Synthesis of Symmetric meso-H-Dipyrrin Hydrobromides from 2-Formyl­pyrroles

Abstract

The reaction of 2-formylpyrroles in acidic methanol gives the corresponding symmetric, meso-H-4,6-dipyrrin hydrobromides. This convenient strategy involves initial deformylation under the acidic conditions, followed by immediate in situ reaction of the resulting α-free pyrrole with the remaining 2-formylpyrrole in solution to give the dipyrrin hydrobromide salt in good yield.

Key words

pyrrole - condensation - coupling - conjugation - dipyrrin

• References and Notes

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• 26 General Procedure for the Synthesis of meso-H-4,6-Dipyrrin Hydrobromides: Aqueous HBr (48%, 1 mL) was added to a solution of 2-formylpyrrole (100 mg, 1 equiv) in MeOH (2 mL). The reaction mixture was then heated at reflux temperature, with stirring, for 1 h or until all starting material was consumed, monitored using thin layer chromatography (30% EtOAc–hexanes). The precipitated product was collected using suction filtration and the residue was washed with Et₂O to yield the respective meso-H-4,6-dipyrrin hydrobromide.
• 27 1,3,7,9-Tetramethyl-2,8-diethyl-4,6-dipyrrin Hydrobromide (2a): The title compound was isolated as a red solid (93 mg, 84% yield); mp 225 °C (dec.). ¹H NMR (300 MHz, CDCl₃): δ = 12.90 (br s, 2 H), 7.01 (s, 1 H), 2.65 (s, 6 H), 2.41 (q, 4 H), 2.25 (s, 6 H), 1.06 (t, 6 H). ¹³C NMR (125 MHz, CDCl₃): δ = 153.9, 141.4, 130.7, 126.3, 118.8, 17.4, 14.6, 13.0, 10.2. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₅N₂: 257.2012; found: 257.2018.