The Thermal and Photochemical Behavior of the Cyclomers Derived from 1,1'-(1,3-Propanediyl)bis(pyridinyl) Diradicals

Article abstract of DOI:10.1246/bcsj.63.1413

Reduction of 1,1'-(1,3-propanediyl)bis(pyridinium) dibromide (5a) with sodium amalgam afforded the meso- and dl-cyclomers formed by intramolecular cyclization of the diradical (2a).The meso-cyclomer (6a) was thermally converted into the dl-cyclomer (7a), while retroversion of 7a into 6a was achieved photochemically.Reduction of the 4,4'-dimethyl (5b) and 4,4'-di-t-butyl (5c) derivatives of 5a similarly afforded the corresponding meso-(6b and 6c) and dl-(7b and 7c) cyclomers, which can be interconverted to each other.Using NMR spectroscopy to follow the reaction theenergies for thermal conversion of meso- to dl-cyclomers were found to be 58.6, 74.5, and 84.5 kJ mol^-1 for 6a, 6b, 6c, respectively.Photodissociation of the cyclomers at -196 deg C gave the diradicals (2a-2c), which showed characteristic triplet ESR spectra.The ESR spectra for both 2a and 2b indicated that there were two conformation, each with different zero-field splitting parameters, in 2-methyltetrahydrofuran glass.In contrast, the ESR spectrum of 2c indicated that it has only one conformation.The 4,4'-bis(methoxycarbonyl) derivative (2d) of 2a also forms cyclomers which, upon photolysis, regenerate the diradical.It was concluded that 1,1'-(1,3-propanediyl)bis(pyridinyl) diradicals are substantially in thermal equilibrium with the cyclomers.