Intramolecular van der Waals Attraction. Conformational Analysis of Di(primary alkyl) Derivatives of Five-and Six-Membered Heterocyclic System.

Article abstract of DOI:10.1021/jo00232a022

Four N,N'-di(RCH2)imidiazoline-2-thiones and nine N,N'-di(RCH2)thiobarbiturates (R = methyl, isopropyl, tert-butyl, 1-adamantyl, phenyl, phenylmethyl, trimethylsilyl, trifluoromethyl, tert-butyl + phenyl) have been synthesized and investigated by dynamic NMR and molecular mechanics calculations (MM2/MMP2).Introduction of two geminal methyl groups in the molecules enabled unequivocal assignments of the stable conformers.Two conformers, syn and anti, were usually found, and in most cases the more "crowded" syn conformer predominated.The thiobarbiturates, but not imidazoline-2-thione, showed a solvent-dependent syn-anti equilibrium.Molecular mechanics calculations satisfactorily reproduced the syn-anti energy difference.Rationalization of the solvent effect and the results of the molecular mechanics calculations provide support for the operation of van der Waals attraction governing the preference for the syn conformation.