Donor-acceptor [2]catenanes based on the tetracationic cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) p-acceptor component are showing considerable promise in devices, ranging in diversity from electronic paper to molecular switches. Their preparation has traditionally been based on the synthesis of a donor-containing macrocycle, followed by the clipping of the CBPQT⁴⁺ ring around a 1,5-dioxynaphthalene or tetrathiafulvalene recognition site. Recently, we demonstrated¹ the viability of an alternative threading-followed-by-clipping approach, wherein the donor thread complexes the preformed acceptor ring and then cyclizes around it, affording the [2]catenane. We now present the third, and conceptually the most elegant method for templating the synthesis of donor-acceptor [2]catenanes. It starts with preformed individual donor and individual acceptor rings and interlocks them into a [2]catenane in a single step in the presence of NBu₄I. Mechanistically, NBu₄I acts as a homogenous nucleophilic catalyst which opens up the acceptor ring. Coordination of the “opened acceptor” with the donor component follows, forming a charge-transfer complex. Another nucleophilic reaction reseals the acceptor ring, expelling the iodide in the process and providing the [2]catenane cleanly and in high overall yields. The reaction rate is dependent on the iodide concentration and, in certain cases, appears to be in an equilibrium between the non-interlocked rings and the [2]catenane. The equilibrium can be shifted towards the [2]catenane by using an excess of either the donor or the acceptor component. This molecular analog of the magician's trick in which two rings “magically” become interlocked, is likely to become a powerful synthetic tool that would facilitate the templated production of novel catenanes, and so bring their applications within very much closer reach.

[1]        Miljanić, O. Š.; Dichtel, W. R.; Mortezaei, S.; Stoddart, J. F. Org. Lett. 2006, 8, 4835–4838.