P—N Rings

Compounds containing P—N Rings. As in previous years, these compounds constitute a sizeable section of the material mentioned in the Report. Compounds with ring systems containing a third element are discussed in the next section. 

The gauche-trans equilibrium in the substituted diazadiphosphetidines (35) has been investigated by means of a detailed and P n.m.r. study. Among the parameters evaluated was J(P—P), which, particularly for the t-butyl derivative, showed a large isomer shift. Substituent effects in the mixed chloride fluorides (36) have been similarly investigated.  

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 

Although antimony trifluoride alone is not a sufficiently powerful fluorinating agent to react with P3N3CI6 and the higher members of this series, admixture with SbCls led to the formation, in moderate yields, of the monofluorides P N Cl2 -iF for n = 3—6. ° The more reactive tetramer and hexamer gave difluorides with non-geminal structures also. 

Details of the mechanism of the reaction of chlorophosphazenes with ortho-dinucleophiles such as catechol or o-aminophenol have been discussed. Although it is not possible to isolate the substituted phosphazene (43a), good evidence is available for its initial formation from isolation of the spiro-phosphorane (44) and an imino-intermediate (45). Complete investigation of this system is thus difficult, but further aspects of the reaction paths were elucidated by studying the reaction of o-aminophenol with the tri-catechol derivative (43b). A non-geminal structure has been assigned, on the basis of P n.m.r. measurements, to the disubstituted 

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The versatile and rich chemistry that is exhibited by these P-N ring systems and their exciting structural features suggest that cyclophosphazenes will continue to attract a lot of attention in the coming years. 

Fig. 15. Conformations of the P— N rings in (a) N6P6X12 and (b) N8P X16 (X - OMe, NMe2). [Adapted from Dougill and Paddock (161a) and Paddock at al. (338) by permission of The Chemical Society. London.]...

Trimeric phosphonitrile fluoride is much less susceptible to nucleophilic attack than the tetramer, as evidenced by the fact that the trimeric fluoride can be washed with water. It is believed that the P—N ring structure remains intact during aqueous hydrolysis, although a defined product has not been isolated. There is no indication of a reaction between (PNF2)s and anihne or piperidine, but while ammonia reacts, no well-defined product... 

P-N Rings.— Cont.) Compound Endocyclic P-N bond lengths (A Further remarks Ref. 

Stretching frequencies of P—N ring bond in cyclic phos phazenea... 

The relative shortness of P-N ring bonds indicates a high bond strength and djt-pjt character. However, the energy values of E(P-N) 72 kcals are found to be only slightly higher than those for single P-N bonds (Chapter 3.4) (Table 7.6). 

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