The Reimer–Tiemann reaction owes its name to two young German chemists, Karl Reimer and Ferdinand Tiemann. In 1876 they isolated and identified hydroxyaldehydes as the principal reaction products of phenol and chloroform in alkaline medium. The scope of this reaction was enlarged in 1884 by von Auwers, who discovered the chlorine-containing cyclohexadienones as by-products in the formylation of alkylphenols. The ring-expansion products, namely, chloropyridines, were first noted by Ciamician when he subjected pyrroles to Reimer–Tiemann reaction conditions. Nearly half a century passed before Woodward recognized that the conversion of an alkylphenol to a substituted cyclohexadienone could lead to the synthesis of terpenes and steroids containing an angular methyl group. Although the method failed as a preparatively useful approach to the synthesis of steroids, an A/B trans-fused hexahydrophenanthrene was prepared using a Reimer–Tiemann reaction. The reaction was last reviewed some 20 years ago.
It is convenient to divide the Reimer–Tiemann reaction into a normal and abnormal transformation depending on the reaction products. A normal Reimer–Tiemann reaction is one in which a phenol (or electron-rich aromatic such as pyrrole) yields one or more aldehydes on treatment with chloroform and alkali. The abnormal Reimer–Tiemann reaction product can be subdivided further into cyclohexadienones and ring-expansion products:
- When ortho– or para-substituted phenols are subjected to the Reimer–Tiemann reaction conditions, 2,2- or 4,4-disubstituted cyclohexadienones may be obtained in addition to the normal products. Several alkylphenols, alkylnaphthols, and tetralols have been converted into cyclohexadienones, whereas some alkylpyrroles are converted to pyrrolines in this manner.
- A variety of five-membered rings yield ring-expansion products when subjected to the Reimer–Tiemann reaction conditions. These products are formed in addition to the normal products.
In addition to aromatic aldehydes, cyclohexadienones, pyrrolines, and ring-expansion products, a variety of other products have been noted, isolated, and identified in a few cases. All these by-products are discussed.