Group 15 systems phosphorus-nitrogen rings

Cyclodehydration of a /J-hydroxyethylamine group to a neighbouring ring-nitrogen is induced by PPA (p. S37), but phosphorus oxychloride may be a more efficient reagent and was recently used in the synthesis of the first member of a tetracyclic ring system. 

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... 

The formulation of more recent APP-based systems also seems to be based on phosphorus-nitrogen synergism. Co-additives are polymeric in nature such as poly(triazine-piperazine) materials, essentially substituted melamine rings linked by piperazine groups, and poly(ethylene-urea-formaldehyde) condensates. The sequence of reactions leading to char formation in these systems is poorly understood. It is likely that both water and ammonia are evolved at certain stages, some phosphorus and probably some nitrogen remain incorporated in the char structure, and some phosphorus ends up as polyphosphoric acid. 

The three nitrogen atoms and the Ruoro-substituted phosphorus atoms are coplanar (within 2.5 pm), but the phenyl-substituted phosphorus atom lies 20.5 pm above this plane. The explanation offered is that the more electropositive phenyl groups cause an expansion of the phosphorus d orbitals, less efficient overlap with the p orbitals of the nitrogen atom, and a weakening of the tr system at that point. This allows27 the ring to deform and the Ph2P moiety (o bend out of the plane. 

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