Vilsmeier-Haack Reaction

The Vilsmeier-Haack Reaction is often reliable method to install a formyl group on aromatic moieties. The active reactant and formylating agent N,N-dimethylchloroiminium ion (Vilsmeier reagent) is commercially available, and it can be generated in situ between most commonly between phosphoryl chloride and DMF or a similar formamide. Oxalyl chloride or thionyl chloride can also be used to generate the active ion. Upon reacting with an aromatic ring, the resulting product furnishes an iminium ion, which is then rapidly converted to the aldehyde during aqueous workup. Like most aromatic substitution reactions, Vilsmeier-Haack reaction tends to operate more efficient on electronically rich aromatic rings, and typically add to the para position or most electron-rich carbon.