A dipyridoimidazolium salt from phenylchlorocarbene and 2,2 '-bipyridyl: Synthesis, reaction mechanism, and effect of rotamerism

Phenylchlorocarbene, generated by photolysis or thermolysis of the phenylchlorodiazirine, reacts with 2,2'-bipyridyl to give, in alkane solvents, the 6-phenyldipyrido[1,2-c:2',1'-e]i-midazolium chloride. We investigated the mechanism of the reaction by flash photolysis. Like the mechanism of formation of 3-phenylindolizine from phenylchlorocarbene + 2-vinyl-pyridine, the process involves generation of the carbene, formation of a pyridinium ylide, and cyclization. of the syn rotamer of this ylide, with the two N atoms of the bipyridyl in a syn relationship. The cyclization product then eliminates Cl-to give a stable aromatic cation. We observed and identified all the intermediate species and measured the rate constants for the successive elementary reactions. This enabled us to define experimental conditions under which the reaction is nearly quantitative (the chemical yield of isolated product is > 90 %). By contrast, use of a polar solvent such as dichloromethane yields a mixture of the products resulting from the cyclization of both the syn- and the anti-bipyridylium ylide rotamers. Semiempirical calculations qualitatively account for the effect of solvent polarity on the relative rates of cyclization of the syn- and anti-bipyridylium ylide rotamers and thus on the nature of the products. These calculations predict that, in the presence of o-phenanthroline, a model for the syn bipyridyl rotamer, phenylchlorocarbene should not give the expected diazacyclopentaphenanthrene, in agreement with the experiment findings. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).