Formation of Alkenes and Alkynes

We shall only be concerned here with those reactions, which have been used in constructions of the carbon skeletons of complex compounds with concomitant regioselective incorporation of the double bond. 1,2-Eliminations are discussed on p. 137ff. 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—0 is then eliminated and a new C C bond is formed. Such reactions take place because the formation of a Y—O bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

Important synthetic aspects in such olefinations of carbonyl compounds are 

Surprisingly carbonyl-substituted carbanions of phosphonates, in which the negative charge is delocalized over two oxygen atoms, are much more nucleophilic than the corresponding phosphoranes. This effect has first been observed by Homer, and has often been utilized in the synthesis of acylated olefins (R.D. Clark, 1975). 

In the reaction of a substituted ylide (R CH-PPh3) with an aldehyde R2CHO, a stereochemical problem arises. Much work has been carried out in order to achieve control of either cis- or Wwis-alkene formation. This work has been reviewed several times with always changing viewpoints (A. Maercker, 1965 L.D. Bergelson, 1964 M. Schlosser, 1970 H. Best-mann, 1979). 

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Out of the above the P elimination is most common. These eliminations result in the formation of alkenes and alkynes. When P phenylethyl bromide is heated with an alcoholic solution of an alkali first a carbanion is formed by the loss of a proton followed by the loss of a halide ion and simultaneous formation of a double bond. 

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