Amide bases reaction with phosphonium salts

The "instant ylides"are solid-phase mixtures of a phosphonium salt with sodium amide (NaNH2, a strong base and the conjugate base of ammonia) and are now commercially available. In the solid phase, no reaction between the strong base and the phosphonium salt occurs. Thus, to generate the desired ylide, the "instant ylide"mixture need only be placed in a suitable solvent. 

The overall sequence of three steps may be called the Wittig reaction, or only the final step. Phosphonium salts are also prepared by addition of phosphines to Michael alkenes (hke 15-8) and in other ways. The phosphonium salts are most often converted to the ylids by treatment with a strong base such as butyllithium, sodium amide, sodium hydride, or a sodium alkoxide, though weaker bases can be used if... 

The reactivity of uronium- (and phosphonium-) based coupling reagents is mainly determined by the type of activated acid derivative formed during activation (see Figure 13.2). Unlike phosphonium salts, uronium salts can react with amines to yield guanidines [84]. This side reaction can interfere with amide formation if more uronium salt than carboxylic acid is used. Illustrative examples of the use of uronium salts are listed in Table 13.6. 

When generating the ylide from the corresponding phosphonium salt, the choice of the method of formation is important for the stereochemistry of the reaction. With the original application of lithium alkyls as bases one equivalent of lithium halogenide is always formed this lowers the stereoselectivity. Not before the development of methods for the preparation of salt-free ylide solutions, such as the sodium amide... 

---