Use in the Wittig reaction

The present preparation illustrates a general and convenient method for the fnms-iodopropenylation of an alkyl halide.4 The iodopropenyl-ated material is not usually stable but is a useful synthetic intermediate. For example, it forms a stable crystalline triphenylphosphonium salt for use in the Wittig reaction, and under Kornblum reaction conditions (DMS0-NaHC03, 130°, 3 minutes) it gives an (E)-a,/9-unsaturated aldehyde.4 In addition to the phosphonium salt described in Note 15, the following have been prepared (4-p-methoxyphenyl-2-butenyl)-triphenylphosphonium iodide [Phosphonium, [4-(4-methoxyphenyl)-2-butenyl]triphenyl-, iodide], m.p. 123-127° (2-octenyl)triphenyl-phosphonium iodide [Phosphonium, 2-octenyltriphenyl-, iodide], m.p. 98° and (2-octadecenyl)triphenylphosphonium iodide [Phosphonium, 2-octadecenyltriphenyl-, iodide], m.p. 50°. 

For the synthesis of alkenes, the Wittig and Horner-Wadsworth-Emmons reactions have become important tools. Triphenylphosphine that is used in the Wittig reaction can be immobilized either on the polymer or can be used in solution for solid-phase chemistry (Scheme 3.18). The Horner-Wadsworth-Emmons reaction for example is used for the synthesis of aldehyde building blocks [261]. 

Next, consider the phosphonium salt you met at the end of Chapter 14 for use in the Wittig reaction, turning aldehydes and ketones to alkenes. It has a 2/ph of 18 Hz. There is no doubt about this structure—it is just an illustration of coupling to phosphorus. There is coupling to phosphorus in the carbon spectrum too the methyl group appears at 5c 10.6 p.p.m. with a l/pc of 57 Hz, somewhat smaller than typical Vph- We haven t yet talked about couplings to 13C we shall now do so. 

Sulfoxides are sometimes drawn as S=0 and sometimes as S+-0 . The second representation might remind you oF the phosphorus ylids used in the Wittig reaction (Chapters j 14 and 31), which can be drawn with a P=CH2 double s, bond or as P+-CH 2-All of these representations are 5- correct—it is a matter of personal choice which you prefer. 

Dimsylsodium (24) functions as a highly basic sulfur ylide. It can be used to convert phosphonium salts to phosphorus ylides for use in the Wittig reaction. Dimsylsodium also reacts with aldehydes and ketones by nucleophilic addition to form epoxides and with esters by nucleophilic substitution to yield p-ketosulfoxides (25) (Scheme 11). The p-ketosulfoxides (25) contain acidic a-hydrogens which can be readily removed to allow alkylation, and the products (26) suffer reductive desulfuration on treatment with aluminium amalgam to yield ketones (27) (Scheme 11) This procedure can, for instance, be applied to the conversion of ethyl benzoate to propiophenone (28) (Scheme 12). 

Treatment of a phosphonium salt 68 with aqueous base instead of the anhydrous bases used in the Wittig reaction leads to a stable phosphine oxide 119. The lithium derivative 120 contains a genuine C-Li bond and reacts stereoselectively with aldehydes to give stable adducts 121 that in turn gives the highly crystalline alcohols syn-122, easily purified by crystallisation and, if necessary, chromatography.22... 

Chloromethylated polystyrene can also be converted to a phosphonium salt for use in the Wittig reaction [193] ... 

The Horner-Wadsworth-Emmons reaction (or HWE reaction) is the reaction of stabilized phosphonate carbanions with aldehydes (or ketones) to produce predominantly -alkenes. In 1958, Horner published a modified Wittig reaction using phosphonate-stabilized carbanions [32]. Wadsworth and Emmons further defined the reaction [33]. Compared to phosphonium ylides used in the Wittig reaction, phosphonate-stabilized carbanions are more nucleophilic and more basic. Likewise, phosphonate-stabilized carbanions can be alkylated, unlike phosphonium ylides. The dialkylphosphate salt by-product is easily removed by aqueous extradion. A reliable and versatile synthesis of a stilbene derivative, 2,2-aryl-substituted cinnamic acid esters, using the Wittig reaction was reported [34—36] (Figure 1.3). 

Why is it important that any aldehyde used in the Wittig reaction be free of carboxylic acid impurities ... 

Wittig reagents Name given to the phosphorus ylides used in the Wittig reaction. 

Phosphonium ylides are typically prepared by the reaction of a phosphonium salt with a base. Non-stabilized ylides require a strong base (such as BuLi) under inert conditions, while stabilized ylides require a weaker base (for example, alkali metal hydroxides in aqueous solution). For more detail on the variety of bases and reaction conditions that can be used in the Wittig reaction, see references [2] and [5],... 

The employment of phosphorus-stabilized carbon nucleophiles for alkene synthesis was initiated by the discovery of the Wittig reaction [10], which provides a convenient method for the preparation of a wide variety of polysubstituted alkenes with complete positional selectivity and generally high levels of geometrical control. Moreover, the phosphonium ylides used in the Wittig reaction are readily formed by the addition of suitable bases to the corresponding phosphonium salts, which are commonly prepared by treating alkyl halides with phosphines. 

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