Dynamics of cyclic allenes. AM1 study of conformational energy surfaces of cycloundeca-1, 2, 5, 6-tetraene and cycloundeca-1, 2, 6, 7-tetraene

摘要

An investigation employing the AM1 semiempirical SCF MO method to calculate structure optimization and conformational interconversion pathways for the two diastereoisomeric forms of cycloundeca-1, 2, 5, 6-tetraene (1) and cycloundeca-1, 2, 6, 7-tetraene (2) has been undertaken. For both compounds, the (+-)- and meso-diastereoisomers are calculated to have fairly similar heats of formation. Two axial symmetrical conformations are found for (+-)-1: conformational interconversion of these forms requires about 36.0 kJ mol~(-1). The unsymmetrical TC conformation of meso-1 is 1.9 kJ mol~(-1) more stable than the TCC form. Conformational racemization of the TC form of meso-1 can take place via the plane-symmetrical CC geometry, and requires about 22.4 kJ mol~(-1). The axial symmetrical crown conformation of (+-)-2 is calculated to be 2.8 kJ mol~(-1) more stable than the unsymmetrical twist form. The most stable conformation of the meso-2 isomer is TCC, which lacks symmetry. Interconversion of the chiral TCC form with its mirror image conformation takes place via TBC (C_1) form, which is 2.6 kJ mol~(-1) less stable than TCC.