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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 ketone or aldehyde during workup. [7] The Vilsmeier–Haack reaction
It is also more expensive than thionyl chloride so it tends to be used on a smaller scale. This reaction involves conversion of N,N-dimethylformamide to the imidoyl chloride derivative (chloromethylene(dimethyl)ammonium ion (CH 3) 2 N + =CHCl), akin to the first stage in the Vilsmeier–Haack reaction. The imidoyl chloride is the active ...
The Vilsmeier reagent is an organic compound with the formula [(CH 3) 2 NCHCl]Cl. It is a salt consisting of the N,N-dimethyliminium cation ([(CH 3) 2 N=CHCl] +) and chloride anion. Depending on the particular reaction, the anion can vary. In typical POCl 3-based reactions, the anion is PO 2 Cl 2 −.
If the nitrogen of the imidoyl chloride has two substituents, the resulting chloroiminium ion is vulnerable to attack by aromatic rings without the need for a Lewis acid to remove the chloride first. This reaction is called the Vilsmeier–Haack reaction, and the chloroiminium ion is referred to as the Vilsmeier reagent.
It is a common solvent in the Heck reaction. [26] It is a common catalyst used in the synthesis of acyl halides, in particular the synthesis of acyl chlorides from carboxylic acids using oxalyl or thionyl chloride. The catalytic mechanism entails reversible formation of an imidoyl chloride (also known as the 'Vilsmeier reagent'): [27] [28]
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The reaction is catalysed by dimethylformamide (DMF), which reacts with oxalyl chloride to give the Vilsmeier reagent, an iminium intermediate that which reacts with the carboxylic acid to form a mixed imino-anhydride.