Wittig type ylides

Trifluoroacetylation of Wittig-type ylides leads to different trifluoromethyl group containing products, depending on the reaction partners and conditions [64, 65, 66, 67] (equations 31-33)... 

Numerous syntheses of 3-deoxy-ulosonic acids are based on the Wittig reaction and its modifications. For this purpose various Wittig-type ylides, shown in Fig.(3), were examined. 

Recent work on the interaction of Wittig-type ylides with esters has been reported in full. One example from this study is the synthesis of esters (99) from... 

Preparation of Trimethylsilyl-substituted Allenes. Kita et al. have explored the addition of stabilized " Mttig reagents to TMSK. Their more recent efforts have led to minor technical improvements with regard to TMSK. Notably, they have demonstrated that use of r-butyldimethylsilyfketene permits reaction with a much wider range of Wittig-type ylides. 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

Among four main directions of the chemical transformations of betaines of the first group, which are presented in Scheme 38, isomerization to metallated ylides (direction A), retro-Wittig type decomposition (direction... 

The use of organotitanium compounds in the synthesis of allenes involves mainly Wittig-type olefmation reactions of carbonyl compounds [86] with titanium ylides. The formation of allenes according to the scheme Q + Q + Q was described by... 

Polyenes are most often synthesized by cross-coupling reactions between unsaturated systems. Typically these reactions require an activated carbon, often in the form of an organometallic reagent. Enolates and phosphonium ylides, Wittig-type reagents, are also commonly employed in carbon-carbon bond formation. Pericyclic rearrangements also result in the generation of new carbon-carbon bonds and will be treated separately. 

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 formation of the phosphoranyl alkyne 19, in the spontaneous room temperature decomposition of the phosphinodiazoketone lm, has been explained in terms of an intramolecular Wittig-type reaction involving the phosphorus-vinyl-ylide form of the phosphinocarbene 2m.39... 

These reactions involve the intermediacy of a bis-ylide which undergoes a Wittig-type reaction with aldehydes or a Michael 1,4-addition to the enone. 

Stabilized telluronium ylides such as dibutyltelluronium carbethoxy, phenacyl/ cyano- and carbamoylmethylide (easily prepared by the reaction of dibutyl teUurides with the appropriate substituted methyl hahdes, followed by treatment with a base), undergo Wittig-type olefmation reactions with a variety of carbonyl compounds, giving the expected olefins in satisfactory yields (method A). This behaviour is in sharp contrast to that of stabilized sulphonium yhdes, which are inert towards carbonyl compounds. 

Dibutyltelluronium benzylide, generated by treatment of the telluronium salt with potassium f-butoxide, behaves similarly to the above-stabihzed ylides, undergoing Wittig-type olefmations with aromatic aldehydes. 

Wittig-type cyclization.2 2-Acyloxy- or 2-benzoylbenzoic acids undergo cycli-zation to 3-chloroflavones or 2-chloroindenones, respectively, when treated with excess (C6H5)3P/CCl4 and N(C2H5)3 in CH2C12 in 40-60% yield. This reaction probably involves formation of a chloro ylide (a) via the acid chloride. 

Wittig-type olefinations can also be performed using selenoaldehydes. Phosphorus ylides initially attack the carbon atom of the selenocarbonyl functionality.405 Aromatic selenoketones undergo reductive dimerization with organo-lithium reagents probably via an electron transfer mechanism.406 Also the addition of organolithium reagents takes... 

On the other hand, tertiary phosphanes react with fiuorinated alkenes to form either vinyl-phosphoranes in the case of perfluorinated alk-l-enes, or ylides when perfluorinated alk-2-enes are used as starting materials. The latter lead to useful building blocks for Wittig-type reactions. 

A variety of reagents are used for the construction of carbon-carbon double bonds from carbonyl compounds. For example, ylides such as Wittig-type reagents or carbanions stabilized by an a-heteroatom react with carbonyls to give the corresponding alkenes. 

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