Theoretical and X‐ray Crystallographic Evidence of a Fluorine‐Imine Gauche Effect: An Addendum to Dunathan’s Stereoelectronic Hypothesis

The preference of β‐fluoroimines to adopt a gauche conformation has been studied by single‐crystal X‐ray diffraction analysis and DFT methods. Empirical and theoretical evidence for a preferential gauche arrangement around the NCCF torsion angle ( ϕ ) is presented (( E )‐2‐fluoro‐ N ‐(4‐nitrobenzylidene)ethanamine: ϕ NCCF =70.0°). In the context of this study, the analysis of a pyridoxal‐derived β‐fluoroaldimine was performed, a species that is implicated in the inhibition of pyridoxal phosphate (PLP)‐dependent enzymes by β‐fluoroamine derivatives. The gauche preference of the internal aldimine (=NCH 2 CH 2 F) that can be rationalized by stereoelectronic arguments does not hold for the corresponding external system (NCHCH 2 F) ( E min when ϕ NCCF =120°). Moreover, the CF bond is lengthened by more than 0.02 Å at ϕ NCCF =±90°, when it is exactly antiperiplanar to the conjugated imine. This activation of the CF σ bond by an adjacent π system constitutes an addendum to Dunathan’s stereoelectronic hypothesis.