Diphenyl-l,3-butadiene

Working with sodium ethoxide Thin-layer chromatography UV/NMR spectroscopy Solventless Wittig reactions 

The Wittig reaction is often used to form alkenes from carbonyl compounds. In this experiment, the isomeric dienes cis, trans-, and trans, fr ns-l,4-diphenyl-l,3-butadiene will be formed from cinnamaldehyde and benzyltriphenyl-phosphonium chloride. 

Two procedures are provided for preparing trans, fr ns-l,4-diphenyl-l,3-butadiene. In Experiment 39B, the reaction uses sodium ethoxide in ethanol solvent as the base, whereas in Experiment 39C, a green chemistry alternative is provided whereby potassium phosphate is employed as the base that is conducted without any solvent. The mechanism of the Wittig reaction in the presence of either sodium ethoxide or potassium phosphate is essentially identical. Sodium ethoxide is shown as the base in the mechanism that follows. In Experiment 39A, an optional procedure is provided for preparing one of the starting materials for the Wittig reaction. 

The reaction is carried out in two steps. First, the phosphonium salt is formed by the reaction of triphenylphosphine with benzyl chloride in Experiment 39A. The reaction is a simple nucleophilic displacement of chloride ion by triphenylphosphine. The salt that is formed is called the Wittig reagent or Wittig salt.  

When treated with base, the Wittig salt forms an ylide. An ylide is a species having adjacent atoms oppositely charged. The ylide is stabilized due to the ability of phosphorus to accept more than eight electrons in its valence shell. Phosphorus uses its 3d orbitals to form the overlap with the 2p orbital of carbon that is necessary for resonance stabilization. Resonance stabilizes the carbanion. 

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Examples 1,3-diphenyl- 1-propene, < /.v-2.3-diphenyl-2-butene, 2,3-diphenyl-l,3-butadiene. 

Ozonolysis as used below is the oxidation process involving addition of ozone to an alkene to form an ozonide intermediate which eventually leads to the final product. Beyond the initial reaction of ozone to form ozonides and other subsequent intermediates, it is important to recall that the reaction can be carried out under reductive and oxidative conditions. In a general sense, early use of ozonolysis in the oxidation of dienes and polyenes was as an aid for structural determination wherein partial oxidation was avoided. In further work both oxidative and reductive conditions have been applied . The use of such methods will be reviewed elsewhere in this book. Based on this analytical use it was often assumed that partial ozonolysis could only be carried out in conjugated dienes such as 1,3-cyclohexadiene, where the formation of the first ozonide inhibited reaction at the second double bond. Indeed, much of the more recent work in the ozonolysis of dienes has been on conjugated dienes such as 2,3-di-r-butyl-l,3-butadiene, 2,3-diphenyl-l,3-butadiene, cyclopentadiene and others. Polyethylene could be used as a support to allow ozonolysis for substrates that ordinarily failed, such as 2,3,4,5-tetramethyl-2,4-hexadiene, and allowed in addition isolation of the ozonide. Oxidation of nonconjugated substrates, such as 1,4-cyclohexadiene and 1,5,9-cyclododecatriene, gave only low yields of unsaturated dicarboxylic acids. In a recent specific example... 

Crude 2,3-diphenyl-l,3-butadiene is unstable. The pure product should be stored in the dark in a refrigerator. The submitters have found it to be stable for at least one year under these conditions. 

This method provides a simple one-step synthesis of 2,3-diphenyl-l,3-butadiene from the readily available diphenyl-acetylene. It illustrates an unusual reaction that has been relatively uninvestigated. The scope of the reaction is unknown, but it would appear that the procedure could be applied to disubstituted acetylenes having aryl substituents that contain functionality that is unaffected by the strong basic conditions of the reaction. 

A conventional preparation of 2,3-diphenyl-l,3-butadiene involves dehydration of ieso-2,3-diphenyl-2,3-butanediol by acidic reagents including acetic anhydride,acetyl bromide, sulfanilic acid, and potassium hydrogen sulfate. Other procedures have been summarized previously. 

PREPARATIVE TECHNIQUE H-H polystyrene has never been obtained directly from styrene monomer. It is synthesized by the selective hydrogenation of l,4-poly(2,3-diphenyl-1,3-butadiene) (PDPB) using potassium/ethanol. PDPB is prepared by the free radical polymerization of 2,3-diphenyl-l,3-butadiene to give a 45% cis, 55% trans structure. H-H PS is then given in the Scune ratio of erythro and threo linkages after the chemical reduction of the internal double bond of the PDPB. ... 

Steric hindrance in achieving a cisoid configuration in 2,3-di-r-butyl-l,3-butadiene 6 could be responsible for its lack of reactivity.However, 2,3-diphenyl-l,3-butadiene 7 gives a quantitative yield of the adduct Not enough steric hindrance is offered, therefore, by chlorine atoms and phenyl groups in the 2 and 3 positions. 

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