C=C Bond Forming Reactions: Part 1

C=C bond forming reactions

1.     Wittig Reaction

This Nobel Prize winning reaction was discovered by George Wittig. It involves the reaction of an aldehyde or ketone with phosphorus ylides, leading to the formation of a double bond between carbon atoms.

The ylide used in this reaction is prepared from triphenylphosphine and an alkyl halide. It may be prepared before the reaction of in situ. Nature of the groups attached to the ylide makes the negative charge stable or unstable. Wittig reaction is chemoselective. It is much faster with aldehydes than with ketones and other carbonyl compounds (reactivity order of electrophilicity of carbonyl carbon). The formation of E or Z alkenes is also dependent on various factors, i.e., solvent, starting compound and stability of the ylide. The driving force for this reaction is the formation of triphenylphosphine oxide. Several variants of Wittig reaction have been introduced since its discovery.

2.     Horner-Wadsworth-Emmons Reaction

Horner-Wadsworth-Emmons reaction (HWE) is a modification of the Wittig reaction. Instead of using the traditional triphenyl phosphine ylides, Horner and Wadsworth made used of the more stabilized phosphonate carbanions. This modification makes the reaction even more selective. Furthermore, the phosphonate anions are more nucleophilic in nature and thus react with almost any type of aldehyde or ketones. Their preparation is also much easier and economical than the phophonium ylides.

The stabilized anion renders the reagent more selective and that is the reason HWE reaction is quite useful for the preparation of E-alkenes. The E-selectivity may be enhanced by increasing the bulk on the phosphonate using larger RO- groups. Using smaller groups may increase the ratio of Z-alkene. A variety of bases may be used in the reaction, such as NaH, KH, NaHMDS, LiHMDS, and KOt-Bu.

3.     McMurry Reaction

The coupling of carbonyl compounds in the presence of Ti leads to the formation of C=C. Ti used in the reaction must be low valent, +3 or +4. A reducing agent in the form of a metal or Zn-Cu is also used in the reaction.

The reaction is basically reductive coupling of carbonyl compounds and is related to pinacol coupling. A pinacolate complex is formed in the first step which collapses into isomeric alkenes along with the formation of titanium oxide, owing to the greater affinity of Ti towards oxygen. The complex is isolable at low temperatures. McMurry reaction is a feasible method for the preparation of sterically hindered substituted alkenes, which would otherwise be difficult with other methods.

The reaction proceeds chemoselectively with aldehydes in the presence of ketones, due to their higher reactivity and electrophilicity.

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