The Reimer–Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols; with the simplest example being the conversion of phenol to salicylaldehyde. The reaction was discovered by Karl Ludwig Reimer and Ferdinand Tiemann. It should be noted that the Reimer in question was Karl Ludwig Reimer (1856-1921) not the better known Karl Ludwig Reimer (1845-1883).
Chloroform (1) is deprotonated by strong base (normally hydroxide) to form the chloroform carbanion (2) which will quickly alpha-eliminate to give dichlorocarbene (3); this is the principle reactive species. The hydroxide will also deprotonate the phenol (4) to give a negatively charged phenolate (5). The negative charge is delocalised into the aromatic ring, making it far more nucleophilic and increases its ortho selectivity. Nucelophilic attack of the dichlorocarbene from the ortho position gives an intermediate dichloromethyl substituted phenol (7). After basic hydrolysis, the desired product (9) is formed.
Hydroxides are not readily soluble in the chloroform, thus the reaction is generally carried out in a biphasic solvent system. In the simplest sense this consists of an aqueous hydroxide solution and an organic phase containing the chloroform. The two reagents are therefore separated and must be brought together for the reaction to take place. This can be achieved by rapid mixing, phase-transfer catalysts, or an emulsifying agent (the use of 1,4-Dioxane as a solvent is an example).
The reaction typically needs to be heated to initiate the process, however once started the Reimer-Tiemann Reaction can be highly exothermic; this combination makes it prone to thermal runaways.
Dichlorocarbenes can also react with alkenes and amines to form dichlorocyclopropanes and isocyanides respectively. As such the Reimer-Tiemann reaction may be unsuitable for substrates baring these functional groups.
Comparison to other methods
The direct formylation of aromatic compounds can be accomplished by various methods such as the Gattermann reaction, Gattermann–Koch reaction, Vilsmeier–Haack reaction, or Duff reaction; however, in terms of ease and safety of operations, the Reimer–Tiemann reaction is often the most advantageous route chosen in chemical synthesis. Of the prior mentioned reactions, the Reimer–Tiemann reaction is the only route not requiring acidic and/or anhydrous conditions. Additionally the Gattermann and Vilsmeier–Haack reactions are not applicable to phenol substrates.
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- Wynberg, Hans (1960). "The Reimer-Tiemann Reaction". Chemical Reviews 60 (2): 169–184. doi:10.1021/cr60204a003. Retrieved 3 January 2014.
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- Dauben, William G. (1982). Organic Reactions, Volume 28. Hoboken, NJ: Wiley-Interscience. p. 347. ISBN 978-0471861416.
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- Rocke, A. J.; Ihde, A. J. (April 1986). "With no Reimer reason: A name reaction with the wrong attribution". Journal of Chemical Education 63 (4): 309. doi:10.1021/ed063p309.