Spectroscopic Studies on the Structure and Conformation of Lewis Acid-Aldehyde Complexes

The complexation of aldehydes with SnCl₄ and BF₃ has been studied spectroscopically. It was determined by HOESY analysis that the aldehyde·SnCl₄ complexes examined prefer the E geometry. This preferential complexation is likely due to steric influences from the substituents on the aldehyde. A qualitative assessment of the relative basicity of various substituted aldehydes was also determined. It was found that both 2-heptynal and n-heptanal were significantly weaker Lewis bases than either (E)-2-heptenal or 4-tert-butylbenzaldehyde. The difference in basicity between (E)- 2-heptenal and 2-heptynal is thought to arise from the electronegativity of the sp carbon atom. Finally the conformation of Lewis acid complexed and neutral (E)-2-heptenal was studied in solution. When complexed to either BF₃ or SnCl₄ or even uncomplexed, (E)-2-heptenal was determined by 1D-NOE studies to be in the s-trans conformation.