The thermal and photochemical behavior of the cyclomers derived from 1,1'-(1,3-propanediyl)bis(pyridinyl) diradicals.

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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 the energies 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, and 6c, respectively. Photodissociation of the cyclomers at −196 °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 conformations, 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.
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