Ylides preparation

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. 

Carbonyls. The stereochemistry of the Wittig olefin synthesis has been reviewed. /i-a/u-Stereoselective olefin synthesis via /3-oxido-ylides is possible only in the presence of soluble lithium salts. Protonation of jS-oxido-ylides prepared from salt-free ylides leads to mixtures of erythro-and r/jr o-betaines and hence to mixtures of cis- and rm/i5-olefins. 

Complexes containing the ylide ligand can be also obtained by reaction of the yellow solution of ylides (prepared by addition of butyllithium to tetrahydrofurane or diethylether solutions of the phosphonium salt) with [AulCfiFs) (tht)]. The weak ligand tht is replaced and the ylide complex is obtained (Equation 3.5) [70]. 

CuCl-catalyzed decomposition of iodonium ylides prepared from /3-keto esters and diacetoxyiodobenzene, has been developed (equation 151)331. 1-Methylbenzvalene is obtained in a good yield by treating a mixture of lithium cyclopentadienide and 1,1-dichloroethane with butyllithium332. The tandem cyclization substitution in l-selenyl-5-hexenyllithiums derived from corresponding selenacetals via selenium/lithium exchange produces bicyclo[3.1.0]hexane derivatives333. 

Epoxide Formation from Ylides Prepared by Means of Bases... 

This variation of the Wittig reaction uses ylides prepared from phosphonates.12 The Horner-Wadsworth-Emmons method has several advantages over the use of phosphoranes. These ylides are more reactive than the corresponding phosphoranes, especially when substituted with an electron withdrawing group. In addition the phosphorus product is a phosphate ester and soluble in water - unlike the Ph3PO product of the Wittig reaction - which makes it easy to separate from the olefin product. Phosphonates are also cheaper than phosphonium salts and can easily be prepared by the Arbuzov reaction from phosphanes and halides. 

A Wittig reaction of aldehyde 1292 with the ylide prepared from triphenylphosphonium bromide 1293 and lithium diisopropylamide gave an ( )/(Z) mixture (1 1, when R = Me, and 5 3, when R = PhS02) of 3-(2-arylethenyl)-l//-indoles 1294 in 70-86% yield (Equation 286) <1999T4341>. 

Other reactions of tetrasubstituted azomethyne ylides prepared from tetracyanoethylene oxide are described, I, (5). 

Sometimes halide displacement is dependent on specific factors of ylide preparation,... 

Cycloimmonium ylides, prepared by interaction of a substituted pyridine with dichlorocarbene generated under phase-transfer catalysis condi-... 

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