Abstract
Modest functional group interactions can play important roles in molecular recognition, catalysis and self-assembly. However, weakly associated binding motifs are often difficult to characterize. Here, we report on the metal-free active template synthesis of [2]rotaxanes in one step, up to 95% yield and >100:1 rotaxane:axle selectivity, from primary amines, crown ethers and a range of C=O, C=S, S(=O)2 and P=O electrophiles. In addition to being a simple and effective route to a broad range of rotaxanes, the strategy enables 1:1 interactions of crown ethers with various functional groups to be characterized in solution and the solid state, several of which are too weak — or are disfavored compared to other binding modes — to be observed in typical host–guest complexes. The approach may be broadly applicable to the kinetic stabilization and characterization of other weak functional group interactions.
Highlights
IntroductionThe N-acylation reaction is effective: mixing together 1.0 equivalents of each of 24-crown-8 1, amine 2, and activated ester 3 in toluene at room temperature spontaneously assembles amide-axle [2]rotaxane 4 in 56% yield, without the need for any other reagents or excess building blocks (Fig. 1)
We reasoned that 4-nitrophenol, formally the other product of the N-acylation reaction, would be deprotonated by 2 and the resulting primary ammonium cation (2H+) would bind strongly to the crown ether preventing it from participating in the active template reaction
We investigated whether the yield of 4 could be improved by the addition of tertiary amines, which when protonated bind more weakly to crown ethers than primary ammonium salts[45] (Supplementary Table 1)
Summary
The N-acylation reaction is effective: mixing together 1.0 equivalents of each of 24-crown-8 1, amine 2, and activated ester 3 in toluene at room temperature spontaneously assembles amide-axle [2]rotaxane 4 in 56% yield, without the need for any other reagents or excess building blocks (Fig. 1). This potentially offers access to kinetically locked systems with unusual combinations of functional groups on the different components forced into close proximity and a 1:1 stoichiometry. Singlecrystal X-ray diffraction of the rotaxanes enabled weak interactions between the crown ether and the newly formed functional groups in the axles to be studied
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