Conformational analysis of meso- and (±)-2,3-dicyano-2,3-dicyclopropylbutane and 1,2-dicyanotetracyclopropylethane

Abstract

The dipole moments of meso-2,3-dicyano-2,3-dicyclopropylbutane meso-1, (±)-2,3-dicyano-2,3-dicyclopropylbutane (±)-1 and 1,2-dicyanotetracyclopropylethane 2 in carbon tetrachloride and benzene have been measured over a range of temperatures. Analyses of the relative permittivity data show that at 25 °C, meso-1 and (±)-1 favour the trans form. However, replacement of the methyl groups in 1 with cyclopropyl moieties completely reverses the trans ⇌ gauche equilibrium, such that 2 exists in 74%  gauche conformation. The experimentally derived values of the energy difference between the gauche and trans conformers and the gauche/trans population quotients were compared with values predicted by molecular orbital calculations. Theory predicts that the conformational preference of the substituted dicyanoethanes is strongly influenced by solvent polarity. The crystal and molecular structures of meso-1 and 2 were determined by single-crystal X-ray diffraction methods. Both compounds exist in the trans conformation in the solid state.