Topically resolved intramolecular CH-pi interactions in phenylalanine derivatives.
NMR spectra of imines and nitrones derived from benzophenone and phenylalanine or tyrosine show clear evidence of an aromatic edge-to-face interaction in solution. At low temperatures the two ortho protons of the edge interacting phenyl ring become topically resolved with the ortho proton NMR signal involved in the CH-pi interactions shifted well upfield (delta 5.4-5.8 at -88 degrees C) of the other ortho signal. Introduction of a para substituent into the phenylalanine ring has a modest effect on the upfield shift. The edge-to-face arrangement also manifests in the X-ray crystal structures of two of these compounds. Barriers to rotation around the syn phenyl-imino bond are also reported (10.5-11.1 kcal mol(-1)).
Jennings
W Brian
WB
Department of Chemistry and Analytical & Biological Chemistry Research Facility, University College Cork, Cork, Ireland. brianj@ucc.ie
McCarthy
Noel J P
NJ
Kelly
Padraig
P
Malone
John F
JF
D001577:Benzophenones / Q000737:chemistry; D018360:Crystallography, X-Ray; D007097:Imines / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry; D010649:Phenylalanine / Q000737:chemistry*; D013816:Thermodynamics
D001577:Benzophenones; D007097:Imines; D009589:Nitrogen Oxides; C015388:nitrones; D010649:Phenylalanine; C047723:benzophenone