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.