Work

Fluorine Labeling of ortho -Phenylenes to Facilitate Conformational Analysis

Year: 2021

Type: article

Abstract: 1 H NMR spectroscopy is a powerful tool for the conformational analysis of ortho -phenylene foldamers in solution. However, as o -phenylenes are integrated into ever more complex systems, we are reaching the limits of what can be analyzed by 1 H- and 13 C-based NMR techniques. Here, we explore fluorine labeling of o -phenylene oligomers for analysis by 19 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 1 H NMR spectroscopy and X-ray crystallography. 19 F NMR experiments complement these methods well. The resolved singlets of one-dimensional 19 F{ 1 H} spectra are very useful for determining relative conformer populations. The additional information from two-dimensional 19 F NMR spectra is also clearly valuable when making 1 H assignments. The comparison of 19 F isotropic shielding predictions to experimental chemical shifts is not, however, currently sufficient by itself to establish o -phenylene geometries.

Source: The Journal of Organic Chemistry

DOI: 10.1021/acs.joc.1c01770