New Geometries for Calix[6]arene‐Based Rotaxanes

Abstract

Understanding the role played by the nature, number, and arrangement of binding sites anchored to a macrocycle remains a topic of current interest for the synthesis of new interwoven species. We report the synthesis and detailed structural characterization of a new calix[6]arene derivative decorated with two phenylureido groups anchored at the diametrical position of the calix upper rim that adopts a 1,2,3‐alternate conformation in solution and in the solid state. Preliminary data on the ability of this host to form redox‐active pseudorotaxanes and rotaxanes are reported.

Thioureidocalix[6]arenes Pseudorotaxanes

“…Mass spectra were determined in ESI mode. TN [13] and DN, [5] axle DOV ditosylate [4c] were synthesized according to reported procedures. The synthesis of DPT and TPT was accomplished by slightly modifying previously reported protocols for DPU and TPU.…”

“…The lower stability of P[DPT(C)�DOV]2OTs with respect to P[TPT(C)�DOV]2OTs can be qualitatively justified on the basis of the lower number of HBD units. On the other hand, though, the lower stability of P[DPT(C)�DOV]2OTs with respect to P[DPU(C)�DOV] 2OTs [5] is counterintuitive, on the sole basis of the nature of the HBD units. Indeed, as suggested by NMR experiments, DPT is mainly present in a 1,2,3-alternate conformation, which is not preorganized to host the bipyridinium guest inside its cavity.…”

“…[12] We started from the known trioctyloxytrinitro (TN) [13] and dioctyloxydinitro (DN) derivatives. [5] Reduction of the nitro groups with hydrazine, in the presence of catalytic amounts of Pd/C (10 mol %), led to the corresponding tri-and di-amino intermediates, which were then reacted with an excess of phenylisothiocyanate, in CH 2 Cl 2 at room temperature. Hence, triphenylthioureido (TPT) and diphenylthioureido (DPT) calix [6]arenes were obtained in good yields (87 % and 82 %, respectively) (Scheme 1).…”

“…To this end, we introduced a novel heteroditopic receptor, the diphenylureido calix [6]arene (DPU) which presents only two HBD moieties (Figure 1b). [5] This wheel, which in low-polarity solvents is present as a mixture of 1,2, 3alternate (1,2,3-alt) and cone (C) conformations ( � 4 : 1), is able to thread bipyridinium salts, and the threading process resulted in two different pseudorotaxane systems, P[DPU(1,2,3-alt)�-DOV]2OTs and P[DPU(C)�DOV]2OTs ( � 1 : 1), in solution (Figure 1). Very recently, our group succeeded to control the conformation of calix [6]arene-based pseudorotaxanes as a function of the counterion associated with the viologen salt.…”

Ion-Pair Selective Conformational Rearrangement of Sulfonamide Calix[6]arene-Based Pseudorotaxanes

“…With the aim of understanding in more detail the role of the binding sites in the conformational control of calix[6]arene-based pseudorotaxanes, our group recently reported on a new 1,4-diphenylureido calix[6]arene (DPU) receptor that adopts a predominant 1,2,3-alternate conformation in low-polarity solvents. 4 Quite unexpectedly, although this latter wheel can form viologen-based pseudorotaxanes, an unselective 1:1 mixture, with cone and 1,2,3-alternate geometries, was obtained. 5 Because one of the most important goals of supramolecular chemistry is the control of a stimulus-induced event, which triggers a conformational preference of synthetic receptors, 6 we speculated that a calix[6]arene bearing more acidic NH-sulfonamide groups (p K a ≈ 14) would be able to respond to the complexation with a selective conformational rearrangement.…”