Phosphoranes special

In this chapter, we consider two aspects of carbon-phosphorus bond formation as they relate to pentacoordinated phosphorus species. The first aspect is the preparation of stable phosphorane species — compounds bearing five bonds to phosphorus with at least one of them being a C-P linkage. At present, this is an area of rather specialized interest, but one that has potential for broader applications. 

The most interesting properties of phosphoranes, i.e. their role as intermediates or transition states of nucleophilic addition reactions of four-coordinate phosphorus compounds and their intramolecular rearrangements according to BPR or TR, have already been fully considered. The synthetic potential of stable phosphoranes has been reviewed by Burger in great detail (B-79MI11702), and only some special aspects need be mentioned in this chapter. 

Aldehydes can be transformed into alkynes by a special phosphorane. 

However, homogeneous catalysts obtained from nickel (II) complexes, e.g. bisallylnickel Ni(All)2, and bis(trimethylsilyl)aminobis (trimethylsilylimino) phosphorane (Me3Si)2NP(=NSiMe3)2 in toluene solution deserve special attention [182] ... 

The special potential for constructing double bonds stereoselectively, often necessary in natural material syntheses, makes the Wittig reaction a valuable alternative compared to partial hydrogenation of acetylenes. It is used in the synthesis of carotenoids, fragrance and aroma compounds, terpenes, steroides, hormones, prostaglandins, pheromones, fatty acid derivatives, plant substances, and a variety of other olefinic naturally occurring compounds. Because of the considerable volume of this topic we would like to consider only selected paths of the synthesis of natural compounds in the following sections and to restrict it to reactions of phosphoranes (ylides) only. 

The double bond is transformed into an aldehyde first. Bishydroxylation and oxidative cleavage creates the aldehyde. Aldehydes can be transformed into alkynes by a special phosphorane. 

As illustrated in Eq. (10), a chiral phosphonium ion can undergo attack by a nucleophile at any one of four different faces or six different edges, thus placing the entering ligand in the a and e positions, respectively. In the general case, when all five ligands are different, and in the absence of special constraints (see Sect. 3.2) 20 isomeric phosphoranes, which are interconnected by 30 pseudorotation steps, are thus produced from both enantiomers of the phosphonium ion. Because of the possibility for reaction via this complex intermediate manifold, interpretation of the stereochemical consequences of... 

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