Missing-link macrocycles:: Hybrid heterocalixarene analogues formed from several different building blocks

The synthesis and structural characterization of hybrid heterocalix[4]arene analogues containing pyrrole, benzene, methoxy-substituted benzene, and pyridine subunits is described. Macrocycles I and 2, examples of calix[2]benzene[2]pyrrole and calix[I]benzene[3]pyrrole systems, respectively, are synthesized by the condensation of pyrrole and an appropriate pheny Ibis (carbinol). Macrocycles 3 and 7, examples of calix[2] benzene[ 1]pyridine [ 1]pyrrole and calix[1]pyridine[3]pyrrole, respectively, are synthesized by the use of a carbene-based pyrrole-to-pyridine ring-expansion procedure. Single-crystal X-ray analysis reveals that compounds la, 1b, and 2b adopt 1,3-alternate conformations in the solid state, whereas compounds 3 and 7 display structures that are best described as "flattened partial cones" in terms of their conformation. (Series a refers to pure benzene-derived systems, whereas series b indicates macrocycles containing 5-mothoxyphenyl subunits). In the solid state, the methoxy-functionalized macrocycles I b and 2b, and the chloropyridine-containing macrocycle 7 exist as dimers. In the case of la and 7, these compounds interact with neutral solvent in the solid state. The conformations of the macrocycles in solution were explored by temperature-dependent proton NMR and NOESY spectral analysis. At 188K, macrocycles I a and 2 a adopt flattened 1,3-alternate conformations, whereas macrocycles 3 and 7 exist in the form of flattened partial-cone conformations. Standard proton NMR titration analyses were carried out in the case of macrocycles I a and 2 a, and reveal that at least the second of these systems is capable of binding fluoride and chloride anions in CD,Cl, solution at room temperature (K-a = 571 and 17 M-1 in the case of 2 a and F- and Cl, respectively).