Phosphorus oxygen bond basicity

In the dissociative model (Fig. 4.2) a phosphorus-oxygen bond is broken, yielding an alcohol or alkoxide (depending on pH) and a metaphosphate ion (PO ). The metaphosphate ion is unstable and highly susceptible to nucleophilic attack. The rate of reaction is dependent on the proton dissociation constant (pKJ of the leaving group, but independent of the basicity of the nucleophile (Wijesekera, 1992). 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

On the basis of correlation analysis by LFER between the chemical structure and spectral data from IR and NMR, Lewis basicity, rate constant of hydrolysis as well as pKa of various organophos-phorus compounds, it is not likely that the benzene ring is conjugated with the phosphoryl group, and that the oxygen of the latter is linked to the phosphorus by a double bond, if the general concepts of conjugation in the chemistry of carbon compounds work as well in the phosphorus series. 

The generation of phosphine (and the phosphide anion) from elemental phosphorus for use in this type of reaction is common. While unreactive with neutral water (although undergoing rapid oxidation with oxygen at 34 °C), elemental phosphorus (white, P4) generates phosphine readily in basic medium. On heating above 400 °C, white phosphorus [P4] generates a different allotropic form known as red phosphorus in which a P-P bond within the fundamental P4 tetrahedron is broken and individual tetrahedra are linked in a polymeric chain. This form of elemental phosphorus is less reactive that white phosphorus, but still is quite useful for synthetic purposes. 

We turn now to the comparison of phosphate esters with esters of other phosphorus oxyacids, namely phosphites, phosphonates and phos-phinates. Phosphites are distinguished from phosphates by a lower formal valence number (l.t. by a basically different bonding structure), whereas in phosphonates and phosphinates the phosphorus is formally pentavalent, as in phosphates. Phosphonates and phosphinates have direct carbon-phosphorus bonding, while the linkage of the organic groups in the phosphate esters is all of the carbon-oxygen-phosphorus type. 

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