Wittig reactions polymers

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

The Wittig reaction has been carried out with polymer-supported ylids (see p. 509). It has also been done on silica gel. " ... 

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

Wittig reaction with polymer-bound phosphonium salts... 

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

For the synthesis of alkenes, the Wittig and Horner-Wadsworth-Emmons reactions have become important tools. Triphenylphosphine that is used in the Wittig reaction can be immobilized either on the polymer or can be used in solution for solid-phase chemistry (Scheme 3.18). The Horner-Wadsworth-Emmons reaction for example is used for the synthesis of aldehyde building blocks [261]. 

Ford, W. T., Wittig Reactions on Polymer Supports, Chap. 8 in Polymeric Reagents and Catalysts, W. T. Ford, ed., Am. Chem. Soc. Symp. Ser., Vol. 308, American Chemical Society, Washington, DC, 1986b. 

Table 2. Comparison of yields in Wittig-reaction on polymer supports...

The G-B bond of boronic esters is inert for functionalization of remote sites other than the B-G bond by metal-catalyzed reactions (Equations (98)-(100)). Gross-coupling reactions of 320478 and 321,479 and titanium-Wittig reaction on polymer resin 323480 have been studied. 

In the following example (Eq. 7) P—QH means styrene polymer where a di-ethylaminoethynyl moiety is built up via a formylation, Wittig reaction with dichloro-methylene phosphorane and elimination-substitution steps. This polymer-bound ynamine is till now unique 18). 

Treatment of the polymer with BuLi and then the aldehyde gives a Wittig reaction (Chapter 31) that releases the alkene product but leaves the phosphine oxide bound to the polymer. 

Thefcphosphine oxide can be reduced back to the phosphine (for example, with Cl- SiH) while still bound to the polymer and the polymer-bound reagent can be used again. Separation of Ph3P=0 from alkene products after a Wittig reaction can be quite a nuisance so the ease of work-up alone makes this an attractive procedure. 

If you need a hint, draw out the reagents that you would add to the polymer to do a Wittig reaction and work out what you would get in each case. 

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... 

An abnormal aza-Wittig reaction is observed in the reaction of a polymer bound iminophosphoranes with electron poor isocyanates, such as p-nitrophenyl isocyanate. With aliphatic isocyanates the regular aza-Wittig reaction occurs. ... 

The aza-Wittig reaction is also used to form polymers with pendant carbodiimide groups. For example, reaction of the polymeric azide 31 with benzyl isothiocyanate and triphenylphosphine gives the corresponding carbodiimide 32, a precursor for oligomeric guanidines." ... 

It was soon realized that the far more readily available 1-chloro and bromo acetylenes could be used too and in fact phenylchloroacetylene gives with lithium dimethylamide in ether, 87 % of the corresponding ynamine Similarly, N-lithium pyrroline affords 67 % of the corresponding ynamine In the following example (Eq. 7) P—CgH4 means styrene polymer where a di-ethylaminoethynyl moiety is built up via a formylation, Wittig reaction with dichloro-methylene phosphorane and elimination-substitution steps. This polymer-bound ynamine is till now unique... 

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