| McMurry reaction | |
|---|---|
| Named after | John E. McMurry |
| Reaction type | Coupling reaction |
| Identifiers | |
| Organic Chemistry Portal | mcmurry-reaction |
TheMcMurry reaction is anorganic reaction in which twoketone oraldehyde groups are coupled to form analkene using atitanium chloride compound such astitanium(III) chloride and areducing 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, includingpotassium,zinc, andmagnesium.[1][2] This reaction is related to thePinacol coupling reaction which also proceeds by reductive coupling of carbonyl compounds.
This reductive coupling can be viewed as involving two steps. First is the formation of apinacolate (1,2-diolate) complex, a step which is equivalent to thepinacol coupling reaction. The second step is thedeoxygenation of the pinacolate, which yields thealkene, this second step exploits theoxophilicity of titanium.
Several mechanisms have been discussed for this reaction.[3] Low-valenttitanium species induce coupling of the carbonyls bysingle electron transfer to the carbonyl groups. The required low-valent titanium species are generated viareduction, usually with zinc powder. This reaction is often performed inTHF 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 andmode 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]
The original publication by Mukaiyama demonstrated reductive coupling of ketones using reduced titanium reagents.[6] McMurry and Fleming coupledretinal to givecarotene using a mixture oftitanium trichloride andlithium aluminium hydride. Other symmetrical alkenes were prepared similarly, e.g. fromdihydrocivetone,adamantanone andbenzophenone (the latter yielding tetraphenylethylene). A McMurry reaction usingtitanium tetrachloride and zinc is employed in the synthesis of a first-generationmolecular motor.[7]
In another example, theNicolaou'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 thedimethoxyethane complex ofTiCl3 in combination with theZn(Cu). The firstporphyrin isomer, porphycene, was synthesised by McMurry coupling.[8]
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