Horner-Emmons Wittig olefination

Olefin formation (Wittig, Horner-Emmons, metathesis)... 

Reaction with aldehydes or ketones (such as benzaldehyde) in the presence of base gave the olefination product 556 (in 84% yield).502 These variations have come to be called the Horner-Wadsworth-Emmons modiflcation of the Wittig reaction, or Horner-Wadsworth-Emmons olefination.503 It is sometimes called Horner-Emmons olefination. 

The mechanism of the Horner-Emmons olefination resembles that of the Wittig reaction via betaine intermediates (Scheme 5), and is shown in Scheme 7 for the reaction of a phos-phonate 23 with an aldehyde 27. 

The simplest preparation of the C3o-ester 1 is by Horner-Emmons olefination of 507 with the Cs-ester 67 [75]. Similar results are obtained by Wittig olefination of 507 with the triphenyl-phosphonium bromide corresponding to 67 [76]. Sodium ethoxide has proved suitable as a base in the ethanol/heptane solvent system. The desired product crystallizes out during the reaction. Thermal isomerization yields a second crystal fraction from the mother liquor. 

The reaction has been extended to include carbanions generated from phosphonates. This is often referred to as the Horner-Wittig or Homer-Emmons reaction. The Horner-Emmons reaction has a number of advantages over the conventional Wittig reaction. It occurs with a wider variety of aldehydes and ketones under relatively mild conditions as a result of the higher nucleophilicity of the phosphonate carbanions. The separation of the olefinic product is easier due to the aqueous solubility of the phosphate by-product, and the phosphonates are readily available from the Arbusov reaction. Furthermore, although the reaction itself is not stereospecific, the majority favor the formation of the trans olefin and many produce the trans isomer as the sole product. 

Syntheses follow a kind of bio-mimetic approach [283, 284] in building up the chain during a sequence of Wittig-type reactions or Horner-Wadsworth-Emmons olefination, adding two carbons to the chain at a time with either methyl- or ethyl-branches. As the final products need to be highly pure (E)-stereoisomers, reaction steps and purification need to be carefully controlled. 

This method, sometimes called the Horner-Emmons, Wadsworth-Emmons, or Wittig-Hor-ner reaction,658 has several advantages over the use of phosphoranes.659 These ylides are more reactive than the corresponding phosphoranes, and when R is an electron-withdrawing group, these compounds often react with ketones that are inert to phosphoranes. In addition, the phosphorus product is a phosphate ester and hence soluble in water, unlike PhjPO, which makes it easy to separate it from the olefin product. Phosphonates are also cheaper than phosphonium salts and can easily be prepared by the Arbuzov reaction ... 

The Wittig, Horner-Wadsworth-Emmons, Peterson, and Julia Olefination Reactions... 

Related reactions Horner-Wadsworth-Emmons olefination, Homer-Wadsworth-Emmons olefination - Still-Gennari modification, Julia-Lithgoe olefination, Takai-Utimoto olefination, Tebbe oiefination, Wittig reaction, Wittig reaction - Schiosser modification ... 

Subsequently, however, we have prepared a series of diazocompounds 21a.b closely ressembling 21, via the Wittig and Horner-Emmons-Wittig (HEW) olefination vide infra, and observed that none of them cyclized even upon heating. It... 

Y = PR3j. Wittig, Horner, Wadsworth- Emmons olefinations OK ... 

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