Pirouetting in chiral [2]catenanes

One of the best known classes of mechanically interlocked molecules is the category of [2]catenanes, which exhibit donor-acceptor interactions between the 1,5-dioxynaphthalene (DNP) units in a crown ether and the bipyridinium units in the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT ⁴⁺). In order to gain an in-depth understanding and appreciation of the stereochemistry and dynamic behavior of these [2]catenanes, chiral analogues - having both the DNP ring, which is capable of displaying planar chirality, and the axially chiral binaphthol (BINAP) moiety (as both enantiomers and as the racemic modification), in a crown ether, in addition to the CBPQT⁴⁺ cyclophane - have been synthesized using a template-directed protocol. Dynamic ¹H NMR spectroscopy shows that (i) the presence of immutable axial chirality, arising from the BINAP moiety in the crown ether component, leads to no induction of diastereoselectivity - the chiral catenanes exist as a mixture of diastereoisomers in solution at low temperatures in the approximate ratio of 1:1, (ii) the barrier (ΔG_c‡) to the interconversion between these two diastereoisomers is 7.9 ± 0.1 kcal mol^-1 at 171 K, and (iii) no induction of diastereoselectivity is observed upon the addition of a chiral solvating agent to the chrial catenanes. The pattern of behavior in the variable temperature ¹H NMR spectra and the low ΔG_c‡ value indicates that the dynamic process involving the interconversion between these two diastereoisomers is one of a pirouetting nature. Of the four possible diastereoisomers, only two, (R)-(pR/pS) or (S)-(pR/pS), are shown to exist in solution.