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Topically resolved intramolecular CH-pi interactions in phenylalanine derivatives
W Brian Jennings1, Noel J P McCarthy, Padraig Kelly
1Department of Chemistry and Analytical & Biological Chemistry Research Facility, University College Cork, Cork, Ireland. brianj@ucc.ie
Organic & Biomolecular Chemistry
|December 22, 2009
Summary
Aromatic edge-to-face interactions were observed in imines and nitrones using Nuclear Magnetic Resonance (NMR) spectroscopy. These CH-pi interactions cause specific proton signals to shift upfield at low temperatures, confirmed by crystal structures.
Area of Science:
- Organic Chemistry
- Supramolecular Chemistry
- Chemical Physics
Background:
- Aromatic interactions play a crucial role in molecular recognition and self-assembly.
- Understanding non-covalent interactions, such as CH-pi interactions, is vital for designing novel molecules and materials.
Purpose of the Study:
- To investigate the presence and characteristics of aromatic edge-to-face interactions in imines and nitrones derived from benzophenone and amino acids.
- To elucidate the structural and electronic consequences of these interactions in solution and solid states.
Main Methods:
- Nuclear Magnetic Resonance (NMR) spectroscopy at variable temperatures.
- X-ray crystallography for structural determination.
Main Results:
- NMR spectra revealed topical resolution of ortho protons at low temperatures, with signals involved in CH-pi interactions shifted significantly upfield (delta 5.4-5.8 at -88 °C).
- Para-substitution on the phenylalanine ring had a minor impact on the observed upfield shift.
- X-ray crystal structures confirmed the edge-to-face arrangement in the solid state.
- Barriers to rotation around the syn phenyl-imino bond were quantified (10.5-11.1 kcal mol(-1)).
Conclusions:
- The study provides clear evidence for aromatic edge-to-face interactions in solution for the studied imines and nitrones.
- These interactions influence the spectroscopic properties and molecular conformation, as supported by both NMR and crystallographic data.
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