Synthesis of 2,2'-Bipyrrole Derivatives and Their Photoluminescence Properties
Toru Okawara, et al. Molecules, 2017, 22(11), 1816.
2,2'-Bipyrroles are building blocks of natural red pigments and have been extensively studied for their luminescent properties and use in synthetic systems. Bipyrrole and its derivatives can be good candidates for developing OLED luminescent materials. For example, new π-extended pyrrole derivatives can be synthesized via aldol condensation reactions using activated methylene compounds. The introduction of vinyl groups into pyrrole or other chromophores can red-shift the absorption or fluorescence wavelengths. In addition, the introduction of ethyl or propyl substituents during pyrrole synthesis can effectively improve the solubility in organic solvents.
Synthesis of bipyrrole derivatives
· Several divinylbipyrroles bearing four ethyl groups were synthesized from3,3',4,4'-tetraethyl-5,5'-diformyl-2,2'-bipyrrole (1) and corresponding activated methylene compounds, malononitrile, 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum's acid), and 1,3-dimethylbarbituric acid, by means of an aldol condensation catalyzed by ammonium acetate.
· The crude product was precipitated from the reaction mixture by addition of water and collected by filtration. The product was purified by column chromatography using neutral alumina and eluting with dichloromethane/acetone. Recrystallization of this fraction from dichloromethane and methanol gave the analytically pure product 2-4 as a crystalline solid.
Synthesis of Carbroles from 2,2'-Bipyrrole by Condensation
Richard A Decréau, et al. Tetrahedron Letters, 2003, 44(16), 3323-3327.
Corroles are porphyrin analogs with bipyrrole ring linkages, and they have been studied as catalysts, PDT photosensitizers, etc. The synthesis of A3 and trans-A2B tris-aryl corroles can be achieved by BF3·Et2O catalyzing the condensation of dipyrromethane-dimethanol and 2,2'-bipyrrole (1:1 ratio).
Synthesis of carroles from 2,2'-bipyrrole
· The reaction was performed in acetonitrile at 0°C or at room temperature, using BF3·Et2O 2 mM as catalyst in the presence of NH4Cl or NaCl 100 mM, or TFA 15-30 mM without salt, 30 min reaction time, followed by DDQ oxidation.
· The condensation of dipyrromethane-dicarbinol and dipyrromethane leading to porphyrin used a 1:1 ratio, similarly the ratio dipyrromethene-dicarbinol 3a-c to 2,2'-bipyrrole was 1:1 since both species contain two pyrrole units.
· Due to the limited solubility of 2,2'-bipyrrole, the maximum affordable concentration of 2,2'-bipyrrole was 1.1 mM. The particular case described here may suffer from the low solubility of 4 which might be overcome by introduction of substituents increasing solubility (e.g. alkyl chains).
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