A pseudorotaxane consisting of a 24-membered crown ether and secondary ammonium salt with the hydroxy group at the terminus was quantitatively acylated by bulky acid anhydride in the presence of tributylphosphane as catalyst to afford the corresponding rotaxane in high yield. Large-scale synthesis without chromatographic separation was easily achieved. The ammonium group in the resulting rotaxane was quantitatively acylated with excess electrophile in the presence of excess trialkylamine. Various N-functionalized rotaxanes were prepared by this sequential double-acylation protocol. 1H NMR spectra and X-ray crystallographic analyses of the rotaxanes showed that the crown ether component was captured on the ammonium group in ammonium-type rotaxane by strong hydrogen-bonding intercomponent interaction. The conformation around the ammonium group was fixed by the hydrogen-bonding interaction. Meanwhile, the conformation of the amide-type rotaxane was determined by the weak CH/pi interaction between the methylene group in crown ether and the benzene ring of the axle component. The N-acylation of ammonium-type rotaxane is useful for the preparation of both functionalized rotaxanes and weak intercomponent interaction-based rotaxanes.
Acidity of a secondary ammonium group in a rotaxane system with a crown ether was unusually lowered. N-Acylation of the ammonium group by acid anhydrides or chlorides proceeded slowly in the presence of excess tertiary amine to give the N-acylated rotaxanes without salt structure.
A pseudorotaxane was prepared from dibenzo-24-crown-8 and a dibenzylammonium salt possessing dimethylphenyl and hydroxy groups at the both termini. Acylation of the pseudorotaxane by 3,5-dimethylbenzoic anhydride in the presence of tributylphosphine afforded corresponding rotaxane in 90% yield. Various rotaxanes were readily synthesized in excellent yields by this method.
The mixing of an acrylate 1a bearing secondary ammonium salt moiety and terminal ditert-butylphenyl group with dibenzo-24-crown-8 (DB24C8) afforded the pseudorotaxane monomer 2a. The association constant K a was 18 L mol -1 in CDCl3 at 25 °C. An equimolar mixture of 1a and DB24C8 was heated at 60 °C for 20 h in benzene (0.5 M) in the presence of AIBN to give side-chain-type polyrotaxane in 66% yield. The incorporation ratio of the rotaxane unit, which is denoted by R, in the polyrotaxane was 0.42. The R value increased as the polymerization temperature decreased or the dielectric constant of the solvent decreased. The effect of the molar ratio [DB24C8]/[1a] on the R value was complicated since DB24C8 itself enhanced the polarity of the system. The R value increased until [DB24C8]/[1a] reached 1.0 in benzene, chlorobenzene, and 1,2-dichloroethane. When [DB24C8]/[1a] > 1, however, the R value increased gradually as [DB24C8]/[1a] increased in polar solvent such as 1,2-dichloroethane, while the R value rather decreased in less polar solvent such as benzene. The copolymerization of 2a and styrene afforded the corresponding copolymer. The composition of the rotaxane unit could be controlled by the feed ratio of styrene, while the ratio of complexation of the ammonium group, i.e., rotaxane content to total acrylate, remained almost constant. The acylative neutralization of the resulting polyrotaxane afforded a nonionic polyrotaxane.
Side chain polyrotaxanes were synthesized from radically polymerizable pseudorotaxane monomers (DB24C8·1) which were composed of secondary ammonium salts having (meth)acryl group at one end and bulky stopper at the other end (1a and 1b) and dibenzo-24-crown-8 (DB24C8). Both radical polymerization of DB24C8·1 and copolymerization of DB24C8·1a with styrene afforded corresponding polymers having rotaxane moieties in the side chains (2a, 2b, and 3).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.