Thermal and photochemical behaviour, structure and bonding of the stereoisomers of 6,7,8,9,14a,14b-hexahydrodipyrido[1,2-a:2′,1′-c][1,4]diazocine

Abstract

The meso and (±) isomers of the title compound have been prepared by reduction of 1,1′-(butane-1,4-diyl)dipyridinium dibromide with sodium amalgam. The meso form converts thermally into the (±) form, which reverts photochemically into the meso form through the diradical intermediate. The diradical generated by photolysis was characterized by EPR and absorption spectroscopy. MNDO calculations indicate that the energetically most favourable meso and (±) conformers possess an extremely long C–C single bond of ca. 1.60 Å at the position joining the two 1,2-dihydropyridine rings, the latter being lower in energy than the former by 5.7 kcal mol^–1. The available experimental facts are reasonably accounted for by the theoretical results.