[2+2] Photodimerization of bispyridylethylenes by a controlled shift of the protonation equilibrium
Irradiation of trans-4,4′-bispyridylethylene in the presence of 1 equiv of concd HCl produced a syn dimer with high selectivity, whereas irradiation in the presence of more than 2 equiv of concd HCl or in the absence of HCl gave a mixture of dimers and by-products with much lower selectivity. This indicated that a suitable amount of acid served as a catalyst for the [2+2] photodimerization of BPEs through cation-π interactions between the pyridinium and pyridine rings.
Pyridyl-substituted Cyclobutanes via Photodimerisation of Azastilbenes
Mono- and diazastilbenes A1 - A9 as well as their mono- and dihydro- or -alkyl quaternary salts are irradiated (UV).Mono salts are smoothly converted into cyclobutanes C in crystalline state and/or in solution.The free bases A or their bis salts, reacting in solution only, from partly or exclusively different products.Their structures together with the constitutions and configurations of the cyclobutanes A are elucidated.Results from the literature are partly confirmed and partly corrected.
Horner, Michael,Huenig, Siegfried
p. 1183 - 1210
(2007/10/02)
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