Intra‐ and Intermolecular C(sp2)–H···O Hydrogen Bonds in a Series of Isobenzofuranone Derivatives: Manifestation and Energetics

Abstract

AbstractDerivatives 2–6 were prepared as models for studying intra‐ and intermolecular C(sp2)–H···O hydrogen bonding. Their X‐ray structures confirm the presence of intramolecular hydrogen bonds in derivatives of the “a” series: the corresponding C···O distances vary between 2.91 and 2.97 Å. The corresponding 13C–1H coupling constants are increased by about 7.5 Hz, and the 1H chemical shifts in CDCl3 are 9.1–10.7 ppm. No intramolecular hydrogen bonds can form in derivatives of the isomeric “b” series. In this series, the chemical shifts of the corresponding aromatic protons exhibit strong solvent dependency; in particular, they are as sensitive as the proton in chloroform to the presence of DMSO. The vinylic protons activated by the electron‐accepting COOR groups behave similarly. Quantum mechanical calculations in the gas phase and in DMSO reproduce the experimental observations. Energies of the intramolecular hydrogen bonds evaluated by two independent approaches vary between 3.7 and 4.4 kcal mol–1 in the gas phase and still amount to at least 2.5 kcal mol–1 in 2a in DMSO. These estimates are practically independent of the computational method (HF, MP2, and DFT B3LYP were employed for derivatives 2). We conclude that the behavior of both activated aromatic and vinylic C(sp2)–H atoms in the studied derivatives is qualitatively and quantitatively similar to the behavior of the C(sp3)–H atom in chloroform. The existence of hydrogen bonds involving these atoms can easily be detected by NMR spectroscopy.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)