Fluorine Labeling of -Phenylenes to Facilitate Conformational Analysis.

H NMR spectroscopy is a powerful tool for the conformational analysis of -phenylene foldamers in solution. However, as -phenylenes are integrated into ever more complex systems, we are reaching the limits of what can be analyzed by H- and C-based NMR techniques. Here, we explore fluorine labeling of -phenylene oligomers for analysis by F NMR spectroscopy. Two series of fluorinated oligomers have been synthesized. Optimization of monomers for Suzuki coupling enables an efficient stepwise oligomer synthesis. The oligomers all adopt well-folded geometries in solution, as determined by H NMR spectroscopy and X-ray crystallography. F NMR experiments complement these methods well. The resolved singlets of one-dimensional F{H} spectra are very useful for determining relative conformer populations. The additional information from two-dimensional F NMR spectra is also clearly valuable when making H assignments. The comparison of F isotropic shielding predictions to experimental chemical shifts is not, however, currently sufficient to establish -phenylene geometries.