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

ortho-Xylyl-[4]cavitands 2 to 6 were synthesized from bromoresorcin[4] arenes 1. Cavitands 2 to 6 show C-4nu symmetry in their H-1 NMR spectra at higher temperatures. Dynamic H-1 NMR studies of 2 to 6 indicate the C-4nu symmetry is due to fast interconversion of rectangular C-2nu isomers. This is supported by the crystal structure of 2a, which shows a C-2nu rectangular structure. The activation energy barriers for 2a and 2b 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 4a and 4b with metal ions raises the energy barrier further, likely via bridging metal cations. The two tetrabromo compounds (2a and 2b) reveal a sensitivity of the activation barrier to the feet of the cavitands. Cavitand 6 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. (C) 2002 Elsevier Science Ltd. All rights reserved.