The self‐association of individual molecules can lead to the formation of highly complex and fascinating supramolecular aggregates. However, for binding motifs which rely only on hydrogen bonds, a combination of several such weak interactions is necessary to observe self‐association in solution. Systems based on four hydrogen bonds in a linear array can be obtained which efficiently aggregate at least in chloroform. Besides the physical‐organic characterization of these aggregates and the factors influencing their stability, such quadruple hydrogen‐bonding motifs can also be used in the field of materials science to synthesize, for the first time, supramolecular polymers through the self‐association of self‐complementary monomers. As the formation of noncovalent interactions is reversible and their strength depends significantly on the chemical environment (for example, solvent, temperature), the macroscopic properties of such polymers can be controlled by variation of these parameters; hence a first step towards intelligent materials with tailor‐made properties is made.
The synthesis of a novel water-soluble guanidiniocarbonyl pyrrole carboxylate zwitterion 2 is described, and its self-association in aqueous solutions is studied. Zwitterion 2 forms extremely stable 1:1 dimers which are held together by an extensive hydrogen bonding network in combination with two mutual interacting ion pairs as could be shown by ESI MS and X-ray structure determination. NMR dilution studies in different highly polar solvents showed that dimerization is fast on the NMR time scale with association constants ranging from an estimated 10(10) M(-1) in DMSO to a surprisingly high 170 M(-1) in water. Hence, zwitterion 2 belongs to the most efficient self-assembling systems solely on the basis of electrostatic interactions reported so far. Furthermore, an amidopyridine pyrrole carboxylic acid 10 was developed as a neutral analogue of zwitterion 2, which also dimerizes with an essentially identical hydrogen bonding pattern (according to ESI MS and X-ray structure determination) but lacking the ionic interactions. NMR binding studies demonstrated that the solely hydrogen-bonded neutral dimer of 10 is stable only in organic solvents of low polarity (K > 10(4) M(-1) in CDCl3 but <10 M(-1) in 5% DMSO in CDCl3). The comparison of both systems impressively underlines the importance of ion pair interactions for stable self-association of such H-bonded binding motifs in water.
A new type of [1]rotaxanes containing two aliphatic bridges between axle and wheel is obtained in 39% yield in a one-step synthesis starting from a [2]rotaxane which contained one sulfonamide group each in both the wheel and the axle. Temperature controlled chemoselective substitution reactions first at these sulfonamide nitrogens and then subsequently at the various other carboxamide nitrogens in the wheel and axle give rise to the formation of an isomeric mixture of three double-bridged [1]rotaxanes which could be separated by HPLC. Structure determination of the main product 3a was possible by NMR experiments supported by molecular modeling calculations. Using different reaction conditions, a double-substituted but not yet bridged [2]rotaxane 4 could be isolated as an intermediate giving further evidence for the assigned structure of 3a and the way of its formation. The shape of this double-bridged [1]rotaxane 3a reminds of a self-intertwining chiral "molecular 8", in which any possible racemization due to deslipping is hindered by the two stoppers originating from the former rotaxane axle. Hence, to the best of our knowledge this is the first example of a molecule in which both concepts, cycloenantiomerism and helical chirality, are realised in one structure. Enantiomer separation of the main product was possible by further HPLC using chiral stationary phases. The Cotton effects of the circular dichrograms are different to those of the already synthesized [1]rotaxanes bearing just one aliphatic bridge between axle and wheel.
New high-yield, threading syntheses of rotaxanes with ester, carbonate and acetal axles are reported. A phenolate anion is bound as a guest by a macrocycle, which acts as a concave template, to form a supramolecular wheeled nucleophile. This nucleophile can then react directly with appropriate components to form different types of rotaxanes. This method was used to synthesize two kinds of diester rotaxanes, which differ in that the ester groups are arranged in opposite directions, according to whether the phenolic functionality was located at the stopper component or at the axle precursor. Use of triphenylacetic acid chloride and ptritylphenol as the complementary reactive axle building blocks led to a rotaxane with only one ester functionality in the axle. This single ester rotaxane contains the shortest rotaxane axle known so far. A rotaxane with a carbonate axle is formed from the reaction between trichloromethylchloroformate and (wheeled) phenolate blocking groups. A similar reaction between dichloromethane, used as both solvent and reagent, and the (wheeled) phenolate stoppers results in the corresponding acetal rotaxane in 81 % yield. The ester, carbonate and acetal axles of the rotaxanes have been hydrolysed; this leads to the release of their wheels.
A practical and highly efficient biocatalytic synthesis of optically active (R)-4-fluorophenylethan-1-ol has been developed based on reduction of the corresponding 4-fluoroacetophenone in the presence of a tailor-made recombinant whole-cell biocatalyst, containing an alcohol dehydrogenase and a glucose dehydrogenase. The reaction proceeds in a pure aqueous solvent media at a substrate concentration of ca. 0.5 M, and gives the desired product with high conversion (> 95 %), good yield (87 %) and with an excellent enantioselectivity of > 99 % ee. In addition, activity tests further showed that also the analogous 2-and 3-fluoroacetophenones are promising substrates.
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