The Peterson olefination (also called the Peterson reaction) is the chemical reaction of α-silyl carbanions (1 in diagram below) with ketones (or aldehydes) to form a β-hydroxysilane (2) which eliminates to form alkenes (3).
Several reviews have been published.
One attractive feature of the Peterson olefination is that it can be used to prepare either cis- or trans-alkenes from the same β-hydroxysilane. Treatment of the β-hydroxysilane with acid will yield one alkene, while treatment of the same β-hydroxysilane with base will yield the alkene of opposite stereochemistry.
The action of base upon a β-hydroxysilane (1) results in a concerted syn elimination of (2) or (3) to form the desired alkene. The penta-coordinate silicate intermediate (3) is postulated, but no proof exists to date.
Potassium alkoxides eliminate quickly, while sodium alkoxides generally require heating. Magnesium alkoxides only eliminate in extreme conditions. The order of reactivity of alkoxides, K > Na >> Mg, is consistent with higher electron density on oxygen, hence increasing the alkoxide nucleophilicity.
The treatment of the β-hydroxysilane (1) with acid results in protonation and an anti elimination to form the desired alkene.
When the α-silyl carbanion contains only alkyl, hydrogen, or electron-donating substituents, the stereochemical outcome of the Peterson olefination can be controlled, because at low temperature the elimination is slow and the intermediate β-hydroxysilane can be isolated.
Once isolated, the diastereomeric β-hydroxysilanes are separated. One diastereomer is treated with acid, while the other is treated with base, thus converted the material to an alkene with the required stereochemistry.
When the α-silyl carbanion contains electron-withdrawing substituents, the Peterson olefination directly forms the alkene. The intermediate β-hydroxysilane cannot be isolated as it eliminates in-situ. The basic elimination pathway has been postulated in these cases.
Acidic elimination conditions are sometimes not feasible as the acid also promotes double bond isomerization.
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The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide called a Wittig reagent. Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes. Most often, the Wittig reaction is used to introduce a methylene group using methylenetriphenylphosphorane (Ph3P=CH2). Using this reagent, even a sterically hindered ketone such as camphor can be converted to its methylene derivative.
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins. The International Union of Pure and Applied Chemistry (IUPAC) recommends using the name "alkene" only for acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloalkadiene, etc.
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