Preparation of phosphonium ylide

Scheme 4 Preparation of phosphonium ylides and general stereochemical outcomes of the Wittig reaction...

Since the pioneering work of H. J. Bestmann and coworkers in the 1960s, acylation of phosphonium ylides is a well known process for the preparation of P-ox-ophosphonium yhdes. The classical way (a) using acylating agents such as acyl chlorides and in situ transylidation is still useful (Scheme 4) [16,17]. 

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). 

Nonstabilized Ylides. These ylides are devoid of electron-v ithdrawing substituents at the anionic center they carry hydrogen or electron-donating alkyl groups on the ylide carbon and phenyl groups on the phosphorus. Nonstabilized ylides are very nucleophilic and react with CO2, O2, and H2O hence they must be handled in an inert atmosphere. Primary as well as secondary alkyl halides may be used for the preparation of phosphonium salts and subsequently ylides. 

Dialkylboron-substituted phosphonium ylides have been prepared by reaction of dialkylchloroboranes with alkylidenephosphoranes (equation 50). The reaction of phosphonium ylides with alkyldichloro-boranes leads to the formation of boron-bridged 1,3-bisylides (equation 51). ... 

Corey and associates (1970) worked out a stereospecific synthesis route for the preparation of juvenile hormone. The essential element of their method is the use of phosphonium-ylides for the stereospecific linkage of three structural units in a single synthesis step. 

Novel vanadium, e.g. (94), and titanium, e.g. (95), complexes of phosphonium ylides have been prepared by the reaction of methylenetriphenylphosphorane with appropriately co-ordinated metal halides. A variety of silver complexes... 

Mixed phosphonium-iodonium ylides 432 represent a useful class of reagents that combine in one molecule the synthetic advantages of a phosphonium ylide and an iodonium salt. The preparation of the tetrafluoroborate derivatives 432 by the reaction of phosphonium ylides with (diacetoxyiodo)benzene in the presence of HBF4... 

Addition reactions of (l-methoxyalkyl)triphenylphosphonium ylides, derived from the corresponding phosphonium salts (82) and n-BuLi, to aldehydes at — 78 °C followed by quenching the reaction mixture with aqueous NH4CI at the same temperature alforded a-hydroxyketones instead of the expected enol ethers.This is the first example of phosphonium ylides acting as an acyl anion equivalent. Flash vacuum pyrolysis (FVP) in a conventional flow system at 10 (84), prepared in a few steps... 

Transfer of substituted aromatic groups from silicon to boron is an efficient synthetic pathway for the preparation of aryl-halogenoboranes and diarylhalogenoboranes with different aryl groups. Substitution reactions of Cbis(trimethylsilyl)amino]--tert-butyl chloroborane have been reported and the reaction of phosphonium ylides with alkyldlchloroboranes investigated. ... 

Phosphonium salts containing a benzyl group may be converted into ylides by the use of only moderately strong bases such as sodium ethoxide. The preparation of benzyli-dene derivatives of aldehydes and ketones is therefore easily done. The procedure below is for the preparation of a substituted butadiene, which in turn is ideally suited for use in the Diels-Alder reaction (see Chapter 8, Section I). 

The quaternary salt is now treated with ethoxide ion in the presence of cinnamalde-hyde so that the ylide reacts in situ as it is produced. A solution of sodium ethoxide is prepared by slowly adding 0.75 g of sodium metal to 100 ml of absolute ethanol in a dry Erlenmeyer flask (hood). In a second flask, the phosphonium chloride is dissolved in 150 ml of absolute ethanol, cinnamaldehyde (2.9 g) is added and the flask is swirled while the ethoxide solution is added a transient orange-red color indicates the formation of the ylide. 

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. 

A related preparation of specific stabilized phosphonium yUdes corresponds to the reaction of triarylphosphines with acetylene dicarboxylic esters in presence of fullerene, which affords a cyclopropanyl-fullerene substituted stabilized phosphonium ylide [9] or the corresponding evolution products [10]. 

Among various modifications of the side chain of stabilized ylides [13,14] can be pointed out the preparation and transformation of the phenyliodonio a-substi-tuted phosphonium yhdes (Scheme 3) [15]. These compounds represent a potentially useful class of reagents, in which the iodonium group can be further substituted by nucleophiles such as PhSLi. 

It must also be pointed out that the alkyne synthesis through the FVP process can also be extended to bis()5-oxo phosphonium ylides) for the preparation of 1.3-diynes compounds [16]. 

Oxidation is an important preparation process in the phosphonium ylide chemistry, which can be performed with a lot of oxidizing agents [29]. 

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. 

Preparation.—It has been shown that the stability of the ylide solution formed from the reaction of dihalogenodifluoromethanes with tertiary phosphines is due to a dynamic equilibrium which lies on the side of phosphine and phosphonium salt (Scheme 1). Ylide solutions generated from chlorodifluoroacetate are unstable since no such equilibrium is possible.1... 

A series of conjugated polyenes capped with chromophores and containing an androstane spacer were synthesized by Wittig or Wittig-type olefinations from epi-androsterone 5150. For example, vinyl carboxaldehyde 52, prepared from 51 in 60% yield as shown in equation 32, was treated with 9-anthrylmethylphosphonium bromide and n-butyllithium to give diene 53. Exocyclic diene 53 was subsequently oxidized to vinyl carboxaldehyde 54. The androsterone vinyl aldehyde intermediate could either be treated with a tetraphenylporphyrinpolyenyl phosphonium ylide, or, as shown below, the phosphonium salt of the androsterone (55) could be reacted with TPP polyeneal 56. The desired all-(E) isomer, 57, was obtained from the ( )/(Z)-isomeric mixture by chromatographic purification. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

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