Pseudorotaxanes and Rotaxanes Incorporating Cycloheptatrienyl Stations − Synthesis and Co-Conformation

Novel [2]rotaxanes containing the tetracationic cyclophane cyclobis(paraquat-4,4′-biphenylene), linked mechanically to a dumbbell-shaped molecular thread incorporating a pair of diaryl cycloheptatriene units, have been synthesised in moderate yields of up to 35% by the acylative endcapping method, with the use of either bulky acyl chlorides or isocyanates. The pseudorotaxanes formed by the threading of the electron-rich molecular threads through the tetracationic ring were characterised by 1H NMR and UV/Vis spectroscopy. The nonplanar, boat-shaped cycloheptatriene rings do not hamper the threading process. According to the 1H NMR spectra, the tetracationic ring undergoes a fast shuttling process between the two cycloheptatriene stations in the rotaxanes. The unexpected signal dispersion of the cyclophane proton resonances is attributable to the existence of two diastereomeric rotaxanes. It was shown that interaction between the recognition sites is strongly influenced by the substitution pattern on the seven-membered ring; the interaction between the electron-deficient ring and a 1,3-diaryl-2,4,6-cycloheptatriene unit is much stronger than that with the regioisomeric 1,4-disubstituted pendant moiety.