Vilsmeier formylation of pyrrole

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PYRROL, THIOPHENE AND FURAN THEORY
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The Vilsmeier formylation involves the reaction of heterocycles rich in electron density with dimethylformamide in the presence of phosphorus oxytrichloride.
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Mechanism:
 
Stage 1. Formation of the Vilsmeier electrophile
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Stage 2. Nucleophilic attack of pyrrole on the Vilsmeier electrophile
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Stage 3. Recovery of aromaticity
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Stage 4. Formation of the immonium salt
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