Nucleophilic substitution of 4H-imidazoles - A key step in the synthesis of fused imidazoles and new chromophores

Article abstract of DOI:10.1002/(SICI)1099-0690(200004)2000:8<1661::AID-EJOC1661>3.0.CO;2-1

The electrophilic properties of the 4H-imidazoles 1 and their protonated derivatives 2 permit the introduction of nucleophilic building blocks, as illustrated by reactions of 1 with selected amines. Depending on the nature of the amine and the substituents R¹ on the heterocycle 1, single (3) or double (4) transamination is observed. The ¹H-NMR spectra of the products, as well as X-ray structure analyses of compounds: 3f and 4c, confirm that the residues at the 4- and/or 5-positions of 1 are exchanged. The tautomerism between 3e-h and 3e'-h' seems to be central to the chemistry of these mixed substituted derivatives. Using orthoesters and acetophenone dimethylacetal as cyclization partners, the imidazo[4,5-d]imidazoles 5 and the 4H-imidazo[4,5- b]pyrazines 6 are obtained, respectively. Reduction of 3e with Zn/HCl or H₂S leads to the air-sensitive, strongly fluorescent leuco compounds 8. Quenching of 8 by addition of aromatic aldehydes results in a condensation reaction and, coupled with the subsequent redox disproportionation, this conversion constitutes an alternative route to imidazo[4,5-d]imidazoles of type 11. The unexpected reaction of 3e-h with Lawesson's reagent allows synthesis of the 6-azapentafulvenes 14. The relevant spectral data show 14 to be members of a new chromophoric system, in which an electron-rich five-membered ring is coupled with an electron-deficient ring.