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This paper describes the syntheses of [2]rotaxanes comprising 23‐ and 26‐membered biphenyl crown ethers as the macrocyclic components and secondary ammonium ions as the dumbbell‐shaped components, and the locking of the dynamic axial chirality of the biphenyl moieties in these structures. Chiral high‐performance liquid chromatography (HPLC) revealed that our [2]rotaxane featuring the 26‐membered crown ether racemized at room temperature, but the racemization of the [2]rotaxane featuring the 23‐membered crown ether did not proceed at room temperature over a period of three days. After separation of the enantiomers of the [2]rotaxane incorporating the 23‐membered crown ether through chiral HPLC, we studied its racemization at elevated temperature. The rate of stereoinversion in dimethylsulfoxide (a polar solvent) was faster than that in o‐dichlorobenzene (a nonpolar solvent), and herein we discuss these kinetic parameters. [ABSTRACT FROM AUTHOR]

A crown ether/amine‐type [2]rotaxane was synthesized and utilized as a probe for the detection of acids and anions. The addition of acids to the amine‐type [2]rotaxane solution generated corresponding crown ether/ammonium‐type [2]rotaxanes, which were purified by silica gel column chromatography as ammonium salts. The isolated yields of the [2]rotaxanes, possessing a variety of anions, depended on the acidity and polarity of the counter anions. The behaviours of the ammonium‐type [2]rotaxanes on thin‐layer chromatography (TLC) silica gel reflected the properties of the counter anions. The treatment of the amine‐type [2]rotaxane with acids afforded the corresponding ammonium‐type [2]rotaxanes bearing several different anions. The ammonium‐type [2]rotaxanes behaved similarly to the purified [2]rotaxanes on the TLC silica gel. Furthermore, we succeeded in the analysis of anions using mixtures of the amine‐type [2]rotaxane and salts in an appropriate solvent. We demonstrated the detection of anions by the combination of TLC and the utilization of the [2]rotaxane probe. [ABSTRACT FROM AUTHOR]

In this study we synthesized two‐ and four‐armed porphyrins – bearing two carboxyl and four 2‐aminoquinolino functionalities, respectively, at their meso positions – as a complementary hydrogen bonding pair for the self‐assembly of a D2‐symmetric porphyrin trimer host. Two units of the two‐armed porphyrin and one unit of the four‐armed porphyrin self‐assembled quantitatively into the D2‐symmetric porphyrin trimer, stabilized through ammidinium‐carboxylate salt bridge formation, in CH2Cl2 and CHCl3. The porphyrin trimer host gradually bound two units of 1,3,5‐trinitrobenzene between the pair of porphyrin units, forming a five‐layer aromatic structure. At temperatures below −40 °C, the rates of association and dissociation of the complexes were slow on the NMR spectroscopic time scale, allowing the 1 : 1 and 1 : 2 complexes of the trimer host and trinitrobenzene guest(s) to be detected independently when using less than 2 eq of trinitrobenzene. Vis titration experiments revealed the values of K1 (2.1±0.4×105 M−1) and K2 (2.2±0.06×104 M−1) in CHCl3 at room temperature. [ABSTRACT FROM AUTHOR]

In this study, we synthesized two porphyrins (P1 and P2) featuring two carboxybenzene and two 2‐aminoquinoline units at meso‐positions, respectively, and these porphyrins quantitatively self‐assembled into a heterodimer (P1⋅P2) through an amidinium‐carboxylate salt bridge formation in aprotic and non‐polar solvents. The capsule‐like dimer, possessing two cofacial porphyrin units, binds electron‐deficient aromatic compounds as guests, forming three‐layer π‐aromatic structures, and NMR and UV/Vis spectroscopies were used to determine the association constants (e. g. 1,3,5‐trinitrobenzene: Ka>104 M−1 in dichloromethane). The association constants and the rates of association and dissociation between the dimer and the guests were related to the electron deficiency of the aromatic guests. Capsule hotel: A self‐assembled heterodimer capsule comprising a pair of porphyrin units was constructed by using amidinium‐carboxylate salt bridge formation. The capsule binds electron‐deficient aromatic compounds, forming three‐layer π‐aromatic structures. The association constants and the rates of association and dissociation were related to the electron‐deficiency of the aromatic guests. [ABSTRACT FROM AUTHOR]

In this study, we synthesized [2]rotaxanes possessing three recognition sites-a dialkylammonium, an alkylarylamine, and a tetra(ethylene glycol) stations-in their dumbbell-like axle component and dibenzo[24]crown-8 (DB24C8) as their macrocyclic component. These [2]rotaxanes behaved as four-state molecular shuttles: i) under acidic conditions, the DB24C8 unit encircled both the dialkylammonium and alkylarylammonium stations; ii) under neutral conditions, the dialkylammonium unit was the predominant station for the DB24C8 component; iii) under basic conditions, when both ammonium centers were deprotonated, the alkylarylamine unit became a suitable station for the DB24C8 component; and iv) under basic conditions in the presence of an alkali-metal cation, the tetra(ethylene glycol) unit recognized the DB24C8 component through cooperative binding of the alkali-metal ion. In addition, we observed that the [2]rotaxanes exhibited selective recognition for metal cations. These shuttling motions of the macrocyclic component proceeded reversibly. [ABSTRACT FROM AUTHOR]

Strecker synthesis has long been considered one of the prebiotic reactions for the synthesis of α-amino acids. However, the correlation between the origin of chirality and highly enantioenriched α-amino acids through this method remains a puzzle. In the reaction, it may be conceivable that the handedness of amino acids has been determined at the formation stage of the chiral intermediate α-aminonitrile, that is, the enantioselective addition of hydrogen cyanide to an imine. Herein, an enantiotopic crystal surface of an achiral imine acted as an origin of chirality for the enantioselective formation of α-aminonitriles by the addition of HCN. In conjunction with the amplification of the enantiomeric excess and multiplication of enantioenriched aminonitrile, a large amount of near enantiopure α-amino acids, with the l- and d-handedness corresponding to the molecular orientation of the imine, is reported. [ABSTRACT FROM AUTHOR]

In this study we synthesized two acid-/base-controllable [2]rotaxanes featuring aminodiazobenzene and aminocoumarin units, respectively, as chromophores and dibenzo[24]crown-8 and dibenzo[25]crown-8 units, respectively, as their macrocyclic components. Each [2]rotaxane contained N-alkylarylamine (ammonium) and N, N-dialkylamine (ammonium) centers as binding sites for their crown ether components. The absorption patterns of the chromophores were dependent on the position of the encircling macrocyclic component and the degree of protonation, with three distinct states (under acidic, neutral, and basic conditions) evident for each [2]rotaxane. The mixed [2]rotaxane system displayed stepwise and independent molecular shuttling behavior based on the degree of protonation of the amino groups in response to both the amount and strength of added acids or bases; as such, the system provided five different absorption signals as outputs that could be read using UV/Vis spectroscopy. [ABSTRACT FROM AUTHOR]

A branched [4]rotaxane containing three switching arms with both secondary ammonium cation and aniline binding sites for threaded crown ethers was prepared. 1 H NMR spectroscopy was used to show the translational isomerisation of the [4]rotaxane, and different absorption maxima and the oxidation potential for the [4]rotaxane possessing a 1,3,5-tris(4-aminophenyl)benzene centre were monitored by ultraviolet spectroscopy and electrochemical studies, respectively, as the [4]rotaxane was exposed to encircling dibenzo[24]crown-8 species. [ABSTRACT FROM AUTHOR]

Abstract: This Letter describes the formation of pseudo[3]rotaxanes containing calix[4]-bis-crowns, exhibiting a 1,3-alternate conformation and large crown cavities, and secondary ammonium ions. The first and second association constants of pseudo[3]rotaxane formation are moderate (K 1 =175, K 2 =100M−1) and are higher than that of the corresponding pseudo[2]rotaxane (K =24M−1), consisting of a calix[4]-mono-crown and the same secondary ammonium ion. [Copyright &y& Elsevier]