Quantitative Study of Interactions between Oxygen Lone Pair and Aromatic Rings:  Substituent Effect and the Importance of Closeness of Contact

• Published in: Journal of organic chemistry Vol. 73; no. 2; pp. 689 - 693
• Main Authors: Gung, Benjamin W, Zou, Yan, Xu, Zhigang, Amicangelo, Jay C, Irwin, Daniel G, Ma, Shengqian, Zhou, Hong-Cai
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 18.01.2008; Amer Chemical Soc
• Subjects: Alicyclic compounds; Alicyclic compounds, terpenoids, prostaglandins, steroids; Benzene Derivatives - chemistry; Chemistry; Chemistry, Organic; Crystallography, X-Ray; Electrons; Exact sciences and technology; Hydrophobic and Hydrophilic Interactions; Magnetic Resonance Spectroscopy - methods; Models, Molecular; Molecular Conformation; Organic chemistry; Oxygen - chemistry; Physical Sciences; Preparations and properties; Science & Technology; Stereoisomerism; ANION-PI INTERACTIONS; RECOGNITION; COMPLEXES; ATOMS; HOST; HEXAFLUOROBENZENE INTERACTION; QUADRUPOLE-MOMENT; STACKING INTERACTIONS; CENTER-DOT-PI; BINDING; Substituent effect; Equilibrium constant; Molecular structure; Benzene derivatives; Prediction; Theoretical study; Molecular interaction; NMR spectrometry; Experimental study; X ray diffraction; Free energy; Quadrupole moment; Model compound; Aromatic compound; Fluorine Organic compounds; Moller Plesset partition; Electron acceptor; Electron donor; Low temperature; Functional group; Electrostatic model
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo702170j

Current models describe aromatic rings as polar groups based on the fact that benzene and hexafluorobenzene are known to have large and permanent quadrupole moments. This report describes a quantitative study of the interactions between oxygen lone pair and aromatic rings. We found that even electron-rich aromatic rings and oxygen lone pairs exhibit attractive interactions. Free energies of interactions are determined using the triptycene scaffold and the equilibrium constants were determined by low-temperature 1H NMR spectroscopy. An X-ray structure analysis for one of the model compounds confirms the close proximity between the oxygen and the center of the aromatic ring. Theoretical calculations at the MP2/aug-cc-pVTZ level corroborate the experimental results. The origin of attractive interactions was explored by using aromatic rings with a wide range of substituents. The interactions between an oxygen lone pair and an aromatic ring are attractive at van der Waals' distance even with electron-donating substituents. Electron-withdrawing groups increase the strength of the attractive interactions. The results from this study can be only partly rationalized by using the current models of aromatic system. Electrostatic-based models are consistent with the fact that stronger electron-withdrawing groups lead to stronger attractions, but fail to predict or rationalize the fact that weak attractions even exist between electron-rich arenes and oxygen lone pairs. The conclusion from this study is that aromatic rings cannot be treated as a simple quadrupolar functional group at van der Waals' distance. Dispersion forces and local dipole should also be considered.