Conformational properties of simple monocyclic conjugated enediynes

An investigation employing the AM1 semi-empirical SCF MO method to calculate structure optimization and conformational interconversion pathways for Z-cyclonona-3-ene-1,5-diyne (1), Z-cyclodeca-3-ene-l,5-diyne (2), and Z-cycloundeca-3-ene-1, 5-diyne (3) has been undertaken. The plane-symmetrical half-chair conformation of 1 is calculated to be 23.8 kJ mol⁻¹ more stable than the planar C_2v transition-state geometry. Compound 2 has two energy-minimum conformations with the twistboat (C₂) form being 31.1 kJ mol⁻¹ more stable than the twist (C₁) geometry. The calculated energy barrier for interconversion of C₂ and C₁ conformations is 37.4 kJ mol⁻¹. The unsymmetrical twist-boat form of 3 is calculated to be the most stable conformation. The twist-boat geometry can undergo two degenerate processes to achieve a time-averaged symmetry of C_2v.