Reactions with Polymer-Bound Phosphanes

Another nucleophilic entity widely used both in solution and on polymer support is phosphorus. The handling of phosphanes both pre- and post-synthesis is considered problematic, pre due to their facile oxidation (in the case of electron-rich phosphanes), post due to the formation of the corresponding oxide that makes purification of the desired product troublesome. In this case, the use of a polymer-bound phosphane shows many advantages over the solution phase reaction the phosphane is easier to handle, and purification of the desired product along with removal of the undesired phosphane oxide is simplified. 

R1 R OH Q 3 PPh2 r1 CCI4, reflux R R = alkyl, aryl or H R1 = alkyl or aryl 

Another well established reaction in solution-phase chemistry using a nucleophilic phosphane is the Staudinger reaction. A polystyrene-supported alternative proved to be less efficient, however. The hydrolysis of the intermediate imino- 

Nucleophilic phosphanes can also be used for olefin isomerization. Nitroolefins having a substituent on the nitro group, readily available via the nitro aldol reaction followed by dehydration, often give a mixture of (E)- and (Zj-isomers. By adding catalytic amounts of polymer-supported triphenylphosphane, the (Z) -isomer could be isomerized to the corresponding (Ej-isomer, in most cases with total (E)-selectivity [30]. 

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Another well-known process that utilizes a nucleophilic phosphane is the Mit-sunobu reaction, that is, the reaction between an acidic partner and an alcohol, typically facilitated by an azodicarboxylate and a phosphane. Two options are possible, anchoring of the electrophilic part to the solid support, dealt with in the next section, or anchoring of the nucleophilic phosphane. Georg et al. used polystyrene-bound triphenylphosphane and DEAD (diethyl azodicarboxylate) in their synthesis of aryl ethers [31]. Alcohols were reacted successfully with electron-rich and electron-deficient phenols, giving the desired products in good yield and purity. More recently, Wilhite and coworkers disclosed an efficient protocol for the synthesis of pyridine ethers using ADDP [l,l -(azodicarbonyl)dipiperidine] and polymer-supported triphenylphosphane (Scheme 6.9) [32], Both methods eliminate purification problems caused by triphenylphosphane oxide, but chromatography is still needed. 

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