Alkenes, Wurtz reaction

The procedure provides a good example of a high-yield intramolecular Wurtz reaction. Internjolecular Wurtz reactions normally do not give high yields of coupled products and are accompanied by formation of alkenes and alkanes corresponding to the alkyl halide.7 In contrast, intramolecular reactions of... 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Thus leading to the disproportionation—alkane (62) + alkene (63)— that is often observed as a side-reaction to the normal Wurtz coupling. 

One of the benefits of the Barbier reaction over the Grignard reaction is generally the production of products with little or no R-R dimer formation. However, because of the radical nature of the reaction Wurtz-type dimer formation can be the primary reaction in the absence of an electrophilic coreagent. Some Russian chemists have reported the dimerization of halogenated alkenes and alkanes using iodine-activated magnesium with silver bromide [Eqs. (27) and (28) 64]. 

Common synthetic pathways to obtain mono-, di-, tri- and tetraalkylsilanes are reactions of the respective halogenosilanes with lithiated alkyl compounds, with alkyl-magnesium halides (Grignard reactions) and with alkyl halides in a Wurtz-Fittig-type reaction, as well as the addition of silanes to alkenes, i.e. hydrosilylation. 

Metallated alkenes such as alkenyllithium or Grignard species undergo addition reactions with various electrophiles. Reaction with primary alkyl bromides or iodides is possible. Wurtz self-coupled products can be avoided if the alkenyllithium species is generated by tin-lithium exchange, or by insertion of lithium metal into... 

As can be seen, the reaction affords excellent yields of the desired products and tolerates a large number of functionalities, including esters, amides, ethers, alkenes, free alcohols, nitriles, and ketals. The electrolysis of two different carboxylic acids, as originally described by Wurtz, generates three products the two symmetrical and the nonsymmetrical dimers (Scheme 4). 

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