Isatogens: crystal structure, electron density calculations, and 13C nuclear magnetic resonance spectra

Abstract

The structures of 2-phenylisatogen (1a) and 2-(2-pyridyl)isatogen (1b) have been determined by X-ray crystallography. In 2-phenylisatogen the isatogen ring and the phenyl ring are almost fully co-planar. In contrast there is a dihedral angle of 45.5° between the isatogen ring and the 2-pyridyl ring in 2-(2-pyridyl)isatogen. The nitrogen atoms in the two ring systems are in a transoid relationship. Non-bonded separation distances in both compounds suggest strong dipolar associations involving the N–O bonds. MNDO Calculations using the experimental geometries of these two compounds show a high electron density at the C(2) position and at the nitrone oxygen atom O(1). ¹³C N.m.r. spectra for a number of isatogens are reported and analysed. There is no obvious relationship between the calculated total valence electron densities and chemical shift values in these compounds. A σ_p⁺ value of 0.343 has been calculated for the isatogen ring when attached to a phenyl ring. This value corresponds most closely to that for the phosphonate mono-anion. The structures, as well as chemical and biological activities of isatogens are discussed in the light of these results.