Isatogens: further studies of crystal structure, electron density, and 13C and 15N NMR spectra

Abstract

The structures of 2-(pyrimidin-2-yl)(1c), 2-methoxycarbonylisatogen (1d), and 2-(1-ethyl-benzimidazol-2-yl)isatogen (1e) have been determined by X-ray crystallography. The dihedral angle between the isatogen ring and the plane of the 2-substituent varies as 43°(1c), 15°(1d), and 48°(1e). The transoid arrangement of the isatogen O(1) atom and the pyridine-type nitrogen atom N (1 5), previously reported in (1b), is preserved in (1e) despite the increased steric effects arising from the ethyl group in this molecule. There are surprising variations in the lengths of the O(1)–N(1)–C(2)–C(3)–O(3) bonds in the isatogen moiety and the C(2)–C(10) bond, this last having very little double bond character. MNDO calculations using the experimental geometries show, as before, that the C(2) atom carries a negative charge and indicate that there is a strong stereoelectronic effect arising from the centres of highest electron density in the isatogen ring O(1) and the nitrogen atoms, N(11) and N(18) in (1c) and (1e), which are remote from each other. The energies of the LUMO and HOMO orbitals indicate that the reaction between these isatogens and dipolarophiles shows the same regioselectivity reported in an earlier paper. Natural abundance ¹⁵N NMR spectra of isatogens are reported for the first time. The chemical shifts of the N(1) atom in the isatogens (1a-e) correlate negatively (r=–0.96) with ¹³C shifts of the C(3) atom, providing further evidence of the highly polarizable nature of the five-membered ring in the isatogen system.