Chenfeng Ke

Catalytic Enantiodifferentiating Photocyclodimerization of 2‐Anthracenecarboxylic Acid Mediated by a Non‐Sensitizing Chiral Metallosupramolecular Host

In remarkable contrast to the recent progress in asymmetric syntheses in the ground state, its counterpart in photochemistry, or “photochirogenesis”, is still a challenge for chemists, mostly because of the short-lived weak interactions available in the excited state. [1] The success in conventional asymmetric synthesis owes largely to the use of chiral transition metal catalysts. [2] A similar approach seems to be applicable to the photochemical asymmetric synthesis, but chiral metal complexes have rarely been employed in chiral photochemistry, with the exception of a few attempts. [3] The lack of success is probably due to the photoinduced electron transfer occurring between metal and ligand or substrate in the complex, resulting in the quenching of excited substrate or dissociation of chiral ligand. The only method to achieve catalytic photochirogenesis is the use of a chiral sensitizing system. [4, 5] Therefore, the development of a novel nonsensitizing, yet catalytic, photochemical route to chiral compounds should greatly expand the range of photochirogenesis. Herein, we report the first catalytic enantiodifferentiating photoreaction mediated by a metallosupramolecular host. This system enables us not only to critically control the orientation and enantioface selectivity of substrate accommodated in a chiral host, but also to accelerate the photoreaction with a catalytic amount of host, thus providing a convenient strategy to achieve the catalytic photochirogenesis

A Polymeric Pseudorotaxane Constructed from Cucurbituril and Aniline, and Stabilization of Its Radical Cation

Polymeric (pseudo)rotaxanes constructed through the encapsulation of covalent polymers, especially p-conjugated polymers, by macrocyclic molecules have attracted increasing interest in the fields of both supramolecular chemistry and polymer chemistry because of their fascinating properties, which are different from those of the original covalent polymers. The early investigations on such polymeric (pseudo)rotaxanes usually involved cyclodextrins or crown ethers. The corresponding cucurbit[n]uril (CB[n], n is usually 6–8) systems have also recently been investigated as a consequence of their considerable binding abilities towards many cationic guests. The first attempts to construct main-chain cucurbituril-threaded polymers were performed by Kim and co-workers as well as by Tuncel and Steinke. Recently, Garcia and co-workers synthesized a polypseudorotaxane consisting of the conjugated polymer poly(phenylene vinylene) threaded through multiple cucurbit[7]uril (CB[7]) molecules, and investigated its enhanced luminescenece properties. However, many studies have been performed on conducting polymers to explore their potential applications in material chemistry and electrochemistry. Among the wide variety of conducting polymers known, polyaniline (PANI) is widely regarded as one of the eminent species because of its high conductivity (up to 1000 Scm ) and its wide applications in electronic and optical materials. In general, numerous radical cations exist in the emeraldine state, that is, the conductive doped form, of PANI, and the stabilization of these radical cations is very important to the stability and conductivity of PANI. The research groups of Goux, Han, and Dawn reported, respectively, the stabilization of PANI radical cations by modulation of the pH value, modification with propylthiosulfonate groups, and complexation with DNA. Recently, Anderson and co-workers reported the stabilization of a radical cation in a cucurbituril/oligoaniline rotaxane: they found that complexation of an oligoaniline with CB[7] could stabilize its radical cation. However, studies on the threading of CB[7]s on long PANI chains and the resulting stabilization of the radical cation have so far not been reported to the best of our knowledge. Herein, we report the construction of a polypseudorotaxane 1 by threading a PANI chain through numerous CB[7] cavities. We have also used microscopy, EPR, and cyclic voltammetry to investigate the morphology and stabilization of PANI radical cations. The results showed that, compared with free PANI, the CB[7]/PANI polypseudorotaxane had higher water solubility and the radical cation had greater stability as a result of the complexation with CB[7]. These effects will allow the further application of PANI-based supramolecular assemblies in many fields of material chemistry. The good host–guest complexation ability of cucurbiturils with aniline derivatives enabled polypseudorotaxane 1 to be conveniently prepared by a polycondensation reaction of aniline in the presence of CB[7] (Scheme 1) by using a

Critical stereocontrol by inter-amino distance of supramolecular photocyclodimerization of 2-anthracenecarboxylate mediated by 6-(ω-aminoalkylamino)-γ-cyclodextrins

A series of 6-(ω-aminoalkylamino)-6-deoxy-γ-cyclodextrins (CDs) 5–8 with varying inter-amino distances were synthesized to control the stereoselectivity of [4+4] photocyclodimerization of 2-anthracenecarboxylate (AC). Complexation behavior of these CD hosts with AC was studied in aqueous solutions by UV-vis and circular dichroism spectral titration. Supramolecular photocyclodimerization of AC mediated by the CDs was performed in water as well as in water–methanol mixture to reveal that aminopropylamino-CD 6 leads to the formation of head-to-head photodimers 3 and 4 in highest yields, while aminobutylamino-CD 7 affords chiral syn-head-to-tail and anti-head-to-head photodimers 2 and 3 in highest enantioselectivities. These results indicate that the inter-amino distance critically control the products stereo- and enantioselectivities through the electrostatic interactions of two anionic AC guests with the dicationic sidechain attached to the γ-CD rim and can be used as a convenient, yet effective, tool for manipulating the stereochemical outcomes of supramolecular photochirogenesis.

pH-Controlled Supramolecular Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate with Capped γ-Cyclodextrins

γ-Cyclodextrins capped with p-cresolbisbenzimidazole at the primary rim were synthesized and used as chiral hosts for mediating the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate. The use of 6A,6E-capped γ-cyclodextrin led to the formation of the anti-head-to-head photodimer in –5% enantiomeric excesses (ee) at pH 11 but in 28% ee at pH 6 with accompanying switching of product chirality, for which a pH-responsive conformational change of the capping moiety is likely to be responsible.

Fluorescence Sensing and Selective Binding of L- and D-Tryptophan-Modified Permethylated β-Cyclodextrins for Aliphatic Oligopeptides

Two tryptophan-modified permethylated beta-cyclodextrins, 6I-L-Trp-6I-deoxy-2I,3I-di-O-methyl-hexakis(2II-VII,3II-VII,6II-VII-tri-O-methyl-beta-cyclodextrin (3) and 6I-D-Trp- 6I-deoxy-2I,3I-di-O-methyl-hexakis(2II-VII,3II-VII,6II-VII-tri-O-methyl)-beta-cyclodextrin (4), were synthesized, and their binding behaviors were investigated with the aliphatic oligopeptides, Leu-Gly, Gly-Leu, Gly-Pro, Glu-Glu, and Gly-Gly. Fluorescence spectrophotometric studies indicated that 3 and 4 can act as efficient fluorescence sensors for aliphatic oligopeptides. Due to their intermolecular co-inclusion binding mode with substrates, 3 and 4 not only afforded high binding constants of up to 10(3)-10(4) M(-1) for guest oligopeptides but also good molecular selectivities of up to ca. 7 for Gly-Gly/Leu-Gly and Glu-Glu/Gly-Gly pairs.