Memory of Induced Helical Chirality

The macromolecular helicity in poly(phenylacetylene)s 36-38 (Fig. 11.9) induced by chiral amines is not static, but dynamic in nature, so that the ICD due to the helical chirality immediately disappears when exposed to a stronger acid. However, we observed an unusual, but interesting macromolecular helical memory in this dynamic helical polymer system; the helical conformations of 36-38 induced by an optically active amine such as (R)-46 were found to be retained, namely ''memorized'', after the chiral amine was completely removed and replaced by achiral amines, e.g., 65 and 66 for 36 and diamines, such as ethylenedi-amine, for 37 and 38 in DMSO (Fig. 11.13) [98-100]. The macromolecular helic-ity memory was not transient, but lasted for an extremely long time (over two years), suggesting that the thermodynamically controlled, dynamic helical conformations transform into kinetically controlled, static ones after the helicity memory is assisted by achiral amines.

The noncovalent helicity induction combined with the helicity memory is a versatile method to produce either a right- or left-handed helix with an excess of the preferred helix-sense. However, the helix-sense is predetermined by the chirality of the enantiomeric amines used. Accordingly, the opposite enantiomeric helicity memory requires the opposite enantiomeric amine, followed by replacement with achiral amines. However, both enantiomeric helices with the mirror image to each other can be produced with a high efficiency from a helical poly(phenylace-tylene) (67) induced by a single enantiomer (Fig. 11.14) [101]. This ''dual memory'' of enantiomeric helices is based on the inversion of the macromolecular helicity with temperature (see Section 11.4). The poly(phenylacetylene) folds into a one-handed helix induced by (R)-46 at 25 °C in DMSO. The helix-sense further inverts at 65 °C, as evidenced by the Cotton effect inversion. These diastereomeric right- and left-handed helices of 67 obtained at 25 and 65 °C can be further memorized by an achiral diamine such as 68 at these temperatures, resulting in the perfect mirror image Cotton effects and identical absorption spectra [101]. The chiral amplification concept can be applied to this system; a 35% ee of 46 induced as an intense ICD as that with 100% ee of 46 at 25 °C and 65 °C after helicity inversion. Subsequent replacement of the nonracemic 46 yielded the enantiomeric helices of 67 with an excess single-handedness.

The pendant phosphonate complexed with 68 appears to be achiral, but can be converted into its methyl ester with diazomethane, resulting in the generation of

.A aaL " J A A A ^
Storage

Fig. 11.14 Schematic illustration of an induced one-handed helicity in optical inactive 67, helix inversion with temperature, subsequent memory of the diastereomeric macromolecular helicity at different temperatures with achiral 68, and storage of the induced helicity and helicity memory by enantioselective esterification with diazomethane.

Storage

Memory

Storage

Fig. 11.14 Schematic illustration of an induced one-handed helicity in optical inactive 67, helix inversion with temperature, subsequent memory of the diastereomeric macromolecular helicity at different temperatures with achiral 68, and storage of the induced helicity and helicity memory by enantioselective esterification with diazomethane.

a phosphorus stereogenic center with optical activity (Fig. 11.14) [102]. The esterification proceeded enantioselectively when 67 had a helical conformation induced by 46 or macromolecular helicity memory assisted by 68. Although the enantioselectivity was low, the pendant chirality is significantly amplified in the polymer backbone at low temperatures, resulting in a higher optical activity as an excess single-handed helix than that expected from the ee of the pendant groups; the helix-sense excess of the polymer reached 62% ee at —95 ° C.

A macromolecular helicity memory has been achieved in organic solvents, but was unsuccessful in water, because dynamic helical polymers retain their helicity memory only when complexed with achiral molecules, such as achiral amines; therefore, the memory in water is lost. The recently developed layer-by-layer (LbL) assembly technique has made possible the macromolecular helicity memory in water (Fig. 11.15) [103]. A negatively charged helical poly(phenylacetylene) 37b induced by a chiral amine ((S)-69) in water showing a full ICD was deposited on a quartz substrate. Subsequently, an achiral positively charged vinylpolymer such as the hydrochloride of poly(allylamine) (PAH) was LbL assembled. The (S)-69 molecules used for the helicity induction in 37b were automatically removed during the LbL assembly process, resulting in optically active multilayer thin films with a macromolecular helicity memory after repeating the alternative deposition

11.3 Helical Polymers with Low Helix Inversion Barriers 353

Fig. 11.15 Schematic illustration of the LbL self-assembly of a charged poly(phenylacetylene) with induced macromolecular helicity. (A) An excess of the one-handed helical sense is induced in 37b with the optically active (S)-69 in water. (B) An induced helical 37b can be LbL assembled with an achiral polyelectrolyte having opposite charges (PAH), resulting in multilayer thin films with an induced macromolecular helicity memory on a substrate.

Fig. 11.15 Schematic illustration of the LbL self-assembly of a charged poly(phenylacetylene) with induced macromolecular helicity. (A) An excess of the one-handed helical sense is induced in 37b with the optically active (S)-69 in water. (B) An induced helical 37b can be LbL assembled with an achiral polyelectrolyte having opposite charges (PAH), resulting in multilayer thin films with an induced macromolecular helicity memory on a substrate.

cycle. When a positively charged, induced helical 39-HCl was used instead, the alternative deposition of an achiral vinylpolymer with opposite charges produced a similar thin film with a macromolecular helicity memory [103].

Although the chiral memory effect has also been observed in other dynamic supramolecular systems [104-108], the use of achiral guests is essential for the maintenance of the memory effect. In the absence of the achiral guest, the memory will be instantly lost. However, the sodium salt of helical 57 (57-Na) with an excess helical sense induced by (S)-69 was found to be automatically memorized after complete removal of the optically active amine in water (Fig. 11.16A) [109]. In sharp contrast to the conformational memory of the induced helical poly(phe-nylacetylene)s, the helix formation of 57-Na may be accompanied by configura-tional isomerization around the C=N double bonds (syn-anti isomerization) (Fig. 11.16B) into one single configuration upon complexation with the chiral amine, which may force the polymer backbone to take an excess helical sense. This is an unprecedented example of the synthesis of a static helical polymer after polymerization through specific noncovalent chiral interactions. The significant advantage of this helicity memory over that of helical poly(phenylacetylene)s is that there is no longer need to use the achiral chaperoning amines to retain the helic-

Fig. 11.16 Schematic illustrations of a helicity induction in 57-Na upon complexation with (S)-69 and memory of the induced macromolecular helicity after complete removal of (S)-69 (A), probably through syn-anti isomerization of the C=N bond (B), modification of the pendants with macromolecular helicity memory (C), and the replication of the macromolecular helicity (D).

Fig. 11.16 Schematic illustrations of a helicity induction in 57-Na upon complexation with (S)-69 and memory of the induced macromolecular helicity after complete removal of (S)-69 (A), probably through syn-anti isomerization of the C=N bond (B), modification of the pendants with macromolecular helicity memory (C), and the replication of the macromolecular helicity (D).

ity in the polymer, and therefore, further modifications of the side groups such as oligoglycines and crown ethers can be possible along with maintaining the macromolecular helicity memory (Fig. 11.16C).

The negatively charged 57-Na with a macromolecular helicity memory can serve as the template for further helicity induction in a different, dynamically ra-cemic helical polymer with opposite charges in water ("helicity-replication"), resulting in biomimetic interpolymer helical assemblies with a controlled helicity in water (Fig. 11.16D) [110]. Although the helical 57-Na no longer has any chiral components and stereogenic centers, the helical chirality of the polymer is efficiently transformed into a dynamically racemic, cationic polyelectrolyte 39-HCl through electrostatic interaction, resulting in the appearance of an ICD in the 39-HCl chromophore region due to an excess one-handed helicity

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