Polymer-supported triphenylphosphine

Scheme 4 Microwave-assisted Wittig reactions using polymer-supported triphenylphosphine in methanol...

Weik and Rademann have described the use of phosphoranes as polymer-bound acylation equivalents [65]. The authors chose a norstatine isostere as a synthetic target and employed classical polymer-bound triphenylphosphine in their studies (Scheme 7.54). Initial alkylation of the polymer-supported reagent was achieved with bromoacetonitrile under microwave irradiation. Simple treatment with triethyl-amine transformed the polymer-bound phosphonium salt into the corresponding stable phosphorane, which could be efficiently coupled with various protected amino acids. In this acylation step, the exclusion of water was crucial. 

In a related study, Srivastava and Collibee employed polymer-supported triphenyl-phosphine in palladium-catalyzed cyanations [142]. Commercially available resin-bound triphenylphosphine was admixed with palladium(II) acetate in N,N-dimethyl-formamide in order to generate the heterogeneous catalytic system. The mixture was stirred for 2 h under nitrogen atmosphere in a sealed microwave reaction vessel, to achieve complete formation of the active palladium-phosphine complex. The septum was then removed and equimolar amounts of zinc(II) cyanide and the requisite aryl halide were added. After purging with nitrogen and resealing, the vessel was transferred to the microwave reactor and irradiated at 140 °C for 30-50 min... 

Very recently a novel one-pot three-step Wittig reaction using microwave irradiation and polymer-supported triphenylphosphine has been reported [57]. By use of... 

Scheme 12.15 Microwave-mediated one-pot, three-component Wittig olefinations utilizing polymer-supported triphenylphosphine.

Scheme 2.2 Polymer-supported triphenylphosphine equivalents for use in Wittig reactions.

In this context, it was shown that polymer-supported triphenylphosphine as a ligand for metal-based oxidation is an alternative catalytic system [72]. 

A PEG-star supported triphenylphosphine analog (66) was synthesized and employed in Mitsunobu reactions. Four phenolethers were prepared within 3-18 h reaction time and 68-93% yield. Upon completion of the reactions, the formed polymer supported triphenylphosphine oxide was isolated by precipitation from diethyl ether in > 85% yield. The reagent could be recycled by means of alane reduction (73%). 

The palladium-catalyzed, microwave-assisted conversion of 3-bromopyridine to 3-cyanopyridine using zinc cyanide in dimethylformamide (DMF) has been reported <2000JOC7984>. Substoichiometric quantities of copper or zinc species improve both conversion rate and efficiency of Pd-catalyzed cyanation reactions <1998JOC8224>. A modification of this procedure uses a heterogeneous catalyst prepared from a polymer-supported triphenylphosphine resin and Pd(OAc)2 the nitriles were obtained from halopyridines in high yields <2004TL8895>. The successful cyanation of 3-chloropyridine is observed with potassium cyanide in the presence of palladium catalysts and tetramethylethylenediamine (TMEDA) as a co-catalyst <2001TL6707>. 

In conclusion, we have shown that attachment of transition metal complexes to polymer supported triphenylphosphine leads to air stable, versatile immobilised catalysts that are as active as their homogeneous analogues and have the advantage that they can be re-used numerous times. Work is currently underway to exploit the activity of other polymer-supported organometallic complexes in metal-mediated organic synthesis. 

Polymer-supported Wittig reagents have recently been developed as an extension to the traditional reagents.29 For example, polystyryldiphenylphosphine has been developed in an attempt to replace the use of triphenylphosphine in the preparation of phosphoranes (see Protocol 1). The hope is that these polymer-bound regents will overcome the practical problem of removing the triphenylphosphine oxide by-product formed in Wittig reactions. Polymer supported phosphonates and Wittig substrates have also been prepared for use in solid phase synthesis and combinatorial chemistry.30... 

One of the main drawbacks of the Wittig reaction is the formation of unwanted triphenylphosphine oxide. A new route, which makes use of polymer-supported triphenylphosphine and microwave dielectric heating has been developed (Scheme 13), which yields the required alkene without the triphenylphosphine oxide. An alternative strategy for separation of the product alkene from unwanted phosphine oxide by-product is to carry out the Wittig reaction in a fluorous solvent using a perfluorinated ylide such as (45). One drawback of this... 

Acid amide-triphenylphosphine dihalide adducts (4) have found wide application in organic synthesis. - Synthetic equivalents are adducts (5) from acid amides and triphenylphosphine/CCU, which are prepared in situ from the educts. - With these reagents the following transformations have been performed dehydration of amides or aldoximes to nitriles, preparation of isonitriles from secondary form-amides, preparation of imidoyl halides from amides or acylhydrazines and preparation of ketene imines from amides. Using polymer-supported triphenylphosphine the work-up procedure is much easier to achieve. Triphenylphosphine can be replaced by tris(dialkylamino)phosphines. - Instead of CCI4 hexa-chloroethane, hexabromoethane or l,l,2,2-tetrabromo-l,2-dichloroethane can be used " the adducts thus formed are assumed to be more effective than those from the triphenylphosphine/CCU system. 

Other phosphorus compounds, e.g. PBrs, mixtures of PBrs and Br or PCI3/CI2 and phenyltetra-chlorophosphorane (PhPCU) have been used to convert amides to imidoyl halides. The formation of imidoyl halides proceeds under mild conditions by action of triphenylphosphine/CCU or triphenylphos-phine dihalides on secondary amides or ketoximes. The work-up procedure can be facilitated by use of polymer-supported triphenylphosphine/CCU. By the action of POCI3, phenyl- or methyl-phos-phonic acid dichloride on aminocarboxylic acid thiol esters in the presence of triethylamine the imidoyl chlorides (205 equation 114) are formed in moderate to good yields. The mechanism of this reaction has not been established. The methylene dialkylchlorophosphorane (206 equation 115) allows the conversion of secondary aromatic amides to imidoyl chlorides under very mild conditions. ... 

Polymer-supported triphenylphosphine has not only been used for the preparation of polymer-supported Wittig reagents, but also as a reagent for Mitsu-... 

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