Adducts of various acyclic enaminoketones and enaminoaldehydes with the Lewis acids boron trifluoride and triphenylboron were prepared. The adducts were characterized by NMR (1H, 13C, 11B) and IR spectroscopy, FAB-MS, and X-ray crystal structure analysis of the adducts of (E)-3- diethylamino-3-phenyl-2-propenal with BF3 (4a) and BPh3 (5a), respectively. The adduct formation occurs at the oxygen atom of the enaminocarbonyl compound and gives rise to a betainic structure with expressed equalization of the bond orders in the enaminocarbonyl moiety. The gas-phase structures of complexes 4a and 5a and of the corresponding free enaminoaldehyde were determined computationally by RHF and DFT methods, and a Natural Bond Orbital (NBO) analysis was undertaken.
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