Phosphorus-lead bonds

The only compounds containing lead-phosphorus bonds are Pb[(Ph2P)2CH]2 and Pb[(Ph2P)2C(SiMe3)2]2, formed as orange crystals from PbCb and the appropriate lithium salt... 

Intramolecular alkyl transfer is a fundamental problem with this reaction this problem can be addressed with modification in structure of the reagents. Neutral trivalent phosphorus reagents do react with carbonyl compounds at much lower temperatures, but lead to several types of pentacoordinated phosphorus products.190-198 More will be noted about the use of such pentacoordinated phosphorus species for carbon-phosphorus bond formation in Chapter 5. 

In this section we will survey the use of transition metal-catalyzed additions, at times accompanied by rearrangement processes, that lead to the generation of new carbon-phosphorus bonds. 

The breaking of carbon-to-phosphorus bonds is by itself not a useful reaction in homogeneous catalysis. It is an undesirable side-reaction that occurs in systems containing transition metals and phosphine ligands and that leads to deactivation of the catalysts. Two reaction pathways can be distinguished, oxidative addition and nucleophilic attack at the co-ordinated phosphorus atom (Figure 2.35). 

In continuation of our efforts to explore the utility of the SAMP/RAMP hydra-zone methodology, we developed the first asymmetric synthesis of a-phosphino ketones via formation of a carbon-phosphorus bond in the a-position to the carbonyl group [70]. The key step of this asymmetric C—P bond formation is the electrophilic phosphinylation of the ketone SAMP hydrazone 87, giving rise to the borane-adduct of the phosphino hydrazone 88 with excellent diastereoselectiv-ity (de = 95-98%). Since these phosphane-borane adducts are stable with respect to oxidation, the chemoselective cleavage of the chiral auxiliary by ozonolysis leading to the a-phosphino ketones (R)-89 could be accomplished with virtually no racemization. Using RAMP as a chiral auxiliary, the synthesis of the enantiomer (S)-89 was possible (Scheme 1.1.25). 

It has been known for many years that transition metals catalyze reactions of coordinated phosphines (2). Known reactions of phosphines as ligands include carbon-hydrogen lx)nd cleavage (cyclometalation), as well as direct carbon-phosphorus bond cleavage. Such metal-catalyzed reactions of phosphines lead to formation of new metal complexes which can affect catalyst properties. A known example is the reaction of triphenylphosphine to propyldiphenylphosphine during the rhodium-catalyzed propylene hydrogenation or hydroformylation (5). 

The process may be complicated, however, by reaction of intermediates I and II with unreacted phosphorus to gi e more complex phosphides which in turn arc broken down by the organometallic reactant. Ileac-tions of organoalkali compounds with phosphorus may also proceed tlirougli such intermediates, but the greater reactivity of the alkali derivatives evidently permits further phosphorus-phosphorus bond cleavage leading to more complex ions. 

Instability in the supposedly stable phosphorus-carbon bond displays itself not only in unfortunate ways, leading as it does to side reactions and the formation, in synthesis, of unwanted by-products, but also in a constructive manner, forming the basis of reaction sequences of outstanding value in synthesis, as for example in alkene-forming reactions. Instability is an inherent property of (a-hydroxyalkyl)phosphonic acids which manifests itself in phosphorus-carbon bond cleavage as a result of the action of heat or of alkali, and which can lead either to dissociation into precursors or to rearrangement to phosphates (a-oxoalkyl)phosphonic derivatives are susceptible to attack by nucleophiles, a process which also results in carbon-phosphorus bond fission. 

Although the syntheses of compounds containing phosphorus-phosphorus bonds are well established, the incorporation of this diatomic unit into polycyclic molecules is not easy to accomplish. The method presented here is a single-step reaction starting with silyl-substituted urea and phosphorus trichloride. Variation of the urea substituents and the use of RPCI2 instead of PCI, demonstrate the scope of this reaction, which leads to bicyclic —X P as well as spirobicyclic... 

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