Alkyltriphenylphosphonium

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 alkyltriphenylphosphonium salt products are ionic and crystallize in high yield from the nonpolar- solvents in which they are prepared. After isolation, the alkyltriphenylphosphonium halide is converted to the desired ylide by deprotonation with a strong base ... 

Triphenylphosphine reacts with alkyl halides to form alkyltriphenylphosphonium salts. Upon reaction with strong bases, the salts release a proton to form an ylide (alkylidenetriphenylphosphorane), which is capable of reacting with aldehydes or ketones providing an unambiguous route to olefins. Since there are virtually no... 

Alkyltriphenylphosphonium salts normally require extremely strong bases for the generation of the ylide. In this experiment, the alkyl group is methyl and the base required to generate the ylide is -butyl lithium. 

Phosphonium ylides are usually prepared by deprotonation of phosphonium salts. The phosphonium salts that are used most often are alkyltriphenylphosphonium halides, which can be prepared by the reaction of triphenylphosphine and an alkyl halide. The alkyl halide must be reactive toward Sw2 displacement. 

Alkyltriphenylphosphonium halides are only weakly acidic, and a strong base must be used for deprotonation. Possibilities include organolithium reagents, the anion of dimethyl sulfoxide, and amide ion or substituted amide anions, such as LDA or NaHMDS. The ylides are not normally isolated, so the reaction is carried out either with the carbonyl compound present or with it added immediately after ylide formation. Ylides with nonpolar substituents, e.g., R = H, alkyl, aryl, are quite reactive toward both ketones and aldehydes. Ylides having an a-EWG substituent, such as alkoxycarbonyl or acyl, are less reactive and are called stabilized ylides. 

Gough, S.T.D. and Trippett, S., The reduction of alkyltriphenylphosphonium halides with lithium aluminum hydride, /. Chem. Soc., 83, 4263, 1961. 

The Wittig reaction of lithium a-(dimethylamino)-alkoxydes and a C15 alkyltriphenylphosphonium salt was used by Wang et al. to elaborate the ethylenic linkage of retinol [79]. This in situ method offers the unique advantage in its application to labile aldehydes, which otherwise would become isomerised or self-condensed, Fig. (44). 

Alkyltriphenylphosphonium salt Base Triphenylphosphonium ylide Conjugate acid... 

Azidoalkyl a-diazoketones have been reacted with A-phenylmaleimide, yielding, for example, the 4,6-dioxo-3a,4,6,6a-tetrahydro-l//,5//-pyrrolo[3,4-6 ]-1,2,3-triazole (163) <84KGS514>. Similarly, 2-azido-A-arylacetamides have been reacted with /V-phenylmaleimide to yield, for example, 4,6-dioxo-3a,4,6,6a-tetrahydro-l//,5//-pyrrolo[3,4-6 ]-l,2,3-triazole (164) <92MI 703-oi>. Various co-azido-alkyltriphenylphosphonium bromides have also been reacted with TV-phenylmaleimide giving, for example, 4,6-dioxo-3a,4,6,6a-tetrahydro-l//,5//-pyrrolo[3,4-6 ]-l,2,3-triazole (165) <92BSF308>. 

The phosphorus-stabilized carbanion is an ylide (pronounced ilL-id )—a molecule that bears no overall charge but has a negatively charged carbon atom bonded to a positively charged heteroatom. Phosphorus ylides are prepared from tri-phenylphosphine and alkyl halides in a two-step process. The first step is nucleophilic attack by triphenylphosphine on an unhindered (usually primary) alkyl halide. The product is an alkyltriphenylphosphonium salt. The phosphonium salt is treated with a strong base (usually butyllithium) to abstract a proton from the carbon atom bonded to phosphorus. 

Reacts with alkyltriphenylphosphonium salts to yield alkylidenephosphoranes (Wittig reagents (Section 19.11). 

Because the active reagent, an ylide, is unstable, it is generated in the presence of the carbonyl compound by dehydrohalogenation of an alkyltriphenylphosphonium bromide with phenyllithium in dry ether in a nitrogen atmosphere. The existence of the four-membered ring intermediate was proved by nmr in 1984. ... 

At a mercury cathode the propensity for cleavage from a phosphonium salt in methanol increases in the sequence methyl < ethyl < -butyl phenyl < tert-huiy < allyl < benzyl however, at a lead cathode the tendency for loss of an alkyl group from an alkyltriphenylphosphonium group is much higher than at a mercury cathode ethyltriphe-nylphosphonium chloride is thus reduced in methanol at a mercury cathode to a 5 3 mixture of ethyldiphenylphosphine and triphenylphosphine, whereas 91% triphenylphos-phine was isolated at a lead cathode [196]. 

Reduction of alkyltriphenylphosphonium salts in aprotic media leads to the formation of ylids [202,203]. The formation of ylids as a result of the electrogeneration of bases (EGB) [204] is discussed in Chapter 30. 

A new Wittig protocol involves the simultaneous in situ formation of aldehydes, generated from the spontaneous decomposition of a-amino alcohols, and ylides, formed by deprotonation of alkyltriphenylphosphonium salts, which react together affording olefins. ... 

A new route to so-called instant ylides has been reported. These are obtained from powder mixtures of alkyltriphenylphosphonium salts and potassium hydride in a 1 1 mole ratio. Addition of tert-butylmethyl ether results in the generation of the triphenylphosphino-alkanide. These simple phosphonium salt/ KH mixtures are said to offer little advantage over sodium amide based instant ylides. However, those containing hetero-substituted phosphonium salts have a much longer shelf life than their sodium amide counterparts. 

Phosphorus ylides are prepared from a nucleophilic substitution reaction between alkyl halides and triphenylphosphine (PPh3). The resulting alkyltriphenylphosphonium salt is then deprotonated by reaction with a strong base (e.g. BuLi) to form the ylide. 

A series of alkyltriphenylphosphonium salts has been prepared in this way in good yield with dimethylformamide as solvent and hydrogen bromide as acid and, after deprotonation, can be used in the same solution for Wittig olefination.319... 

Preparation and Structure. - The equilibrium acidities and homolytic bond dissociation energies of a number of alkyltriphenylphosphonium salts (1) and the oxidation potentials of the corresponding ylides have been determined. The triphenylphosphonium substituent lowers the pKa value of the a-hydrogens by approximately 25 units (142 kJmol" ) while the effect of the corresponding triphenylarsonium substituent is some 4 pKa units less. 

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