Phosphorus-Oxygen Systems

A brief comparison of phosphorus-nitrogen bond lengths relative to phosphorus-oxygen systems leads to the suggestion... 

Using P NMR, the intermediacy of the quasiphosphonium ion bearing the phosphorus-oxygen linkage has been demonstrated experimentally. In the absence of added acid, the same product formation occurs, but the intermediate quasiphosphonium species has not been detected experimentally. However, evidence of an intermediate oxyphosphorane (see Section 5.3) has been demonstrated under these conditions, as it has under acidic conditions, and such an intermediate has independently been shown to provide the usual products of the reaction. Evidence is available in other systems for the equilibrium of oxyphosphoranes with oxygen-containing quasiphosphonium salts. 

In the catechol-substituted compound (37), which was synthesized from (F3PNMe)2 and the di-lithium derivative of catechol, n.m.r. measurements showed that the fluorine atom at the substituted phosphorus atom occupies an equatorial position. The reason for such behaviour in this and in similar compounds is probably associated with the lower ring strain in the phosphorus-catechol system when the oxygen atoms occupy axial and equatorial positions. A new diazadiphosphetidine (38) has been prepared by the oxidation of the A A -compounds (39) with biacetyl, and although, at low temperatures, an excess of biacetyl leads to the A A -product (40), this compound is converted into the monophosphazene (41) within 24 hours at room temperature The thermodynamic instability of the dimer (40) follows from the presence of bulky substituents on the nitrogen atoms and electron-donating groups at phosphorus. McjSi... 

Among the possible ring systems based on phosphorus, oxygen and carbon, the dioxaphosphinanes (dioxaphosphorinanes), dioxaphospholanes and dioxaphospholes have been most studied (Table 6.25). 

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