McMurry reaction

The McMurry reaction is a reaction name and named after the explorer John E. McMurry. Here, two aldehydes or ketones to give an alkene in the presence of a titanium chloride such as titanium (III ) chloride, and coupled to a reducing agent. The McMurry reaction is mainly used for preparation of symmetrical alkenes and is particularly suitable for the synthesis of sterically demanding substituted alkenes is an important process and can be called back reaction of the ozonolysis.

Reaction

Aliphatic and aromatic aldehydes or ketones ( cyclic ketones) can be reductively dimerize to high yields of symmetrical alkenes. Taking as the low-valent titanium as a reducing agent, by reduction of the titanium (III ) chloride ( TiCl 3 ), or titanium (IV ) chloride ( TiCl 4 ) with for example, lithium aluminum hydride ( LiAlH4 ) or zinc or magnesium is obtained.

Reaction mechanism

The reaction mechanism of the McMurry reaction proceeds, depending on the reducing agent used, different and not necessarily on metallic titanium ( 0) as a low-valent titanium compound, as has long been suspected. It was shown, for example, that the use of lithium aluminum hydride ( LiAlH4 ) as a reducing agent, a Hydridtitanchlorid [ HTiCl (THF ) 0.5] x is formed as reactive titanium species.

In the variant with zinc as the reducing agent, now widely used because of their high yields, however, it could be shown that the reduction of titanium (III ) chloride ( TiCl 3 ) (or titanium ( IV) chloride TiCl 4 ) after the formation of a titanium (III) - carbonyl complex takes place and the formation of a titanium ( II) leads carbonyl complex. In a first step, then a Pinakolatbildung, in the second step deoxygenation of Pinakolates to the alkene. Case of aromatic aldehydes and ketones the Pinakolatbildung a carbenoid intermediate runs, while the case of aliphatic aldehydes and ketones the Pinakolatbildung proceeds via a radical intermediate.

Unsymmetrically substituted olefins from mixtures of two different ketones with Mc Murry reaction, only in unsatisfactory yields accessed when one of the components is present in excess.

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

Examples

The original publication describes the coupling of the retinal carotene with titanium (III ) chloride / lithium aluminum hydride. Similarly tetraphenylethylene is synthesized from benzophenone. Other products that are described in the original publication, are adamantanone and civetone. An application of the McMurry reaction of titanium (IV ) chloride and zinc is concerned with the synthesis of a nano- engine.