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Theoretical and X-ray crystallographic evidence of a fluorine-imine gauche effect: an addendum to Dunathan's stereoelectronic hypothesis

By C. Sparr, E. Salamanova, W.B. Schweizer, H.M. Senn and R. Gilmour

Abstract

The preference of &#946;-fluoroimines to adopt a <i>gauche</i> conformation has been studied by single-crystal X-ray diffraction analysis and DFT methods. Empirical and theoretical evidence for a preferential <i>gauche</i> arrangement around the NCCF torsion angle (&#966;) is presented ((<i>E</i>)-2-fluoro-<i>N</i>-(4-nitrobenzylidene)ethanamine: &#966;(NCCF) =70.0°). In the context of this study, the analysis of a pyridoxal-derived &#946;-fluoroaldimine was performed, a species that is implicated in the inhibition of pyridoxal phosphate (PLP)-dependent enzymes by &#946;-fluoroamine derivatives. The <i>gauche</i> preference of the internal aldimine (=NCH<sub>2</sub>CH<sub>2</sub>F) that can be rationalized by stereoelectronic arguments does not hold for the corresponding external system (N==CHCH<sub>2</sub>F) (<i>E</i><sub>min</sub> when &#966;NCCF=120°). Moreover, the C--F bond is lengthened by more than 0.02&#197; at &#966;NCCF=&#177;90°, when it is exactly antiperiplanar to the conjugated imine. This activation of the C--F &#963; bond by an adjacent &#960; system constitutes an addendum to Dunathan's stereoelectronic hypothesis

Topics: QD
Year: 2011
DOI identifier: 10.1002/chem.201100644
OAI identifier: oai:eprints.gla.ac.uk:50909
Provided by: Enlighten
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