Phosphazenes basicity

As discussed in Chapter 3, some other inorganic polymers also contain phosphorus atoms.47 They are derived from the basic phosphazene structure described in Chapter 3, and are obtained by the ring-opening polymerization of heterocyclic compounds in which one of the disubstituted phosphorus atoms is replaced by another moiety. Specifically, introduction of a carbon atom can yield poly(carbophosphazenes), with the repeat unit shown in 6.49. Alternatively, replacement with a sulfur atom can yield a poly(thiophosphazene) (6.50). Relatively little is known about these polymers at the present time 47... 

The highly basic phosphazenes thus provide an alternative to organolithium bases such as lithium dialkylamides and alkyllithiums. Milder reaction conditions and better solubility are two key advantages associated with phosphazenes. Another important consideration is the lack of a coordinating cation naked enolates and other anions obtained with phosphazenes often exhibit enhanced reactivity, relative to their lithium salts. 

The synthesis of poly(organophosphazenes), POPs, is a research area that has involved a lot of effort in the past by many scientists active in the phosphazene domain. There are several important reasons for this, basically related to the high cost of the starting products [44] used to prepare POPs, to difficulties in carefully controlling the reactions involved in the preparative processes [38] and to the need for accurately predicting both molecular weight and molecular weight distribution of the POPs produced [38,45]. 

As described in several review articles [409,452-454] and books [10,13,15], this is basically due to the inherent features of the d -p bond in phosphazenes, which allows the permanent overlapping of the 2pj orbital of the skeletal nitrogens with any one of the 3p orbitals of the phosphorus atoms [455]. Such a high chain flexibihty generated very low glass transition temperatures in these polymers, which can reach values of about -100 °C when suitable flexible substituent groups (e.g. n-butanol) are present on the skeletal phosphorus [274]. 

Of course, not all the phosphazene polymers that have been synthesized are equally important. Many of them, in fact, have a mere academic or speculative interest, and will not be described in this article. A few other classes of POPs, however, do occupy an important place in phosphazene history, and have been seriously considered for industrial development and commercialization. These polymers are basically those in which the properties of the inorganic -P=N- skeleton overlap to the highest extent those of the phosphorus side substituents. In the successive sections of this article we will describe in some detail the most important classes of polyphosphazenes that fulfil this condition. 

As already reported in Table 6, the solubility of phosphazene polymers is strongly influenced by the nature of the substituent groups attached at the phosphorus atoms along the -P=N- skeleton. Water-solubility, for instance, can be induced in polyphosphazenes by using strongly polar substituents (e.g. methylamine [84], glucosyl [495], glyceryl [496], polyoxyethylene mono-methylether [273] or sulfonic acid [497,498] derivatives), or may be promoted by acids or bases when basic (amino substituents like ethylamine [499]) or acid (e.g. aryloxy carboxylate [499] or aryloxy hydroxylate [295]) substituents are exploited. 

This result has been further substantiated by the work of Koppel et al. <2001PCA9575>. They conducted a theoretical study of the basicity of phosphorus imines and ylides. Verkade bases 6-8 were included. They showed basicities comparable to commercially used organic superbases (/-BUP4 phosphazene imine), with compound 8 in particular giving calculated basicities similar to Li3P and Li20. 

For example, there is considerable interest in the preparation of sulfur-nitrogen polymers that have organic substituents on the sulfur atoms.56 This could help alleviate the intractability problem mentioned, and could also give rise to a series of polymers that parallel the phosphazenes in their structural variability. A series of polymers with S-N backbones of this type has been prepared, but with oxygen atoms as some of the substituents. The basic structure has the repeat unit -RS(=0)(N)- or -FS(=0)(N)-but they have not yet been studied in detail. The fluorine-containing polymer, however, is known to be a tough elastomer which is unaffected by water, acids, or bases up to 100 °C.7... 

A great deal of basicity data is available on substituted chlorophosphazenes. None has hitherto been published on fluoro-phosphazenes. A priori, one surmises a greater electron-withdrawing inductive effect for fluorine, which oould be compensated for by the greater potential of fluorine to back-donate by a mesomsric effect. Data is new compared on structurally related chloro- and... 

The electron-withdrawing ability of the fluorine atom can draw electron density from an adjacent nitrogen atom, making a distant nitrogen more basic. A second effect is the shortening of the P—Cl bond in a P(F)C1 vs. a PCI2 center (32). Thus, an incoming Lewis acid will interact with the phosphazene at a site that is distant from the P(F)C1 site. 

A dinuclear compound, (Me2N)3P=N-P(=NH)(NMe2)2, with a high nitrogen phosphorus ratio is the first member of a series of strongly basic, branched polyamino-imino phosphazenes (Scheme 43). ... 

Computational studies concerning theoretical approaches to the intrinsic basicity of neutral nitrogen bases have been reported, including those of phos-phoranimines. The non-ionic phosphazene bases BEMP (112), BTPP (113) and (114, R = Ph) appear to be excellent catalysts for the Michael addition reactions. Thus the yield of the coupling reaction of ethyl isocyanoacetate with l,2-bis(4-bromomethylphenyl)ethane is increased by the addition of the phosphazene base BEMP. Polymer-supported BEMP (P-BEMP) has been applied for the allylation of 2H-benzo[d]l,3-dioxolan-5-ol by allyl bromide. " Cyclodehydration of 1,2 diacylhydrazines by tosyl chloride in the presence of P-BEMP leads to excellent yields of 1,3,4,-oxadiazoles. Addition of P-BEMP also improves the yield of the Hofmann elimination step in the synthesis of tertiary mines using REM resin (polymer-bound acrylate ester). ... 

Phosphazene bases represent a class of non-ionic materials which exhibit exceptionally high degrees of basicity, this being dependent upon the structural composition of the molecules [1, 2]. The core structural unit of non-ionic phosphazene bases is derived from a nitrogen basic center double-bonded to a pentavalent phosphorus (PI unit). This peralkylated triaminoiminophosphorane, through oligomerization, can result in a structurally diverse range of phosphazene bases (Fig. 1). 

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