Ketones acyloin coupling reactions

Another important coupling reaction uses esters as the electron-accepting species and leads to a-hydroxy ketones (acyloin coupling). Sodium, potassium (less frequently) or sodium-potassium alloys are commonly used as electron donors in nonpolar solvents such as toluene or xylene. The first detectable reaction intermediate after the primary reductive step is the enediolate which can be trapped with tri-alkylsilyl chloride. This method is widely used to synthesize highly nucleophilic alkenes and/or protected acyloins (Scheme 12) [50, 51]. 

A second important reductive coupling procedure involves the reduction of esters to a-hydroxy ketones (acyloins). This reaction is usually carried out with sodium metal in an inert solvent. Diesters undergo intramolecular reactions and this is also an important method for preparation of medium and large carbocyclic rings. There has been considerable discussion of the mechanism of the acyloin condensation but the picture may be complicated by the possibility that the reaction is a heterogeneous one, taking place on the surface of the reacting metal. 

One typical radical reaction is a coupling reaction. Oxidative decarboxylation coupling reaction of carboxylic acids by electrolysis (Kolbe electrolysis), intramolecular coupling reaction of diesters with Na (acyloin condensation), formation of pinacols from ketones or aldehydes with Na or Mg are well known classical methods [1,2]. Recently, oxidative... 

Intramolecular free radical cyclization of dicarboxylic esters leads to a-hydroxy ketones (acyloins). Reductive coupling of dicarbonyl compounds provides 1,2-diols ipinacols) and further reaction of these yields cycloalkenes (McMurry reaction). These cyclization reactions are especially valuable for the preparation of medium and large rings that are not readily accessible by other methods. 

An intramolecular reductive coupling of ketones with nitriles has been reported for acyclic and monocyclic substrates y9-amino nitriles were isolated from acyclic malononitrile adducts [87]. The Sml2-initiated reductive cyclization of cyclic a-cyanoalkyl-substituted ketones leads to acyloin products in high yields. In this instance further irradiation of the reaction mixtures was performed to afford complete conversion (Scheme 27) [88]. More applications have been collected in several excellent reviews [89-92]. 

A useful relative of the acyloin condensation is the McMurry olefination reaction. l in this reaction, ketones or aldehydes are treated with Ti(0) (TiCls + LiAlH4 TiCls + K TiCl3 + Li)2i5 to give alkenes.2 6 Reductive coupling is possible with a wide variety of ketones and aldehydes. i 3,217 Retinal (276), for example, was coupled with this reagent to give P-carotene (277) in 85% yield. ... 

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