A series of new rotaxanes with axles different in length was prepared. Following the synthetic protocol utilizing a known anion template effect (Scheme 1), surprisingly low yields in the order of 2 ± 5% were obtained (Scheme 3), which furthermore significantly depended on the nature of the stopper (Fig. 1). Variations in the synthetic procedures and computational results from Monte Carlo simulations allowed us to analyze the origin of these findings: The rotaxane wheel 3 acts as a noncovalently bound −protecting group× for the stopper nucleophile. The protection of the nucleophilic phenolate O-atom depends much on the steric demands of the stoppers (see 2 vs. 10) which induce different conformations of the wheel. Based on this model, an improved synthetic scheme is suggested. 1. Introduction. ± The synthesis of rotaxanes, catenanes, and other types of mechanically interlocked molecules [1] strongly relies on the operation of efficient template effects [2]. There exist different apparoches including inter alia those based on the tetrahedral [3] or octahedral [4] coordination geometry of metal ions, p-donor/pacceptor interactions [5], and H-bonding involving ammoniums ions [6] or neutral amides [7]. The latter is believed to play a crucial role in the formation of a trefoil knot with twelve amide bonds [8]. Recently, Vˆgtle and co-workers reported [9] the highyield rotaxane synthesis shown in Scheme 1, which is based on the recognition of phenolate stopper 2 À within the macrocyclic rotaxane wheel 3. Two H-bonds bind the trityl phenolate with a surprisingly high binding constant of K > 10 5 m À1 [9] [10] so that the equilibrium is shifted far to the side of the stopper-wheel complex 2 À ¥ 3. If the axlecenter piece 1 is added to the reaction mixture, the semi-axle 4 is formed ± either in a direct reaction of 1 with free 2 À , or in a reaction of 1 and 2 À ¥ 3 followed by deslipping that occurs due to the much lower strength of the H-bonds formed between the wheel and the semi-axle ether O-atom. Finally, the semi-axle 4 reacts with 2 À ¥ 3 to yield the rotaxane 5 ¥ 3 in up to 95% yield.For deslipping experiments [11], we attempted to synthesize rotaxanes with axlecenter pieces of different lengths, i.e., 9a ± j (obtained from 6 and 7a ± j via 8a ± j, see Scheme 2), and stoppers of intermediate size such as 10 (Scheme 3). Surprisingly, the rotaxane yields decreased dramatically for the rotaxanes discussed here, even to below 5%. Instead, large amounts of the free axle were isolated as the by-product. Three questions arise from these findings: i) Why does the yield of rotaxane depend so much on the nature of the stopper? ii) If this is due to an unfavorable competition between axle and rotaxane formation, why is the free axle formed so much faster than the rotaxane if 10 is applied as the stopper instead of 2? iii) What is the influence of the center piece?
Several macrocycles of the Hunter-Vögtle type have been identified as superior host compounds for the detection of small amounts of acrylamide. When coated onto the surface of a quartz microbalance, these compounds serve as highly sensitive and selective sensor-active layers for their use in electronic noses. In this study, differently substituted macrocycles were investigated including an open-chain analogue and a catenane. Their structure and functional groups are correlated with their observed affinities to acrylamide and related acids and amides. The much smaller response of the open-chain compound and the almost absent sensor response of the catenane suggest that binding occurs within the cavity of the macrocycle. Theoretical calculations agree well with the experimental data even though they do not yet take into account the arrangement of the macrocycles in the sensor-active layer. The lower detection limit of acrylamide is 10 parts per billion (ppb), which is impressively low for this type of sensor. Other related compounds such as acrylic acid, propionamide, or propionic acid show no or significantly lower affinities to the macrocycles in these concentration ranges.
A new pathway for the supramolecular synthesis of oligocatenanes is developed. It is based on a combination of most suitable macrocyclic structural units, obtained from tert-butyl-substituted isophthalic acid and terephthalic acid building blocks. These structural parts guarantee, on the one hand, the solubility of the catenanes and their intermediates, and, on the other hand, the preferred formation of larger ring sizes of the macrocycles to be intertwined. Acting as monotopic and ditopic concave templates, the tetra-and octalactam macrocycles were submitted to threading procedures to yield higher-order catenanes of the amide type. By repetition of the threading steps, it was possible to isolate multiply mechanically connected [n]catenanes up to n 4 composed of various macrocyclic units.
Building bridges: The wheels of [3]rotaxane were intramolecularly tethered to yield the first member of cyclochiral, interlocked architectures named “bonnanes” (after the city Bonn, Germany; see scheme). The enantiomers and meso form were separated on an immobilized chiral phase and characterized.
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