McMurry reaction

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The McMurry reaction is a name reaction in chemistry and named after the discoverer John E. McMurry , who published it in 1974. Here, two ketones or aldehydes are converted into an alkene in the presence of a titanium chloride, such as. B. titanium (III) chloride , and a reducing agent coupled. The McMurry reaction is mainly used to produce symmetrical alkenes and is an important process, especially for the synthesis of sterically demanding substituted alkenes, and can be referred to as the reverse reaction of ozonolysis .

reaction

Reaction scheme for the McMurry reaction

Aliphatic and aromatic aldehydes or ketones (including cyclic ketones) can be reductively dimerized in high yields to give symmetrical alkenes . Here is low-valent titanium as a reducing agent prepared by reduction of titanium (III) chloride (TiCl 3 ), or titanium (IV) chloride (TiCl 4 ) with z. B. lithium aluminum hydride (LiAlH 4 ) or zinc or magnesium is obtained.

Reaction mechanism

The reaction mechanism of the McMurry reaction varies depending on the reducing agent used and does not necessarily involve metallic titanium (0) as a low-valent titanium compound, as was long suspected. So could z. For example, it can be shown that when lithium aluminum hydride (LiAlH 4 ) is used as the reducing agent, a hydride titanium chloride [HTiCl (THF) 0.5 ] x is formed as a reactive titanium species.

In the case of the variant with zinc as the reducing agent, which is often used today because of its high yields, it was shown that the reduction of titanium (III) chloride (TiCl 3 ) (or titanium (IV) chloride TiCl 4 ) only takes place after the formation of a titanium ( III) carbonyl complex takes place and leads to the formation of a titanium (II) carbonyl complex. In a first step, pinacolate is then formed , and in the second step, the pinacolate is deoxygenated to form the alkene. In the case of aromatic aldehydes and ketones , pinacolate formation takes place via a carbenoid intermediate, while in the case of aliphatic aldehydes and ketones, pinacolate formation takes place via a radical intermediate.

Unsymmetrically substituted alkenes from mixtures of two different ketones can only be obtained in unsatisfactory yields with the Mc-Murry reaction if one of the components is present in excess.

If the reactants are not heated and the reaction time is kept short, it is possible that only the first reaction step occurs. Only 1,2-diols are obtained as reaction products. In this case it is a simple Pinacol coupling under McMurry conditions. An example of the reaction under these conditions is the taxol synthesis according to Nicolaou .

Examples

The original publication describes the coupling of retinal to carotene with titanium (III) chloride / lithium aluminum hydride. Similarly, tetraphenylethylene is synthesized from benzophenone. Other dimerizations described in the original publication are those of adamantanone and dihydrozibetone. One application of the McMurry reaction with titanium (IV) chloride and zinc deals with the synthesis of a nanomotor .

Example of a McMurry reaction

Comparison and alternatives

Crossed McMurry couplings between two different carbonyl compounds are usually restricted by the possibility of homodimerization - a complication known from simple aldol reactions. The intramolecular versions of the McMurry coupling can, however, be very effective for the preparation of macrocycles or sterically strained olefins, especially at high dilution. Alternatives for coupling two different carbonyl compounds are Takeda olefination or Barton olefination , in which the carbonyl group of one of the two reactants is converted into a sulfur derivative beforehand.

Individual evidence

  1. ^ A b John E. McMurry and Michael P. Fleming: New method for the reductive coupling of carbonyls to olefins. Synthesis of β-carotene . Journal of the American Chemical Society . Volume 96, 1974, pp. 4708-4709, doi : 10.1021 / ja00821a076 .
  2. ↑ Jerry March: Advanced Organic Chemistry . 4th edition, Wiley-Interscience, ISBN 0-471-60180-2 .
  3. ↑ Michel Ephritikhine: A new look at the McMurry reaction . In: Chem. Commun. 1998, pp. 2549-2554, doi : 10.1039 / a804394i .
  4. ↑ a b Alois Fürstner and Borislav Bogdanovic: New developments in the chemistry of low-valent titanium . In: Angewandte Chemie , Volume 108, 1996, pp. 2583-2609. doi : 10.1002 / anie.19961082104 .
  5. ↑ a b Borislav Bogdanovic and Andreas Bolte: A comparative study of the McMurry reaction utilizing [HTiCl (THF) 0.5 )] x , TiCl 3 (DME) 1.5 -Zn (Cu) and TiCl 2 * LiCl as coupling reagents . In: J. Organomet. Chem. Vol. 502, 1995, pp. 109-121, doi : 10.1016 / 0022-328X (95) 05755-E .
  6. ↑ Matthijs KJ ter Wiel, Richard A. van Delden, Auke Meetsma, and Ben L. Feringa: Light-Driven Molecular Motors: Stepwise Thermal Helix Inversion during Unidirectional Rotation of Sterically Overcrowded Biphenanthrylidenes . In: J. Am. Chem. Soc. . 127, No. 41, 2005, p. 14208. doi : 10.1021 / ja052201e .

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