Phosphazene bases

DBU IS most widely employed as the base m the Barton-Zard reaction Stronger nonionic bases such as PrMeNCH-,CH-j,N and the phosphazene base Csiipplied by Flukai are more effective to induce pyrrole formadon in the reaction of nitroalkenes v/ith isocyano esters than DBU Sterically hindered nitroalkenes are converted into the corresponding pyrroles using these bases, as shovm in Eq 10 35, but DBU is not effective in this transformation... 

This strategy using super nonionic strong base is also applied to less reac compounds The reaction of Tnitronaphthalenev/ith ethyl isocyanoacetate proceeds very slowly in the presence of DBU, but this reaction is accelerated by the use of phosphazene base to give the corresponding pyrrole in reasonable yield fEq 10 36 ... 

Table 1.1 Synthesis of aryl-vinyl epoxides by use of chiral sulfide 1 a phosphazene base.

ETP2 base, see phosphazene base 2 Evans auxiliary, see Darzens reaction 13 exo cyclization mode 272... 

Phi = NTs ([N-(p-toluenesulfonyl)iminio]phe-nyliodinane) 79 phomactin A 307 (+)-phonomactin 277 ( )-phoracantholide 299 phosphazene base 3 f. phosphazine base 31 phospholene epoxide 243 phosphoramidite 261 phosphoramidite ligand 247 piperidinephosphonate 103 pipermethystine 302 P K 177... 

Table 11 Structure and properties of phosphazene-based ionic conductors ...

Poly[bis(methylamino)phosphazene], for instance,has been radiation cross-linked [640] for the preparation of phosphazene-based membranes. MEEP, a... 

Fig. 7.8 Polystyrene-bound dehydrating agent (left) and polymer-supported phosphazene base PS-BEMP (right) utilized for oxadiazole synthesis.

Similar treatment of 5-nitrobenzothiadiazole 97 with ethyl isocyanoacetate and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave the thiadiazolo[3,4-< ]isoindole 93 (21%), but when a phosphazene base was employed the major isolated product was the pyrimidine fV-oxide 98 (46%) (Equation 13) <1996J(P1)1403, 1996TL8391>. 

Allcock s research led to the development of poly-phosphazene-based PEMs by his small molecule studies of the sulfonation of cyclic trimeric phosphazenes and the surface chemistry of polyphosphazene macromolecules. In a 1993 report, he described the sulfonation of aminophosphazenes with 1,3—propanesultone. While these specific materials are not necessarily ideal as PEMs, this study demonstrated a novel technique for creating sulfonated polyphosphazene materials that may provide more control over the sulfonated polymer product than wholesale sulfonation of a base polymer by a strong sulfonating agent. 

Schwesingefs phosphazene base 2-tert-butylamino-2-diethylamino-l,3-dimethyl-perhydro-l,3,2-diazaphosphorine (PS-BEMP has a pKb = 27.5 in MeCN) has been immobilized and shown to have immense utility in the N- and O-alkylation of many weakly acidic heterocycles. Kim et al. has made extensive use of this reagent in the multi-step synthesis of a small collection of guanines possessing potential antiviral activity [90]. The generic procedure involved the direct alkylation of the purine moiety (20) (Scheme 2.64), promoted by PS-BEMP, resulting in a mixture... 

A very interesting and preparatively useful new development in the realm of bases is that of compounds like phosphazene-base P4-t-Bu 4 and phosphazene-base P4-t-0ct 523 24 which are commercially available (Fluka AG, Buchs, Switzerland). 

These new, uncharged amine bases are strong enough (pXa of 4 in acetonitrile is 42.62) to deprotonate esters or lactones, where they can even be used with advantage over the more conventional bases like LDA or metal hcxamethyldisilazanides. For example, the alkylation of dimethyl isopropylidene tartrate with n-alkyl iodide is not possible with the latter bases but can be achieved with phosphazene-base P4-t-Bu or phosphazene-base P4-t-Oct24. With phos-phazene-base P4- -Bu the yield of 8 is 41 % with 28 % of recovered starting material 6, whereas with phosphazene-base P4-i-Oct the yield of 8 is 71 % with only 9% of 6 left unreacted. (For alkylations of 6 with other more active electrophiles see later.)... 

In the same publication, it was reported that this cyclo-dehydration could also be affected by using tosyl chloride and the polymer-supported phosphazene base PS-BEMP, and again, microwave heating was found to be advantageous (Scheme 6.25). In utilising this protocol, no scavenger purification strategy was deemed necessary and the authors note that this is the most efficient 1,3,4-oxadiazole synthesis of the three polymer-supported methods described. 

A fast catalyst for the ring-opening polymerization of D4 in solution or in the bulk has been reported by Molenberg and Moller13. They report the use of a phosphazene base in combination with methanol as the initiator, compound 3. 

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