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

Abstract

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 36.0kJ mol−1. The unsymmetrical TC conformation of meso-1 is 1.9kJmol−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 22.4kJmol−1. The axial symmetrical crown conformation of (±)-2 is calculated to be 2.8kJmol−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 (C1) form, which is 2.6kJmol−1 less stable than TCC.