4H-Imidazoles and Imidazoles from Anionic and Dipolar Electrocyclization Reactions of 2,4-Diazapenta-1,3-dienes



1-Amino-2,4-diazapenta-1,3-dienes 5 are synthesized by the activation of N-acylamidines 2 with trifluoromethanesulfonic anhydride and subsequent condensation with amino compounds 4. When the amino compounds 4 possess electron-withdrawing substituents (e.g. alkoxycarbonyl groups, fluorenyl derivatives) this condensation leads instead to 4H-imidazoles 6 and imidazoles 7. These reactions are considered to be 1,5-dipolar electrocyclization reactions of 1,5-dipoles that are tautomers of the 2,4-diazapenta-1,3-dienes 5. Deprotonation of 5b,f by the use of strong organic bases yields the corresponding imidazoles 7b,c after amine elimination during the anionic 1,5-electrocyclization reaction. Results of quantum chemical model calculations (RHF, MP2, SCS-MP2, G3, DFT) performed on the parent 2,4-diazapentadienyl anion 9a, its lithium compound 9a-Li, the corresponding 1,5-dipole 12 and their substituted derivatives are in accordance with the suggested reaction mechanism. X-ray data are given for the 2,4-diazapenta-1,3-dienes 5a·F3CSO3H, 5c, 5d·F3CSO3H, 5e,f and the spirocyclic 4H-imidazole derivatives 6bd. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)