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

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 cyclou ndeca-1,2,6,7-tetraene (2) has been undertaken. For both compounds, the (+/ -)- and meso-diastereoisomers are calculated to have fairly similar hears o f formation. Two axial symmetrical conformations are found for (+/-)-1: con formational 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 t han 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 k J mol(-1). The axial symmetrical crown conformation of (+/-)-2 is calculate d to be 2.8 kJ mol(-1) more stable than the unsymmetrical twist form. The m ost stable conformation of the meso-2 isomer is TCC, which lacks symmetry. Interconversion of the chiral TCC form with its mirror image conformation t akes place via TBC (C-1) form, which is 2.6 kJ mol(-1) less stable than TCC . (C) 1999 Elsevier Science B.V. All rights reserved.