Pseudorotaxanes and rotaxanes incorporating cycloheptatrienyl stations - Synthesis and co-conformation

Novel [2]rotaxanes containing the tetracationic cyclophane cyclobis(paraqua t-4,4'-biphenylene), linked mechanically to a dumbbell-shaped molecular thr ead incorporating a pair of diaryl cycloheptatriene units, have been synthe sised in moderate yields of up to 35% by the acylative endcapping method, w ith the use of either bulky acyl chlorides or isocyanates. The pseudorotaxa nes formed by the threading of the electron-rich molecular threads through the tetracationic ring were characterised by H-1 NMR and UV/Vis spectroscop y. The nonplanar, boat-shaped cycloheptatriene rings do not hamper the thre ading process. According to the H-1 NMR spectra, the tetracationic ring und ergoes a fast shuttling process between the two cycloheptatriene stations i n 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 i nfluenced by the substitution pattern on the seven-membered ring; the inter action between the electron-deficient ring and a 1,3-diaryl-2,4,6-cyclohept atriene unit is much stronger than that with the regioisomeric 1,4-disubsti tuted pendant moiety.