Polymer-supported phosphazene base

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

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. 

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). ... 

In more recent work a different polystyrene supported dehydrating agent (Fig. 16.4a) was used for synthesis of 1,3,4-oxadiazoles under thermal and microwave conditions [107]. Addition of a homogeneous base, for example guanidine, improved the cyclodehydration reaction and led to completion under thermal and, even more significantly, microwave conditions. This required subsequent purification steps to remove the base for clean and quantitative isolation of the desired products, however. On the other hand, use of a polymer supported phosphazene base (Fig. 16.4b) as a base additive circumvents the need for additional purification of the reaction mixture for product isolation. 

Polymer-supported phosphazene bases (of which one example is commercially available) can be used to deprotonate a phenol group for allylation in the generation of an intermediate during an efficient synthesis of the natural product carpanone [48]. Such a system was also employed for a dehydration step in the synthesis of 1,3,4-oxadiazoles [49]. 

Scheme 18.4 Use of a polymer-supported phosphazene base in a key step in synthesis of the C/D...

Scheme 18.4). For preparation of the C/D spiroketal fragment (9), we needed to transform the chloroepoxide (5) to a second homologated epoxide (8) via an intermediate chlorohydrin (6) (Scheme 18.4). Solution-phase bases were used initially but led to alternative aUyUc alcohol products however, the polymer-supported phosphazene base (7 PS-BEMP) was more successful and allowed the epoxide (8) to be isolated following filtration in 87% yield. 

Schwesinger, R., et al. Extremely Strong, Uncharged Auxiliary Bases Monomeric and Polymer-Supported Polyaminophosphazenes (P2-P5), Liebigs Ann. 1996,1055-1081. A review on phosphazenes. 

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