Phosphonium ylides resonance forms

The structural analyses in this chapter present a fairly unihed description of the bonding in phosphonium ylides. The bonding in these compounds can best be described by the ylide resonance form A with a small contribution from the ylene form B. The evidence for this conclusion is summarized below. 

In phosphonium ylides, as we shall see in Section II. A, the phosphorus atom is tetracoordi-nate and the unique (anionic) carbon is tricoordinate. For exactly the same reasons as in the oxides2, all previous discussions3-14 of the P—C bond have been in terms of a resonance hybrid between a dipolar form 1A and a double bond form IB ... 

Figure 11.1 shows the parent compound Me3P -CH2 of the phosphonium yhdes as A and the most simple triarylphosphonium ylide, Ph3P CII2 , as B. The ionic representation of the ylides in Figure 11.1 represents only one of two conceivable resonance forms of such... 

Application of the Wittig reaction in the carbohydrate field is accompanied by certain difficulties. A correct choice of the initial sugar components is the main problem, owing to the basicity of phosphoranes and, especially, to the drastically basic conditions employed with phosphonium ylides (2a). It is not surprising, therefore, that protected (acetalated and aeetylated) aldehydo sugars and resonance-stabilized phosphoranes were used at first,3-5 although partially protected, and even unprotected, aldoses were shown to be amenable to the reaction with various resonance-stabilized phosphoranes, thanks to the presence of the carbonyl form in the mobile equilibrium. The latter reactions, however, are extremely complicated (see Section IV, p. 284). 

It was soon found that it was possible to form ylides of the C15 ylide type (6), which were resonance-stabilized by conjugated double bonds, from the phosphonium salts, using alcoholates, also in protic solvents, such as alcohols. In addition, they are so retarded in their reactivity that they are no longer hydrolyzed by water... 

The phosphonium salt is more acidic than usual because its conjugate base, the ylide, is stabilized by resonance involving the double bonds. Therefore, methoxide ion, a weaker base than usual, can be used lo form the ylide. Reaction of the ylide with the aldehyde that has its hydroxy group protected as an ester produces vitamin A acetate. The acetate group can readily be removed to complete the synthesis of vitamin A (see Section 10.2). 

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