McMurry reaction

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McMurry reaction
Named after John E. McMurry
Reaction type Coupling reaction
Identifiers
Organic Chemistry Portal mcmurry-reaction
The McMurry reaction of benzophenone

The McMurry reaction is an organic reaction in which two ketone or aldehyde groups are coupled to form an alkene using a titanium chloride compound such as titanium(III) chloride and a reducing agent. The reaction is named after its co-discoverer, John E. McMurry. The McMurry reaction originally involved the use of a mixture TiCl3 and LiAlH4, which produces the active reagents. Related species have been developed involving the combination of TiCl3 or TiCl4 with various other reducing agents, including potassium, zinc, and magnesium.[1][2] This reaction is related to the Pinacol coupling reaction which also proceeds by reductive coupling of carbonyl compounds.

Reaction mechanism

This reductive coupling can be viewed as involving two steps. First is the formation of a pinacolate (1,2-diolate) complex, a step which is equivalent to the pinacol coupling reaction. The second step is the deoxygenation of the pinacolate, which yields the alkene, this second step exploits the oxophilicity of titanium.

A proposed mechanism when TiCl4 and Zn(Cu) are used for the coupling of benzophenone, as proposed in a reference.[3] Note that the mechanism may vary when different conditions are used.

Several mechanisms have been discussed for this reaction.

single electron transfer to the carbonyl groups. The required low-valent titanium species are generated via reduction, usually with zinc powder. This reaction is often performed in THF because it solubilizes intermediate complexes, facilitates the electron transfer steps, and is not reduced under the reaction conditions. The nature of low-valent titanium species formed is varied as the products formed by reduction of the precursor titanium halide complex will naturally depend upon both the solvent (most commonly THF or DME) and the reducing agent employed: typically, lithium aluminum hydride, zinc-copper couple, zinc dust, magnesium-mercury amalgam, magnesium, or alkali metals.[4] Bogdanovic and Bolte identified the nature and mode of action of the active species in some classical McMurry systems,[5] and an overview of proposed reaction mechanisms has been published.[3] It is of note that titanium dioxide is not generally a product of the coupling reaction. Although it is true that titanium dioxide is usually the eventual fate of titanium used in these reactions, it is generally formed upon the aqueous workup of the reaction mixture.[4]

Background and scope

The original publication by Mukaiyama demonstrated reductive coupling of ketones using reduced titanium reagents.

molecular motor.[7]

McMurry coupling to a molecular motor
Porphycene, first porphyrin isomer, synthesis from bipyrrole dialdehyde through McMurry coupling reaction

In another example, the

Zn(Cu). The first porphyrin isomer, porphycene, was synthesised by McMurry coupling.[8]

Further reading

References

  1. ISBN 0471936235
    .
  2. ISBN 0471936235
    .
  3. ^
    doi:10.1039/a804394i
    .
  4. ^
    doi:10.1002/anie.199624421
    .
  5. doi:10.1016/0022-328X(95)05755-E
    .
  6. doi:10.1246/cl.1973.1041
  7. PMID 14653742.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  8. doi:10.1002/anie.198602571
    .

External links
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