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Lewis Acids as Activators in CBS‐Catalysed Diels–Alder Reactions: Distortion Induced Lewis Acidity Enhancement of SnCl 4
Author(s) -
Nödling Alexander R.,
Möckel Robert,
Tonner Ralf,
Hilt Gerhard
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201602394
Subject(s) - lewis acids and bases , chemistry , adduct , homo/lumo , reactivity (psychology) , medicinal chemistry , catalysis , lewis acid catalysis , computational chemistry , stereochemistry , organic chemistry , molecule , alternative medicine , pathology , medicine
The effect of several Lewis acids on the CBS catalyst (named after Corey, Bakshi and Shibata) was investigated in this study. While 2 H 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 Δ δ ( 2 H) 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 SnCl 4 as Lewis acid. The SnCl 4 –CBS adduct was much more reactive than the Δ δ ( 2 H) values predicted and gave similar reaction rates to those observed for the prominent AlBr 3 –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 SnCl 4 –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 SnCl 4 as Lewis acid in Diels–Alder reactions.