Vilsmeier-Haack reaction

The Vilsmeier-Haack reaction (also called the Vilsmeier reaction) is the chemical reaction of a substituted amide (1) with phosphorus oxychloride and an activated arene (3) to produce an aryl aldehyde or ketone (5). The reaction of a substituted amide with phosphorus oxychloride gives a substituted chloroiminium ion (2), also called the Vilsmeier reagent. The initial product is an iminium ion (4b), which is hydrolyzed to the corresponding aromatic ketone or aldehyde.



For example, benzanilide and dimethylaniline react with phosphorus oxychloride to produce an unsymmetrical diaryl ketone. Similarly, anthracene can be formylated exclusively at the 9-position.



Reaction mechanism
The reaction of the amide with phosphorus oxychloride produces an electrophilic iminium cation. The subsequent electrophilic aromatic substitution produces an iminium ion intermediate, which is hydrolyzed to give the desired aryl ketone or aryl aldehyde.



Applications
One recent application of this reaction involved a new synthetic route to tris(4-formylphenyl)amine from triphenylamine which by known procedures only resulted in a modest chemical yield of 16%. It was found that this low yield was caused by deactivation of the remaining benzene ring by the imine groups on the other two phenyl groups in the third formylation step. The procedure was modified by taking the reaction to di-imine compound followed by hydrolysis to the di-formyl compound and then (with final position reactived) a separate formylation to the tri substituted compound.