title: Theoretical and X-ray crystallographic evidence of a fluorine-imine gauche effect: an addendum to Dunathan's stereoelectronic hypothesis.
abstract: 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 (N=CHCH(2)F) (E(min) when φ(NCCF) =120°). Moreover, the C-F 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 C-F σ bond by an adjacent π system constitutes an addendum to Dunathan's stereoelectronic hypothesis.
authors: Swiss Federal Institute of Technology (ETH) Zürich, Laboratory for Organic Chemistry, Department of Chemistry and Applied Biosciences, Zürich, Switzerland.
authors: Christof
authors: C
authors: Sparr
authors: Evdokiya
authors: E
authors: Salamanova
authors: W Bernd
authors: WB
authors: Schweizer
authors: Hans Martin
authors: HM
authors: Senn
authors: Ryan
authors: R
authors: Gilmour
mesh_terms: D018360:Crystallography, X-Ray / Q000379:methods; D005461:Fluorine / Q000737:chemistry*; D006845:Hydrocarbons, Fluorinated / Q000737:chemistry*; D007097:Imines / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D013237:Stereoisomerism
chemical_list: D006845:Hydrocarbons, Fluorinated; D007097:Imines; D005461:Fluorine