Topological transformation of a trefoil knot into a catenane
Topological transformation of a zinc-templated trefoil knot, Zn-TK, into a zinc-templated catenane, Zn-C, was studied. The net reaction 2 Zn-TK → 3 Zn-C was accomplished in 89% yield by heating a solution of Zn-TK in D2O. Kinetic investigation by 1H NMR spectroscopy and high resolution mass spectrometry revealed that the mechanism is complex, involving a large pool of intermediates that form after imine bond cleavage. Bromide ions, which can occupy the central cavity of Zn-TK, inhibited the reaction. Two similar transformations were also studied, one of a cadmium-containing trefoil knot, Cd-TK, into a cadmium-containing catenane, Cd-C, and the other of Cd-TK into Zn-C. The latter transformation could be achieved in one step at high temperature or in two steps via transmetallation to form Zn-TK at room temperature followed by topological conversion of Zn-TK to Zn-C at high temperature.