Application of NMR spectroscopy of chiral association complexes 11—Rotation about the C(sp2)—C(aryl) Bond in 2,6-Difluorobenzamides Studied by 19F NMR

The energy barrier to rotation about the C(sp²) C(aryl) single bond in non-planar N,N-dimethyl-, N,N-tetramethylene-, N,N-diisopropyl-2,6-difluorobenzamides and in N,N-dimethyl-2-chloro-6-fluorobenzamide was investigated by ¹⁹F{¹H} NMR in the presence of an optically active shift reagent. The free energy of activation was calculated from the coalescence temperature of the fluorine signals; the calculation was accomplished using the simple, approximative equation and, also, by the graphical procedure including the (eigen) line width, i.e. without the broadening caused by two-site exchange. A distinct effect of the size of the N-substituents on the ΔG_c‡values was observed, in accordance with expectation.