Phase Transfer Wittig Reactions

The major advantage of the phase transfer method in Wittig reactions is increasing the convenience of the reaction. Concentrated aqueous alkali is obviously easier to handle than is /r-butyllithium or sodium hydride and such solvents as dichloromethane and benzene are more readily removed after the reaction is complete than is DMSO. 

The phase transfer Wittig reaction can therefore be characterized by saying that the sequence 1.1 —1.3 is altered only in the identities of the base and the solvent. 

An interesting feature of the phase transfer Wittig reaction is that it succeeds in the absence of any added catalyst. Apparently, the phosphonium salts which are reactants in this system are also effective as phase transfer agents. It is less apparent why certain phosphonates should show the ability to catalyze these reactions and this is discussed in Sect. 1.3. In the phosphonium salt case, proton transfer must occur at the interface followed by dissolution of the ylid in the organic medium where it then undergoes the normal Wittig reaction. 

It has recently been shown that crown ether catalyzed Wittig reactions give product distributions typical of the so-called salt-free reaction [6]. Using potassium r-butoxide or potassium carbonate as base, the yields of olefin were high and the isomer ratio was quite solvent dependent. The condensation of ethylidenetriphenyl-phosphine with benzaldehyde, for example, produced over 90% of i3-methylstyrene in either tetrahydrofuran or dichloromethane, but the olefin was 85% cis in the former case and 78% trans in the latter [5]. 

The Wittig reaction wherein a phosphonate is substituted for the more traditional phosphonium salt is called either the Wittig-Horner [8] or Wittig-Emmons [9] reaction. The synthesis of alkenes via base catalyzed phosphonate addition to an aldehyde or ketone has been accomplished under phase transfer conditions [10—13]. 

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The Homer-Wadsworth-Emmons reaction is an important variant of the Wittig reaction and involves using a phosphonate ester in place of a phosphonium salt. Like the phase-transfer Wittig reaction just discussed, these reactions may be easily performed in the undergraduate laboratory. In one of the procedures that follows, the phosphonate ester 12 is deprotonated with potassium tert-butoxide in the polar, aprotic solvent N,N-dimethylformamide, (CH3)2NCHO (DMF), to provide the resonance-stabilized, nucleophilic phosphonate anion 13 (Eq. 18.7). 

Purpose To synthesize a mixture of stilbenes by a phase-transfer Wittig reaction, to determine the major product, and to perform their geometric isomerization. 

The phase transfer Wittig-Horner-Emmons reaction has been used to prepare examples of o, i3-unsaturated sulfides, sulfoxides, and sulfones [19]. Although these reactions are discussed in Sect. 14.3, we have summarized in Table 13.7 the results of transformations according to equation 13.10. Note that this reaction yields sulfur compounds on condensation with aldehydes only if the starting phosphonate is sulfur substituted. 

Table 14.2. Phase transfer Wittig-Horner-Emmons reactions Phosphonate Carbonyl compound...

The chloromethyl derivatives of 1 and 2 can be converted to the corresponding phosphonium salts by treatment with triphenyl-phosphine.19 A subsequent phase transfer catalysed Wittig reaction of these salts with formaldehyde introduced pendant vinyl groups. 

The Witting reaction has been investigated in aqueous conditions.305 Wittig olefination reactions with stabilized ylides (known as the Wittig-Homer or Homer-Wadsworth-Emmons reaction) are sometimes performed in an organic/water biphase system.306 Very often, a phase-transfer catalyst is used. Recently, the use of water alone as solvent... 

The synthesis of quadrupolar chromophores has also been achieved from 2,6-DTT-dicarboxaldehyde 117. Push-push (i.e., bis-donor) compound 118 was prepared via a double Wittig reaction carried out under solid-liquid phase transfer conditions. Pull-pull (i.e., bis-acceptor) compounds 119 were obtained from a symmetrical bis-aldehydes via a double Horner-Emmons-Wittig condensation (Scheme 9) <2002SM17, 1999CC2055>. 

Catalyzed Wittig reactions. Wittig reactions of cyclopropylidenetriphenyl-phosphorane with carbonyl compounds proceed in low yield under standard conditions. Reactions conducted at 62° in THF with 1 (10 mole %) as the phase-transfer catalyst result in alkylidenecyclopropanes in 60-95% yield. ... 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

Wittig reactions. Dehmlow and Barahona-Naranjo2 have examined use of phase-transfer catalysts in addition to a base in Wittig reactions and conclude that best results are obtained with solid potassium r-butoxide in benzene without an extra catalyst. Solid NaOH is superior to aqueous NaOH. KF and K2C03 are not generally useful. 

E)-ct, -Enones. The arsonium ylides formed from these salts react with aldehydes under phase-transfer conditions to form (E)-a,(3-enones in 71-99% yield (equation I). This Wittig olefination is particularly useful for reactions of unstable aldehydes because of the mild conditions. 

Quaternary phosphonium salts are organophosphorous compounds used as Wittig olefination reagents, phase transfer catalysts, electrolytes, ionic liquids, and as surface active reagents. Their preparation involves the C-P bond formation in tertiary phosphines. We envisaged that addition of phosphines to unsaturated compounds should be preferable as compared to the conventional method using a substitution reaction of organohalogen compounds (Scheme 1). In this chapter, we describe our recent study on this subject. 

Wittig-Horner reaction. The reagent reacts with a wide range of aldehydes, aliphatic, aromatic, and a,P-unsaturated, in the presence of a base to form (E)-a-methyl-a, -unsaturated esters stereoselectively. The reaction can be carried out either with potassium t-butoxide in DMF or with K,COj in QH /HjO and Aliquat 336 as phase-transfer agent. 

These phosphines are typically used as ligands for catalytic reactions such as hydroformylation, hydrogenation, dimerization, and other metal-catalyzed reactions. Other uses for the phosphorus derivatives are Wittig chemistry (vitamin A and E), phase-transfer catalysis, epoxy curing, extractants, and agricultural and pharmaceutical intermediates. 

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