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Reimer–Tiemann reaction

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Title: Reimer–Tiemann reaction  
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Reimer–Tiemann reaction

The Reimer–Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols;[1][2][3][4][5] with the simplest example being the conversion of phenol to salicylaldehyde. The reaction was discovered by Karl Reimer[6] and Ferdinand Tiemann. The Reimer in question was Karl Reimer (1845-1883) not the less known Carl Ludwig Reimer (1856-1921).[7]

The Reimer-Tiemann reaction

Contents

  • Reaction mechanism 1
    • Reaction conditions 1.1
    • Scope 1.2
  • Comparison to other methods 2
  • References 3

Reaction mechanism

The mechanism of the Reimer-Tiemann reaction

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.

Reaction conditions

Hydroxides are not readily soluble in the chloroform, thus the reaction is generally carried out in a 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.

Scope

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 bearing 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.[3] Additionally the Gattermann-Koch and Vilsmeier–Haack reactions are not applicable to phenol substrates.

References

  1. ^
  2. ^
  3. ^ a b
  4. ^
  5. ^
  6. ^ Karl Reimer, Berichte der deutschen chemischen Gesellschaft 9, S. 423–424 (1876). - [1]
  7. ^ see "Karl Reimer Nachruf 1883.pdf" at Commons and text on page 101.
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