Vilsmeier-Haack Reaction

The Vilsmeier Reaction allows the formylation of electron-rich arenes.

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Mechanism

The formylating agent, also known as the Vilsmeyer-Haack Reagent, is formed in situ from DMF and phosphorus oxychlorid:

An electrophilic aromatic substitution leads to α-chloro amines, which are rapidly hydrolyzed during work up to give the aldehyde: