Phosphorus yhdes

Wittig reaction (Section 17 12) Method for the synthesis of alkenes by the reaction of an aldehyde or a ketone with a phosphorus yhde... 

The synthetic potential of phosphorus ylides was initially developed by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphorus yhde with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond ... 

Stabilized species in the phosphorus yhde category are normally generated in a two-step sequence beginning with the formation of the quaternary phosphonium salt. This is usually accomplished with ease by the reaction of triphenylphosphine with the appropriate haloalkane. Salt formation is followed by deprotonation at the carbon adjacent to phosphorus using an appropriate base to generate a zwitterionic species stabilized by the adjacent functionality (illustrated in equation 20). The resultant phosphorus species reacts with an introduced carbonyl compound to generate an intermediate oxaphosphatane that undergoes decomposition to produce alkene and phosphine oxide at relatively low temperatures (equation 21). 

Fig. 5.17 The phosphorus resonances in phosphafulvene-based phosphorus yhdes...

Other phosphorus containing superbases are phosphines and phosphorus yhdes. The most detailed studied class is Verkade s proazaphosphatranes (cyclic azaphosphines). Because of the extraordinary basicity and relatively weak nucleophilicity of nonionic proazaphosphatranes, they have been found to be efficient catalysts and promoters for many reactions... 

The second row elements, particularly phosphorus and sulfur, stabilize the adjacent carbanion. The cause of stabihty is due to delocalization of negative ciarge of carbanion by a vacant d-orbital (pit-dit bonding) of phosphorus and sulfur, for example, sulfur and phosphorus yhdes. Ylides are more stable than the corresponding simple carbanions (Scieme 3.7). 

These highly air-sensitive complexes apparently owe their high stability to the incorporation of chelating phosphorus yhde ligands. The neutral complexes show dynamic behavior in solution producing dimeric, trimeric and polymeric species in addition to the monomers. This could be demonstrated with the di-tert-butyl phosphonium derivative of lutetium (Schumann and Reier, 1982a) ... 

Shapiro, P. J. Jiang, R Jin, X. Twamley, B. Patton, J. T. Rheingold, A. L. Zwitterionic phosphorus yhde adducts of boron-bridged ansa-zirconocene complexes as precatalysts for olefin polymerization. Eur. J. Inorg. Chem. 2004, 3370-3378. 

In summary, we have explored three carbon nucleophiles in this section. We started with Grig-nard reagents, and then we moved on to ylides (phosphorus yhdes and sulfur ylides). We have seen that phosphorus ylides and sulfur ylides produce very different products ... 

Rhodium-catalyzed hydroformylation of terminal alkenes in the presence of stabilized phosphorus yhdes was found to initiate a domino hydroformylation-Wittig olefination process. When monosuhstituted acceptor-stabilized phosphorus ylides were employed, a hydrogenation step follows the Wittig olefination to give a domino hydroformylation-Wittig olefination hydrogenation process (Equation 7.19) [166]. 

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