The formation of macrocycles containing two imidazolium rings such as 1.2X and 2.2X is anion-directed through hydrogen bonding. The template effect exerted by the chloride anion in the ring-closure reaction of the monocationic model intermediate 9+ to yield the [1(4)]imidazoliophane 2(2+) has been evaluated. This effect was quantified following the kinetics of the macrocyclization by using a UV-vis technique. The rate of the ring closure of monocation 9+ is increased up to 10 times, in the presence of Bu4NCl 0.04 M. This finding confirms that the template effect is operative in the macrocyclization leading to dicationic [1(4)]imidazoliophanes.
The template effects exerted by bis(p-phenylene)[34]crown-10 (3) and by 1,5-dinaphto[38]crown-10 (4) in the ring-closure reaction of the trication 2(3+) to yield the [2]catenanes 7(4+) and 8(4+) have been quantitatively evaluated in acetonitrile at 62 degrees C by UV/visible spectroscopy. The rate of ring closure of the trication 2(3+) dramatically increases in the presence of the templates 3 and 4, up to approximately 230 times at [3] approximately equals 0.1 molL(-1), and up to approximately 1,900 times at [4] approximately equals 0.01 molL(-1). The outcome of kinetic selection experiments, in which the two crown ethers compete for the incorporation into the catenane structure, has been discussed in the light of the obtained results. It has been shown that the product ratio of catenanes obeys the Curtin-Hammett principle only if the concentrations of the templates are equal and much greater than that of the substrate. Analysis of the rate profiles has shown that the 1,5-dioxynaphthalene unit, present in the template 4, has a greater affinity than the 1,4-dioxybenzene unit, present in the template 3, for the electron-deficient pyridinium rings present in both the transition-state and substrate structures. Ab initio calculations at the 3-21G and 6-31G(d) levels of theory indicate that the greater affinity of the 1,5-dioxynaphthalene unit cannot be explained on the basis of greater pi-pi stacking and [C-H...pi] interactions, but rather on the basis of the model of apolar complexation in which the solvent plays a major role.
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