1,3,4-oxadiazoles, polymer bound

Scheme 7.94 Synthesis of 1,3,4-oxadiazoles utilizing a polymer-bound Burgess reagent.

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). ... 

Ley and coworkers used this method to carry out the cyclization of 1,4-disubstituted-(thio)semicarbazides, forming 2-amino-l,3,4-oxadiazoles (Scheme 6.6). A polymer-bound tertiary base was needed to facilitate the ring closure [16], In summary, the most obvious advantages of the CC14 or CBr4/PS-TPP (polystyrene-supported triphenylphosphane) system are the ease of purification... 

Very recently more comprehensive research on the solid-supported preparation of substituted oxadiazoles and thiadiazoles was reported [108], Initially, a set of ten oxadiazoles was prepared by microwave-mediated cyclization of semicarba-zides. In the presence of polymer-bound DCC the desired compounds were obtained within 1 h at 140 °C (Scheme 16.70a). To find a different pathway a set of nine polymer-bound bases was used in an optimization sequence employing sul-fonyl acid chlorides. If the reactions were performed in THF under the action of microwave irradiation for 1 h, PS-BEMP was found to be the most effective base. A set of twenty sulfonyl acid chlorides was used in this microwave procedure to afford the corresponding compounds in moderate to good yields and excellent purity... 

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

Main-chain polymers, such as polymer 57 shown in Table 6.11, incorporating a higher concentration the 2,5-disubsituted-l,3,4-oxadiazole moiety directly bound to the polymer backbone were prepared as ETLs. The glass transition temperature can be controlled by varying the relative concentration of the monomer units in copolymers. Polymers incorporating other... 

Hydrophobic fluorescent probes have been bound onto copolymers of 2-dialkylaminoethyl methacrylate-N-vinyl-2-pyrrolidone and compared with effects of dye binding l and in an amphiphilic copolymer of jl-styrenesulphonate with butyl methacrylate containing vinyl carbazole as a fluorescent probe only monomer fluorescence was observed especially in water where the polymer formed latex particles . two molecular populations have been observed for an oxadiazole dye in poly(l-trimethylsilyl-l-propyne)353 while a... 

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