Synthesis of Cyclic Phosphazenes

A re-examination of the reaction of ammonia with phosphorus penta-chloride, which may be expressed  

In another study, P n.m.r. was used to follow the progress of the reaction between ammonium chloride and phosphorus pentachloride in yA/j-tetrachloroethane, and the mechanism of the reaction considered in some detail. The rate of hydrogen chloride evolution and changes in the 

The way in which this salt is formed is still not known with any certainty, but it may involve transient formation of a monophosphazene, 03 = NH. It may then be that Cl3P=NH effects expansion of cationic phosphazene chains  

Cyclization could then occur by PCI4 elimination  

The factors affecting the preparation of the cyclic chlorophosphazenes from phosphorus pentachloride and ammonium chloride continue to receive attention. For example, the yields and reaction times for the preparation of the series, (NPCla) ( — 3—7), varied with the fineness of the ammonium chloride, the nature and volume of the solvent, and added catalysts such as phosphoryl chloride. A procedure, giving due consideration to these factors, was described for the preparation of N3P3CI6 in good yield (88% of cyclic products) and in a relatively short time (2J h). The cyclic chlorophosphazenes can be obtained in even shorter times ca. 10 min) by addition of four moles of pyridine to remove the hydrogen chloride formed  

The first synthesis of a phosphazene that forms part of a four-membered ring has been accomplished  

Examples of phosphazenes which form part of a five-membered ring have 

A new route to a cyclic monophosphazene in a six-membered ring (22) involves the addition of an isocyanate to a nitrilimine. Hydrolytic studies on these cyclophosphazenes were also reported. 

Further variations on the theme of the preparation of chlorocyclophospha-zenes are described in a recent patent. This involved the formation of ammonium chloride in situ, before heating to effect the reaction with phosphorus pentachloride. 

Very neat preparations of alkyl- and aryl-cyclotri- and arylcyclotetra- 

Compounds containing a phosphazene linkage which forms part of a four-membered ring are elusive. It has now been shown that (70), claimed as the jfirst phosphazene of this type, is in fact a mixture of the six-membered ring systems (71) and (72) in 

The reactions of A-chloroalkylphosphazenes with hydroxylamine hydrochloride to give (73) proceed in a similar manner. Derivatives of (73) were obtained by reactions with arylamines, with sodium aryloxides, and with acetic (or formic) acid. The latter is noteworthy in that the proton of the hydroxy-group in (74) does not migrate to nitrogen. 

Publications dealing with improvements in the synthesis of NsPaClg and N4P4CI are still appearing. For example, yields from the reaction between PClg and NH4CI 

The compounds Bu2SnCl2, (BuO)4Ti, and MePhSiCla act as catalysts for the preparation of chlorocyclophosphazenes from ammonium chloride and phosphorus pentachloride. Yields of the methylchlorocyclophosphazenes (NPClMe)3,4 from the reaction 

An alternative route to the tetramer (mixed with trimer) is to ammonolyse dimethyltrichlorophosphorane and pyrolyse the product in vacuo  

Several novel syntheses of new types of cyclophosphazenes have been described. For example, the mono- and di-phosphazenes H2N(RaN)2-P=NH (R = Et or Bu ) and (16) may be used to insert mono- and di-phosphazene units, respectively, into the triazine molecule (17). Thus in refluxing benzene solution ammonia was evolved leaving the cyclo-phosphazatriene (18) and the cyclodiphosphazatriene (19). and 

The cyclization of linear diphosphazenes can be effected by reaction with trisdimethylaminophosphine. As in the reactions previously reported involving the same diphosphazene and phosphites, (21) was present as the P—H rather than the N—H tautomer. In a related series of reactions it 

The structure of the major product from the reaction of chlorodiphenyl-phosphine with hydrazine hydrochloride has now been shown to be (23 X = Cl), rather than the linear phosphazene (H2N-Ph2P=N Pha-P=N-Ph2PNH2) Cl . A number of other salts based on [(23) X = I, 

Yet more improvements in the synthesis of chlorocyclophosphazenes have appeared. Yields in the PC15-NH4C1 reaction are increased by the use of heavy-metal salts as catalysts,93 but similar results may also be achieved by the use of acid-treated montmorillonite clay.94 The use of surfactants can also improve yields of cyclic products.95 

Methylcyclophosphazenes N3P3Me6 and N4P4Me8 are more conveniently prepared and separated using methylamine hydrochloride, rather than ammonium chloride, in the reaction with Me2PCl3.98 The initial product from this reaction is probably (Me2PClNMe)2, which on pyrolysis gives a mixture of N3P3Me6 MeCl and 

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The polymeric phosphazenes are treated in chapter (see Polyphosphazenes) A recent monograph covers the chemistry of polyphosphazenes (nomenclature, synthesis of cyclic monomers , ring opening polymerization, condensation polymerization, substitution, polymer properties, and applications more than 1000 literature citations). Other reviews have also been published recently. Sulfur-containing polyphosphazenes have also been described. ... 

The synthesis of poly(phosphazene) derivatives by single or multistep reactions of poly(phosphazene) precursors continues to represent the major pathway to new members of this class of materials. The reaction of an oxyanion with (NPClj) is the most common synthetic procedure in this area. Protected glyceral poly(phosphazenes) analogous to the cyclic homologs, 28, have been prepared and may be crosslinked by exposure to yi radiation. Acidic deprotection yields the free glyceral substituted polymers which undergo slow hydrolysis. The deprotected polymers can be crosslinked using... 

The earliest and most well-developed route to polyphosphazenes involves the thermal ROP of cyclic phosphazenes bearing halogen substituents at phosphorus. Use of this method and subsequent halogen replacement with alkoxides has led to the synthesis of ferrocene- and ruthenocene-containing polyphosphazenes 31 and 32 with molecular weights (d/w) in excess of 2 x... 

An alternative type of condensation synthesis involves the decomposition of organophos-phorus azides. First reported by Haber and coworkers in the 1960s,61 these syntheses follow the pathway shown in reaction (24), in which elimination of nitrogen takes place in solution with the direct formation of either a cyclic phosphazene or a polymer. Matyjaszewski and coworkers62 have used this method to produce the nearly insoluble (NPPh2) , which is difficult to prepare by other approaches, and [NP(Ph)(p-MePh)] which is soluble in aromatic solvents. However, phosphorus azides are potential detonators, and they must be prepared and used in solution and not isolated in the solid state. 

In addition to classical substituents such as NH2, NHR, NR2, OR, SR in various combinations, in recent years some more exotic groups have been attached to phosphazene rings. Among the novel cyclic phosphazenes the carbo-rane substituted derivatives are worth mentioning. Other unusual derivatives include chiral cyclophosphazenes, cyano-, ferrocenylphenoxy-, and ferrocenylhydrazone-cyclophosphazenes. Another interesting development is the synthesis of crown ethers with chlorocyclophosphazene subunits (Scheme 54). ... 

Activity in the phosphazene area has increased over that reported in volume 21 as indicated by an increase of fifty-five in the number of citations. Three particular areas deserve special mention. These are the use of the aza-Wittig reaction in the construction of heterocyclic rings, the structural diversity and solvent selectivity in macrocycles formed by reactions of long chain diamines (or oxodiamines) with N3P3CI6 and lastly the synthesis of new heterophosphazene polymers by ring opening reactions of cyclic heterophosphazenes. 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

H.R. AUcock, A.G. Scopelianos, Synthesis of sugar-substituted cyclic and polymeric phosphazenes and their oxidation, reduction, and acetylation reactions. Macromolecules 16 (5) (1983) 715-719. 

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