Furan Vilsmeier-Haack reaction

The Vilsmeier-Haack reaction (herein, Vilsmeier reaction ) provides an effective method for the formylation of aromatic systems. The combination of phosphoryl chloride with V-methylaniline or dimethylformamide generates an iminium phosphorus derivative or chloro-iminium cation that is the active electrophile in an electrophilic substitution reaction. The resulting substitution product is an iminium salt 1, which is hydrolyzed on workup with alkali to give the carbaldehyde product 2 (Scheme l).1,2 The method is particularly useful with activated arenes or electron-rich heterocycles, such as pyrroles, furans, thiophenes, and indoles. We had a special interest in the preparation of indole-7-carbal-dehydes, namely, their properties as isosteres of salicylaldehyde. Thus, we became involved in a wide-ranging investigation of 4,6-dimethoxy-... 

Thiophene is also readily acylated under both Friedel-Crafts and Vilsmeier-Haack conditions and similarly to pyrrole and furan gives 2-acylated products. An almost quantitative conversion of thiophene into its 2-benzoyl derivative is obtained by reaction with 2-benzoyloxypyridine and trifluoroacetic acid. The attempted preparation of 2-benzoylthiophene under standard Friedel-Crafts conditions, however, failed (80S139). 

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