McMurry reaction

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.

File:McMurry-Mechanism-Updated.png | issue = 23| year = 1998 | pages = 2549–2554 | doi = 10.1039/a804394i }} Note that the mechanism may vary when different conditions are used.]]

Several mechanisms have been discussed for this reaction. Low-valent titanium species induce coupling of the carbonyls by 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.{{cite journal | title = New developments in the chemistry of low-valent titanium |author1=Alois Fürstner |author2=Borislav Bogdanovic | journal = Angew. Chem. Int. Ed. | year = 1996 | volume = 35 | pages = 2442–2469 | doi = 10.1002/anie.199624421 | issue = 21}} Bogdanovic and Bolte identified the nature and mode of action of the active species in some classical McMurry systems,{{cite journal | title = A comparative study of the McMurry reaction utilizing [HTiCl(THF)_0.5]_x, TiCl₃(DME)_1.5-Zn(Cu) and TiCl₂*LiCl as coupling reagents |author1=Borislav Bogdanovic |author2=Andreas Bolte | journal = J. Organomet. Chem. | year = 1995 | volume = 502 | pages = 109–121 | doi = 10.1016/0022-328X(95)05755-E}} and an overview of proposed reaction mechanisms has been published. 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.

The original publication by Mukaiyama demonstrated reductive coupling of ketones using reduced titanium reagents.Mukaiyama, T.; Sato, T.; Hanna, "Reductive Coupling of Carbonyl Compounds to Pinacols and Olefins by Using TiCl₄ and Zn" Chem. Lett. 1973, 1041. {{doi|10.1246/cl.1973.1041}} McMurry and Fleming coupled retinal to give carotene using a mixture of titanium trichloride and lithium aluminium hydride. Other symmetrical alkenes were prepared similarly, e.g. from dihydrocivetone, adamantanone and benzophenone (the latter yielding tetraphenylethylene). A McMurry reaction using titanium tetrachloride and zinc is employed in the synthesis of a first-generation molecular motor.{{cite journal | title = Increased Speed of Rotation for the Smallest Light-Driven Molecular Motor | author = Matthijs K. J. ter Wiel, Richard A. van Delden, Auke Meetsma, and Ben L. Feringa | journal = J. Am. Chem. Soc. | year = 2003 | volume = 125 | issue = 49 | pages = 15076–15086 | doi = 10.1021/ja036782o | pmid = 14653742| url = https://pure.rug.nl/ws/files/10288186/2003JAmChemSocterWielSupp2b.PDF }}

:Image:McMurryMolecularMotor-2.png{{clear|left}}

File:First Porphycene synthesis.tif isomer, synthesis from bipyrrole dialdehyde through McMurry coupling reaction]]

In another example, the Nicolaou's total synthesis of Taxol uses this reaction, although coupling stops with the formation of a cis-diol, rather than an olefin. Optimized procedures employ the dimethoxyethane complex of TiCl₃ in combination with the Zn(Cu). The first porphyrin isomer, porphycene, was synthesised by McMurry coupling.{{cite journal |last1=Vogel |first1=Emanuel |last2=Köcher |first2=Matthias |last3=Schmickler |first3=Hans |last4=Lex |first4=Johann |title=Porphycene—a Novel Porphin Isomer |journal=Angewandte Chemie International Edition |date=March 1986 |volume=25 |issue=3 |page=257 |doi=10.1002/anie.198602571 |url=https://doi.org/10.1002/anie.198602571|url-access=subscription }}

• {{cite journal | title = New Method for the Reductive Coupling of Carbonyls to Olefins. Synthesis of β-Carotene |author1=John E. McMurry |author2=Michael P. Fleming | journal = J. Am. Chem. Soc. | year = 1974 | volume = 96 | issue = 14 | pages = 4708–4709 | doi = 10.1021/ja00821a076 | pmid=4850242}}

• [https://www.organic-chemistry.org/namedreactions/mcmurry-reaction.shtm McMurry reaction in organic-chemistry.org]
• [http://www.sussex.ac.uk/Users/kapx7/Named%20reactions/Sheets/McMurry%20Coupling.pdf Mcmurry reaction at the University of Sussex]{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}

{{Alkenes}}

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Category:Olefination reactions

Category:Carbon-carbon bond forming reactions

Category:Substitution reactions

Category:Name reactions