Kazuko Nakazono

Size‐Complementary Rotaxane Cross‐Linking for the Stabilization and Degradation of a Supramolecular Network

As cross-linked polymers are widely used fundamental materials, their chemical recycling becomes a quite important issue. A promising strategies is to utilize reversible crosslinking based on dynamic covalent chemistry (DCC). 3] For example, Chen et al. reported the efficient thermal reversibility of cross-linking by Diels–Alder reaction. On the other hand, supramolecular gels formed by intermolecular interactions, such as hydrogen bonds, can be efficiently de-crosslinked by specific stimuli without destruction of covalent bonds. 3d, 5–7] However, supramolecular gels are naturally stable only under limited conditions that are capable of keeping the intermolecular interactions strong. A polyrotaxane network (PRN) is a supramolecular gel stabilized by not only intermolecular interactions but also mechanical restriction. We previously reported a reversibly cross-linkable PRN based on DCC, consisting of poly(crown ether) backbone and bis(ammonium) cross-linker possessing a disulfide bond in its center (Figure 1a). The work gave a new impulse for the chemical recycling of cross-linked polymers, but it suffered from the disadvantage that it is a sluggish reaction. Therefore, we have developed a novel approach that enables the efficient de-cross-linking of PRNs without any cleavage of covalent bonds. Recently, we have found a novel procedure in which the axle component as a cross-linker has an end group the size of which is complementary to the macrocycle cavity placed on the trunk polymer (Figure 1b). The end groups provide an energy barrier to slow the dissociation, thereby kinetically stabilizing the rotaxane skeleton. Thus, the PRN stabilizes the network structure under normal conditions, but it can be de-cross-linked when certain conditions, such as those that accelerate the dissociation of the rotaxane skeletons, are satisfied. Because the decross-linking can be achieved without breaking the covalent bonds, the PRN is selectively degraded so as to not damage the trunk polymer. Furthermore, the stability and de-crosslinking capability of PRNs can be adjusted by the size of the end groups of the axle components. Herein we describe the concept of novel de-cross-linkable network polymers that utilize the size-complementary effect of the rotaxane crosslinks. First, we investigated a model for the size-complementary effect using crown ether/ammonium salt [2]rotaxanes 1 (Scheme 1). According to previous reports, roxaxanes 1 with suitable end groups (R = cyclohexyl, tBu, 4-tBuC6H4) were sufficiently stable to maintain its threaded structure owing to both the bulky end groups and hydrogen bonds between dibenzo [24]crown-8 ether (DB24C8) and the ammonium group. However, 1 dissociated into two parts, axle and wheel, when the hydrogen bonds were disturbed by a stimulus, in accordance with reported results (R = 4tBuC6H4). [12] Details are summarized in Table 1. As we envisioned, the dissociation rate depends on both the size of the end group and the kind of the external stimulus, as discussed below. An investigation into the dissociation behavior of 1 in dimethylsulfoxide (DMSO), a polar solvent that disturbed the hydrogen bonds, showed the decomposition of only 1a among three derivatives. The dissociation rate of 1a to DB24C8 and 2a obeyed first-order kinetics. The half-life (t1/2) was estiFigure 1. Strategy for de-cross-linking a PRN using a) a reversible cleavage of disulfide bond and b) characteristics of a rotaxane crosslink consisting of size-complementary components.

Selective Transformation of a Crown Ether/sec-Ammonium Salt-Type Rotaxane to N-Alkylated Rotaxanes

Versatile functionalization of a crown ether/sec-ammonium salt-type rotaxane was accomplished. The rotaxane underwent reductive N-alkylation with sodium tri(acyloxy)borohydride or sodium tri(acyloxy)borohydride/arbitrary aldehyde in excellent yields. Structural switching based on reversible tert-ammonium/tert-amine conversion by acid and base was demonstrated as a pH-controlled molecular shuttle.

Neutralization of a sec-Ammonium Group Unusually Stabilized by the “Rotaxane Effect”: Synthesis, Structure, and Dynamic Nature of a “Free” sec-Amine/Crown Ether-Type Rotaxane

A fifteen-year riddle has been settled: neutralization, the most popular chemical event, of a crown ether/sec-ammonium salt-type rotaxane has been achieved and a completely nonionic crown ether/sec-amine-type rotaxane isolated. A [2]rotaxane was prepared as a typical substrate from a mixture of dibenzo[24]crown-8 ether (DB24C8) and sec-ammonium hexafluorophosphate (PF(6)) with a terminal hydroxy group through end-capping with 3,5-dimethylbenzoic anhydride in the presence of tributylphosphane as a catalyst in 90% yield. A couple of approaches to the neutralization of the ammonium rotaxane were investigated to isolate the free sec-amine-type rotaxane by decreasing the degree of thermodynamic and kinetic stabilities. One approach was the counteranion-exchange method in which the soft counterion PF(6)(-) was replaced with the fluoride anion by mixing with tetrabutylammonium fluoride, thus decreasing the cationic character of the ammonium moiety. Subsequent simple washing with a base allowed us to isolate the free sec-amine-type rotaxane in a quantitative yield. The other approach was a synthesis based on a protection/deprotection protocol. The acylation of the sec-ammonium moiety with 2,2,2-trichloroethyl chloroformate gave an N-carbamated rotaxane that could be deprotected by treating with zinc in acetic acid to afford the corresponding free sec-amine-type rotaxane in a quantitative yield. The structure of the free sec-amine-type rotaxane was fully confirmed by spectral and analytical data. The generality of the counteranion-exchange method was also confirmed through the neutralization of a bisammonium-type [3]rotaxane. The mechanism was studied from the proposed potential-energy diagram of the rotaxanes with special emphasis on the role of the PF(6)(-) counterion.

Rational control of a polyacetylene helix by a pendant rotaxane switch

Polyacetylene bearing a pendant rotaxane moiety with an optically active wheel component was synthesized to realize reversible structural control of its helical structure by position control of the wheel component. Polyacetylene formed a one-handed helical structure only when the optically active wheel component moved close to the main chain.

Reversible helix–random coil transition of poly(m-phenylenediethynylene) by a rotaxane switch

Pendant rotaxane switch-tethering poly(m-phenylene diethynylene) was synthesized by the polyoxidative coupling of a rotaxane containing an axle-terminal m-diethynylbenzene group and an optically active crown ether. The reversible helix-random coil transition of the polymer was successfully performed by the positional switching of the rotaxane wheel.

A Rational Design for the Directed Helicity Change of Polyacetylene Using Dynamic Rotaxane Mobility by Means of Through‐Space Chirality Transfer

Directed helicity control of a polyacetylene dynamic helix was achieved by hybridization with a rotaxane skeleton placed on the side chain. Rotaxane-tethering phenylacetylene monomers were synthesized in good yields by the ester end-capping of pseudorotaxanes that consisted of optically active crown ethers and sec-ammonium salts with an ethynyl benzoic acid. The monomers were polymerized with [{RhCl(nbd)}(2)] (nbd=norbornadiene) to give the corresponding polyacetylenes in high yields. Polymers with optically active wheel components that are far from the main chain show no Cotton effect, thereby indicating the formation of racemic helices. Our proposal that N-acylative neutralization of the sec-ammonium moieties of the side-chain rotaxane moieties enables asymmetric induction of a one-handed helix as the wheel components approach the main chain is strongly supported by observation of the Cotton effect around the main-chain absorption region. A polyacetylene with a side-chain rotaxane that has a shorter axle component shows a Cotton effect despite the ammonium structure of the side-chain rotaxane moiety, thereby suggesting the importance of proximity between the wheel and the main chain for the formation of a one-handed helix. Through-space chirality induction in the present systems proved to be as powerful as through-bond chirality induction for formation of a one-handed helix, as demonstrated in an experiment using non-rotaxane-based polyacetylene that had an optically active binaphthyl group. The present protocol for controlling the helical structure of polyacetylene therefore provides the basis for the rational design of one-handed helical polyacetylenes.

Thermoresponsive Shuttling of Rotaxane Containing Trichloroacetate Ion

A thermoresponsive rotaxane shuttling system was developed with a trichloroacetate counteranion of an ammonium/crown ether-type rotaxane. Chemoselective thermal decomposition of the ammonium trichloroacetate moiety on the rotaxane yielded the corresponding nonionic rotaxane accompanied by a positional change of the crown ether on the axle. The rotaxane skeleton facilitated effective dissociation of the acid, markedly lowering the thermal decomposition temperature.

Gold‐Catalyzed Enantioselective Synthesis, Crystal Structure, and Photophysical/Chiroptical Properties of Aza[10]helicenes

The enantioselective synthesis of an aza[10]helicene, possessing two pyridone units, has been achieved by the gold-catalyzed intramolecular quadruple hydroarylation of a tetrayne. This aza[10]helicene was successfully converted into a fully aromatic aza[10]helicene, possessing two pyridine units. Structure-photophysical and chiroptical properties relationship in a series of azahelicene isomers has also been disclosed.

Efficient Synthesis of Cyclic Block Copolymers by Rotaxane Protocol by Linear/Cyclic Topology Transformation.

High-yielding synthesis of cyclic block copolymer (CBC) using the rotaxane protocol by linear-cyclic polymer topology transformation was first demonstrated. Initial complexation of OH-terminated sec-ammonium salt and a crown ether was followed by the successive living ring-opening polymerizations of two lactones to a linear block copolymer having a rotaxane structure by the final capping of the propagation end. CBC was obtained in a high yield by an exploitation of the mechanical linkage through the translational movement of the rotaxane component to transform polymer structure from linear to cyclic. Furthermore, the change of the polymer topology was translated into a macroscopic change in crystallinity of the block copolymer.

Thiazolium-Tethering Rotaxane-Catalyzed Asymmetric Benzoin Condensation: Unique Asymmetric Field Constructed by the Cooperation of Rotaxane Components

Asymmetric benzoin condensation of aromatic aldehydes with two kinds of optically active rotaxanes possessing thiazolium salt moieties was studied. A binaphthyl group as the chiral auxiliary was introduced in either the wheel or the axle component of the rotaxanes. Rate of the catalytic benzoin condensation of benzaldehyde with a rotaxane catalyst without the binaphthyl moiety was compared with its axle component to understand the effect of wheel component. Among several solvents used, methanol was the best solvent, which showed the highest yield (98%) of benzoin in the presence of 5 mol% of either the rotaxane and the axle catalysts. The benzoin condensations of aromatic aldehydes catalyzed by the chiral rotaxanes were studied in detail and found to give optically active benzoins with 0–32% e.e. in 5–92% yield depending on the structure of the rotaxane and the reaction conditions employed. From the results, two intrarotaxane chirality transfers are confirmed: (i) through-space chirality transfer from wheel to axle and (ii) through-bond chirality transfer controlled with an achiral wheel. Because these asymmetric reaction fields are specific to the rotaxane structure, the importance and possibility of the “rotaxane field” as a particular reaction field are demonstrated.