Wittig polymer-bound

Although of little general use under normal Wittig conditions, tetra-n-butyl-ammonium iodide aids the reaction between polymer-bound benzylphosphonium salts and aromatic aldehydes with yields generally >90% [17]. 

Wittig reaction with polymer-bound phosphonium salts... 

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). 

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. 

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. ... 

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... 

Pheromones.— Polymer-bound aldehydes (64a) have been used, with free Wittig reagents, to produce sex attractants, on subsequent cleavage, as a mixture of cis-and (ran -alkenes. It is noteworthy that synthesis in the reverse sense, i.e, the use... 

Another route uses polymer-bound acetophenones, for example hydroxy- or carboxya-cetophenones, which are condensed with aldehydes (Fig. 6.19 (B)) [32,33]. A third alternative is to apply immobilized aldehydes in a Wittig reaction, or to condense these aldehydes with acetophenones under base catalysis (Fig. 6.19 (C)) [34,35],... 

The group of M. J. Kurth [36] used polymer-bound enones for the Michael-type addition of aryl thiolates (Fig. 6.20). In the first step 1,4-butanediol was attached to PS-tritylchloride resin. This was followed by an oxidation to the aldehyde, subsequent Wittig reaction and addition of aryl thiolates. Cleavage was performed with formic acid in THF. 

Scheme 6.12 Polymer-bound phosphane-mediated aza-Wittig reaction.

Polymer-bound triphenylphenylphosphine [354] has been converted to a phosphonium salt 8i by reaction with 2-nitrobenzyl bromide. The nitro group was then reduced to an amine and acylated. The resulting product formed a resin-bound ylide in the presence of base, which underwent a Wittig reaction with aldehydes that released alkenes into solution with the phosphine oxide by-product remaining bound to the solid support [355]. 

Preparation.—Conventional quaternization reactions of phosphines with alkyl halides have been used for the preparation of chiral P-hydroxyalkylphosphonium salts for use in prostaglandin synthesis and of the salts (111), (112), and (113). This approach has also been used for the preparation of polymer-bound phosphonium salts for use in subsequent Wittig reactions and of a range of co-dialkylaminoalkylphosphonium salts. The salt (114), of limited thermal stability, is formed on treatment of the parent phenylphosphaferrocenophane (67, R = Ph) with iodomethane. The oxonium salt (115) is converted into the mixed onium salt (116) on treatment with triphenylphosphine. A range of... 

Among other reactions which have proved to be useful for the synthesis of targeted libraries are the Wittig and the closely related Homer-Emmons couplings. These powerful reactions can be carried out with either polymer-bound aldehydes/ketones or polymer-bound ylides. 

Bolli, M.H. and Ley, S.V. (1998) Development of a polymer bound Wittig reaction and use in multi-step organic synthesis for the overall conversion of alcohols to hydroxyamines. J. Chem. Soc. Perkin Trans. 1 2243-2246. 

A more recent report discussed a microwave-assisted tetronate synthesis entailing domino addition/Wittig olefinations of polymer-bound a-hydroxy esters with the cumulated phosphorus ylide Ph3P=C=C=0 [59]. The desired immobilized a-hydroxy esters can be obtained by microwave-mediated ring opening of the corresponding glycidyl esters by OH-, NH-, or SH-terminal polystyrenes of the Merri-... 

Sequential Ugi-4CC and nza-Wittig reaction using a polymer-bound trapping reagent... 

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