Synthesis and conformational dynamics of ortho-xylyl-bridged-[4]cavitands

ortho-Xylyl-[4]cavitands to were synthesized from bromoresorcin[4] arenes . Cavitands to show C4v symmetry in their 1H NMR spectra at higher temperatures. Dynamic 1H NMR studies of to indicate the C4v symmetry is due to fast interconversion of rectangular C2v isomers. This is supported by the crystal structure of , which shows a C2v rectangular structure. The activation energy barriers for and were measured by quantitative 1D NOESY (EXSY) experiments. The results were analyzed by the initial rate approximation and matrix calculations. The activation energy barrier was found to depend strongly on the size of the upper rim group (Br, H, OH, CN). In addition, replacing the phenolic proton of tetrol derivatives and with metal ions raises the energy barrier further, likely via bridging metal cations. The two tetrabromo compounds ( and ) reveal a sensitivity of the activation barrier to the feet of the cavitands. Cavitand may act as a hub for radially expanded cavitands. The advantages of the 1D NOESY experiments over the more commonly used two-dimensional experiments are discussed.