type (P)-tetramers. The structure of the linker was proven to be effective in controlling intramolecular and intermolecular aggregations.