Lewis Acids as Activators in CBS-Catalysed Diels-Alder Reactions: Distortion Induced Lewis Acidity Enhancement of SnCl4

The effect of several Lewis acids on the CBS catalyst (named after Corey, Bakshi and Shibata) was investigated in this study. While H-2 NMR spectroscopic measurements served as gauge for the activation capability of the Lewis acids, in situ FT-IR spectroscopy was employed to assess the catalytic activity of the Lewis acid oxazaborolidine complexes. A correlation was found between the Delta delta(H-2) values and rate constants k(DA), which indicates a direct translation of Lewis acidity into reactivity of the Lewis acid-CBS complexes. Unexpectedly, a significant deviation was found for SnCl4 as Lewis acid. The SnCl4-CBS adduct was much more reactive than the Delta delta(H-2) values predicted and gave similar reaction rates to those observed for the prominent AlBr3-CBS adduct. To rationalize these results, quantum mechanical calculations were performed. The frontier molecular orbital approach was applied and a good correlation between the LUMO energies of the Lewis acid-CBS-naphthoquinone adducts and k(DA) could be found. For the SnCl4-CBS-naphthoquinone adduct an unusual distortion was observed leading to an enhanced Lewis acidity. Energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) calculations revealed the relevant interactions and activation mode of SnCl4 as Lewis acid in Diels-Alder reactions.