Abstract
A phenylacetylene containing the l-valine ethyl ester pendant (PAA-Val) was synthesized and polymerized by an organorhodium catalyst (Rh(nbd)BPh4) to produce the corresponding one-handed helical cis-poly(phenylacetylene) (PPAA-Val). PPAA-Val showed a unique temperature-triggered switchable helix-sense in chloroform, while it was not observed in highly polar solvents, such as N,N′-dimethylformamide (DMF). By heating the solution of PPAA-Val in chloroform, the sign of the CD absorption became reversed, but recovered after cooling the solution to room temperature. Even after six cycles of the heating-cooling treatment, the helix sense of the PPAA-Val’s backbone was still switchable without loss of the CD intensity. The PPAA-Val was then coated on silica gel particles to produce novel chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). These novel PPAA-Val based CSPs showed a high chiral recognition ability for racemic mandelonitrile (α = 2.18) and racemic trans-N,N′-diphenylcyclohexane-1,2-dicarboxamide (α = 2.60). Additionally, the one-handed helical cis-polyene backbone of PPAA-Val was irreversibly destroyed to afford PPAA-Val-H by heating in dimethyl sulfoxide (DMSO) accompanied by the complete disappearance of the Cotton effect. Although PPAA-Val-H had the same l-valine ethyl ester pendants as its cis-isomer PPAA-Val, it showed no chiral recognition. It was concluded that the one-handed helical cis-polyene backbone of PPAA-Val plays an important role in the chiral recognition ability.
Highlights
Biomacromelocules, such as DNA and proteins, possess helical structures, which are known to play key roles in nature and closely linked with specific activities in living systems [1,2]
PPAA-Val was soluble in CHCl3, MeOH, acetone, THF, N,N 0 -dimethylformamide (DMF), N,N’-dimethylacetamide (DMAc), and partially soluble in dimethyl sulfoxide (DMSO) at room temperature
These results indicated that as the chiral stationary phases (CSPs), the regular one-handed helical cis-polyene backbone helical cis-backbone is essential for the high chiral recognition ability of the poly(phenylacetylene) of PPAA-Val played key roles in the chiral recognition by high-performance liquid chromatography (HPLC)
Summary
Biomacromelocules, such as DNA and proteins, possess helical structures, which are known to play key roles in nature and closely linked with specific activities in living systems [1,2]. Cis-Poly(phenylacetylene) prepared by an organorhodium catalyst is a typical dynamic helical polymer [14] Their helix-helix inversions based on the dynamic cis-backbone were studied by Yashima, Freire, and other groups [15]. Only a few studies have reported results regarding the switchable endogenous helix-helix inversion of polyacetylene without the assistance of extraneous molecules Another attractive feature of optically-active poly(phenylacetylene)s is their chiral recognition ability as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). A novel one-handed helical cis-poly(phenylacetylene), in which the L-valine ethyl ester pendants are linked with the polyene main chain by an amide group (PPAA-Val), was designed and synthesized, and the effects of solvent and temperature on their helical structure and CD signals are discussed. PPAA-Val was transformed into PPAA-Val-H by heat treatment [24,25,26,27,28,29,30], and the contribution of the one-handed helical cis-structure of PPAA-Val for its chiral recognition was discussed
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