McMurry reaction

 The McMurry reaction is an organic reaction in which two ketone or aldehyde groups are coupled to an alkene in the presence of a titanium chloride compound such as titanium(III) chloride and a reducing agent ^[1].

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The McMurry reaction is named after its discoverer, John E. McMurry, who is currently professor emeritus at Cornell University.

Reaction mechanism

In 1974, McMurry and Fleming described a ‘new method for the reductive coupling of carbonyls to olefins’ with TiCl₃ and LiAlH₄. This reductive coupling involves two steps. The first step is the formation of a pinacolate (1,2-diolate) (see pinacol coupling reaction ), the second the deoxygenation of the pinacolate to yield the alkene.

McMurry himself proposed, after his discovery of the TiCl₃–LiAlH₄ system (1974), several ‘improved procedures’ by using the TiCl₃–K, TiCl₃–Li (1976) and then TiCl₃–Zn(Cu) reagents (1978)and finally recommended an ‘optimized procedure’ with the TiCl₃(DME)_1.5–Zn(Cu) combination, which gave reproducibly high yields in many cases (1989).

Several mechanisms have been discussed for this reaction ^[2]. Low-valent titanium species are formed before or during the reaction and induce the reaction by single electron transfer to the carbonyl group - which low-valent titanium species is formed depends on the used reagent^[3]. In fact, it was found that TiCl₃ could be reduced by Zn only if its redox potential has been lowered by coordination to the carbonyl substrate. A decisive contribution to the mechanistic investigations was made by Bogdanovic and Bolte, who identified the nature and mode of action of the active species in some classical McMurry systems^[4].

Scope

The original publication concerned the coupling of retinal to carotene with a titanium(III) trichloride / Lithium aluminium hydride system. In a similar fashion tetraphenylethylene is synthesized from benzophenone. Other dimers formed in the original publication are those of adamantanone and civetone. A McMurry reaction with titanium tetrachloride and zinc is employed in the synthesis of a molecular motor ^[5].

References

1. ^ John E. McMurry, Michael P. Fleming (1974). "New method for the reductive coupling of carbonyls to olefins. Synthesis of β-carotene". J. Am. Chem. Soc. 96 (14): 4708-4709. doi:10.1021/ja00821a076.
2. ^ Michel Ephritikhine (1998). "A new look at the McMurry reaction". Chem. Commun.: 2549–2554.
3. ^ Borislav Alois Furstner, Borislav Bogdanovic (1996). "New developments in the chemistry of low-valent titanium". Angew. Chem. Int. Ed. (35): 2442.
4. ^ Borislav Bogdanovic, Andreas Bolte (1995). "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". J. Organomet. Chem. 502: 109-121.
5. ^ Matthijs K. J. ter Wiel, Richard A. van Delden, Auke Meetsma, and Ben L. Feringa (2005). "Light-Driven Molecular Motors: Stepwise Thermal Helix Inversion during Unidirectional Rotation of Sterically Overcrowded Biphenanthrylidenes". J. Am. Chem. Soc. 127 (41): 14208. doi:10.1021/ja052201e.