title: In search of oligo(2-thienyl)-substituted pyridine derivatives: a modular approach to di-, tri- and tetra(2-thienyl)pyridines.
abstract: Herein, we describe our attempts to systematically prepare a series of oligo(2-thienyl)-substituted pyridine derivatives. The crucial starting material, a β-alkoxy-β-ketoenamide, is easily available on a large scale by the reaction of lithiated methoxyallene with thiophene-2-carbonitrile and thiophene-2-carboxylic acid. This three-component reaction is followed by intramolecular cyclization to yield the suitably functionalized 2,6-di(2-thienyl)-substituted pyridine derivates. The two oxygen atoms allow the programmed activation of positions C-3, C-4, or C-5 of the pyridine ring to perform palladium-catalyzed coupling reactions with thiophene-2-boronic acid or 2-(tributylstannyl)thiophene, and alternatively, reductive removal of groups. With this concept, we were able to prepare five pyridine derivatives with 2-thienyl substituents in the 2,6-, 2,3,6-, 2,4,6-, 2,3,4,6-, and 2,3,5,6-positions. 2,3,4,5,6-Penta(2-thienyl)pyridine was not available with our methods. The UV/Vis and fluorescence spectra of all pyridines were recorded and showed a dependence on the substitution pattern and protonation state. For the protonated 2,3,5,6-tetra(2-thienyl)-substituted pyridine, a Stokes shift of about 180 nm with an emission at 515 nm was observed.
authors: Institut für Chemie und Biochemie, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany.
authors: Mrinal K
authors: MK
authors: Bera
authors: Paul
authors: P
authors: Hommes
authors: Hans-Ulrich
authors: HU
authors: Reissig
mesh_terms: D002264:Carboxylic Acids; D002384:Catalysis; D003500:Cyclization; D015394:Molecular Structure; D010165:Palladium / Q000737:chemistry*; D011725:Pyridines / Q000138:chemical synthesis* / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013876:Thiophenes / Q000138:chemical synthesis* / Q000737:chemistry*
chemical_list: D002264:Carboxylic Acids; D011725:Pyridines; D013876:Thiophenes; C550746:2-thiophene carboxylic acid; D010165:Palladium