Preparation of Phosphorus Ylides

Phosphorus ylides are usually obtained from alkyl halides and triphenylphosphine in a two-step sequence. We recall that the nucleophilicity of third-row elements such as sulfur and phosphorus is greater than that of second-row elements because the atoms are more polarizable (Section 10.1). The phosphorus atom of triphenylphosphine is an effective nucleophile. Since it is also a weak base, a competing elimination does not occur, and bimolecular substitution of primary and secondary alkyl hahdes gives good yields. In the first step, the halide ion of the alkyl halide is displaced in an S 2 reaction to yield an alkyltriphenylphosphonium salt. 

Two resonance forms depict the structure of the ylide. The dipolar resonance form is generally used to show the mechanism of the Wittig reaction. This resonance form has a single bond between the phosphorus and carbon atoms. Although positive and n ative charges are located on adjacent atoms, the dipolar form is the major contributor to the yhde structure. The uncharged resonance form has a double bond between the phosphorus and carbon atoms, so the phosphorus atom has 

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Alkyl halide reacts with triphenylphospine to give a phosphonium salt, which is an important intermediate for the preparation of phosphorus ylide (see Section 5.3.2). 

Preparation of phosphorus ylide Phosphorus ylides are produced from the reaction of triphenylphosphine and alkyl hahdes. Phosphorus ylide is a... 

Preparation of phosphorus ylides Elimination of nitrous acid Steric control of elimination reactions. Mdol condensation Rearrangements Reactions with aryl isocyanates Preparation of carboxylic acid chlorides Synthesis of macromolecules... 

Compared with the typical Wittig reaction, the Wittig-type reaction under catalytic conditions provides a way to couple aldehydes with tosylhydrazones for the preparation of substituted stilbenes with good tran -selectivity. The method also avoids the separate preparation of phosphorus ylide. Thus, this method provides an alternative pathway of C=C bond formation and extends the scope of substrates from alkyl halides to various aldehydes. However, more than 1 equiv. of phosphoms reagent is stiU needed. This method has not been well applied to ketmie olelination. 

The second chapter (M. Taillefer and H. J. Cristau) is dedicated to new trends in ylide chemistry. The preparation and the reactivity of phosphorus ylides, C-substituted by heteroatoms is presented, ylides being substituted by groups 1 and 2 elements, by transition metals or by elements of groups 13 to 16. A rich and versatile chemistry is thus reported. 

The first silicon-organophosphorus betaine with a thiolate center (15a) was synthesized by the reaction of stable silanethione (14) with trimethyl-methylenephosphorane (Scheme 8) and characterized by multinuclear NMR spectroscopy.14 Compound 15a is formed under kinetic control and is transformed, under the thermodynamically controlled conditions, into the silaacenaphthene salt (16). The processes presented in this scheme reflect the competition of the basicity and nucleophilicity of phosphorus ylides. Betaine 15b prepared from less nucleophilic and less basic ylide with phenyl substituents at the phosphorus atom is much less resistant toward retro-decomposition compared to the alkyl analog. Its equilibrium concentration does not exceed 6%. 

With no major research on the reaction of phosphorus ylides and aliphatic or alicyclic thiones prior to their investigation, Krapcho et where able to elaborate a useful method for the synthesis of the thietane ring system in addition to the preparation of novel types of thiocarbonyl stabilized ylides (Eq. 1). 

In addition to the phosphorus ylide building blocks which are employed in industrial processes for the preparation of carotenoid terpenes, a large number of phosphorus ylides or phosphonium salts have been developed for laboratory syntheses of natural carotenoids. The monograph Carotenoids by O. Isler7) contains an extensive summary of the examples described in the literature and in the patent literature. 

The Wittig reaction of phosphorus ylides derived from phosphonium salts with carbonyl compounds to form alkenes is widely used in synthetic chemistry. The alkylation of phosphorus ylides offers a route to a variety of phosphonium salts which can be further manipulated. Bestman and coworkers - have been responsible for a large number of reports based upon this and other aspects of phosphorus chemistry. Their work includes many examples of the intramolecular alkylation of ylides derived from phosphonium salts to form cyclic compounds. The salts can be prepared from dihalides either with triphenylphos-... 

Heterocyclic Synthesis. - The reactions of phosphorus ylides with phenan-threne-9,10-quinone (113) have been used to prepare phenanthrene [9,10-x]-fused compounds with four, five, and six membered heterocyclic rings. (E)-4-carbethoxymethylene-l,2,3,4-tetrahydro-2-quinolones 114 have been obtained from the stereoselective reaction of 3-hydroxy-1,2,3,4-tetrahydroquinoline-2,4-diones and ethyl(triphenylphosphoranylidene)acetate. A -trifluoroacetylanilines 115 react with Ph3P=C02Et producing enamine derivatives 116 as a mixture of (E)- and (Z)-isomers. Enamines 116 are useful precursors for the synthesis of indoles and quinolones. 

Mononuclear ylide complexes of gold(I) and gold(III) have been obtained in a variety of ways203,204, as detailed below. The largest number of complexes of both gold(I) and gold(III) has been prepared with phosphorus ylides, but species containing arsenic or sulphoxonium ylides have also been reported. Some prototypes are summarized in Table 7. 

Higher molecular weight oligomers and polymers are obtained when ethylene is reacted with P—0 chelate catalysts synthesized by the oxidative addition of phosphorus ylides to zerovalent nickel compounds in the presence of triphenylphosphine or other ligands. " As shown in eq 9, the catalysts are easily prepared from keto—ylides by oxidative addition of a P—Ph bond to a nickel(O) precursor like Ni(COD)2. " In this manner, a wide... 

Oxidation of stabilized phosphorus ylides. 3-Carotene and other symmetrical carotenoids can be prepared conveniently by oxidation of phosphorus ylides with hydrogen peroxide (307o) and sodium carbonate in aqueous solution. ... 

Generation of Phosphorus Ylides and Phosphonate Anions. NaHMDS isthemostutiUzedbaseforthedeprotonation of a variety of phosphonium salts to generate the corresponding ylides, which then undergo Wittig reaction with a carbonyl compound. More recently, it was shown that such a base is compatible with a variety of other systems. For instance, it was shown that allenes and dienes could be prepared, respectively, from aromatic and alicyclic aldehydes when reacted with (Me2N)3P=CH2 in the presence of 4 equiv of NaHMDS and titanium trichloride iso-propoxide (eqs 30 and 31). ... 

What combination of phosphorus ylide and a carbonyl compound could be used to prepare each of the following alkenes ... 

The preparation and characterization of stable ylides have been recently described in various examples. Two remarkable syntheses of phosphorus ylides were reported by the condensation of electron-poor acetylenes including dimethylacetylene dicarbojylate with triphenylphosphine (Fig. 7).262.263 xhe reactions of nucleophilic active... 

Kawashima and co-workers attempted to prepare the phosphorus ylide-sta-bilized AYC ligand 129 which could, however, not be isolated because of an intramolecular rearrangement giving 130. The intermediate formation of 129 was confirmed by its reaction with sulfur to give 131 and by the formation of the rhodium complexes 132 and 133 (Scheme 1.17). Infrared spectroscopy of the carbonyl complex 133 confirmed the superb donor properties of AYC 129. A palladium complex bearing the phosphorus ylide-stabilized NHC was also prepared. Fiirstner et al. synthesized the analogous... 

Phosphorus ylides are prepared from alkyl halides by a two step sequence The first step is a nucleophilic substitution of the 8 2 type by triphenylphosphme on an alkyl halide to give an alkyltriphenylphosphonium salt... 

The Wittig reaction (Sections 17.12-17.13) Reaction of a phosphorus ylide with aldehydes and ketones leads to the formation of an alkene. A versatile method for the regiospecific preparation of alkenes. 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

Phosphorus ylides like 1 can be prepared by various routes. The most common route is the reaction of triphenylphosphine 5 with an alkyl halide 6 to give a triphenylphosphonium salt 7, and treatment of that salt with a base to give the corresponding ylide 1 ... 

The reactivity of the phosphorus ylide 1 strongly depends on substituents R R. For preparative use R often is a phenyl group. When R or R is an electron-withdrawing group, the negative charge can be delocalized over several centers, and the reactivity at the ylide carbon is reduced. The reactivity of the carbonyl compound towards addition of the ylide increases with the electrophilic character of the C=0 group. R R are often both alkyl, or alkyl and aryl. 

The first important lead toward the application of ylides as an initiator came from the observations of Zweifel and Voelker [47] in 1972. In their experiment, these authors polymerized lactones or unsaturated compounds initiated by phosphorus ylides prepared directly from tertiary phosphines or similar compounds. 

An aldehyde or ketone reacts with a phosphorus ylide to yield an alkene in which the oxygen atom of the carbonyl reactant is replaced by the =0 2 of the ylide. Preparation of the phosphorus ylide itself usually involves reaction of a primary alkyl halide with triphenylphosphine, so the ylide is typically primary, RCH = P Ph)3-This means that the disubstituted alkene carbon in the product comes from the carbonyl reactant, while the monosubstituted alkene carbon comes from the ylicle. 

What carbonyl compound and what phosphorus ylide might you use to prepare i each of the following compounds ... 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

Highly stabilized phosphorus ylides are prepared from acetylenic esters, a carbon-based nucleophile, and triphenylphosphine in aqueous media.40 In acetone-water (2 1) solvent, the reaction proceeds via the conjugate addition of triphenylphosphine to dialkyl acetylenedicarboxy-lates the resulting vinyl triphenylphosphonium salts undergo Michael addition reaction with a carbon-nucleophile to give the corresponding highly stabilized phosphorus ylides. 

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