AM1 study of conformational and configurational energy surfaces of 1,6-, 1,4-, and 3,4-diazacyclonona-1,2,4,5-tetraene

Abstract

An investigation employing the AM1 semiempirical SCF MO method to calculate structure optimization, conformational and configurational interconversion pathways for the two diastereoisomeric forms of 3,4-, 1,6-, and 1,4-diazacyclonona-1,2,4,5-tetraene ( 1–3) has been undertaken. For compound 1, the meso-diastereoisomer is calculated to be 12.8 kJ mol −1 more stable than the ( ± )-isomer. The chair conformation of meso- 1 is 5.3 kJ mol −1 more stable than the boat form. The ground-state conformation of ( ± )- 1, is the axial symmetrical twist-boat conformation which is 4.9 kJ mol −1 more stable than the unsymmetrical twist form. Interconversion of the two diastereoisomeric forms of 1 via CC or CN bond rotation or nitrogen atom inversion is calculated to require more than 200 kJ mol −1. The meso-isomer of 2 is calculated to be 10 kJ mol −1 more stable than the ( ± )-isomer. Configurational interconversion between the meso- and ( ± )-diastereoisomers of 2 requires about 60 kJ mol −1. The chair conformation of meso- 2 is calculated to be 5.9 kJ mol −1 less stable than the boat conformation. The chair and the boat conformations of meso-like- 3 have similar energies. The meso-like diastereoisomer of 3 is calculated to be 11 kJ mol −1 more stable than the ( ± )-isomer. Configurational interconversion of the two diastereoisomers of 3 can take place by nitrogen inversion and requires about 66 kJ mol −1.