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Stereodynamics and Edge-to-Face CH-π Aromatic Interactions in o-Phenethyl-Substituted Biaryls

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posted on 17.03.2006, 00:00 by W. Brian Jennings, Brid M. Farrell, John F. Malone
As part of a search for systems that exhibit intramolecular aromatic edge-to-face interactions, a series of four biaryl compounds containing a phenethyl side chain were prepared. These compounds exist as a slowly interconverting mixture of two atropisomers due to steric hindrance to rotation about the biaryl bond. The more thermodynamically stable isomer exhibits an abnormal shielding of an ortho-proton in solution indicative of an edge-to-face CH-π interaction with the terminal phenyl ring on the side chain. This tilted-T type of geometry was observed in the X-ray crystal structure of one of the compounds. The edge-to-face conformation in solution is estimated by variable temperature NMR studies to be energetically favored by ca. 1.6 kcal mol-1 but entropically disfavored by ca. 5.0 cal K-1 mol-1.