Wittig reaction reagent

A good approach to 1,2-diols is hydroxylation of an olefin with reagents such as OSO4 or KMn04. The olefin can be made by the Wittig reaction so the discoimections arc ... 

First the normal Wittig reaction, and then a second mol of Wittig reagent must be behaving as a carbene equivalent, just as we hoped the sulphur yhd would. 

The Wittig reaction uses phosphorus ylides (called Wittig reagents) to convert aldehydes and ketones to alkenes... 

A useful apphcation of phosphines for replacing a carbonyl function with a carbon—carbon double bond is the Wittig reaction (91). A tertiary phosphine, usually triphenylphosphine, treated with the appropriate alkyl halide which must include at least one a-hydrogen, yields the quaternary salt [1779A9-3] which is then dehydrohalogenated to form the Wittig reagent, methylenetriphenylphosphorane [19943-09-5] an yhde. 

The double bond transposition could also be achieved by the conversion of an intermediate for PGA2 synthesis into a 1,3-diene iron tricarbonyl complex from which PGC2 was synthesized in four steps. The Fe(CO)3 diene complex which survived the Wittig reaction was cleanly removed by Collins reagent in the subsequent step (Ref. 10). 

Much better known are the fluonnatedphosphoranes, which have been widely used m the Wittig reaction for the preparation of fluoroolefms Difluoromethylena tion reactions have been effected by using a variety of conditions Treatment of dibromodifluoromethane with two equivalents of tns(dimethylammo)phosphine m carefully dried tnglyme yields a solution of bromodifluoromethylphosphonium broomide, which very effectively converts ketones to difluoromethylene derivatives A more sensitive reagent is prepared by the addihon of two equivalents of the phosphine to the reaction mixture of fluorohalomethane and a carbonyl compound [39, 40] (equation 40) (Table 14)... 

The Wittig reaction, for which George Wittig received the 1979 Nobel Prize in Chemistry, is an important synthetic procedure for converting aldehydes and ketones into alkenes. The active reagent is a phosphorous ylide which undergoes nucleophilic addition to the carbonyl carbon, e.g., for addition of triphenylphosphinemethylidene to acetone. 

Important and widely used variants of the Wittig reaction are based on carbanionic organophosphorus reagents, and are known as the Wadsworth-Emmons reaction, Wittig-Horner reaction or Horner-Wadsworth-Emmons reaction. As first reported by Horner, carbanionic phosphine oxides can be used today carbanions from alkyl phosphonates 13 are most often used. The latter are easily prepared by application of the Arbuzov reaction. The reactive carbanionic species—e.g. 14 —is generated by treatment of the appropriate phosphonate with base, e.g. with sodium hydride ... 

Aldehydes can be prepared by the Wittig reaction using (methoxymethylene)-triphenylphosphorane as the Wittig reagent and then hydrolyzing the product with acid. For example,... 

The Collins reagent in CH2CI2 oxidizes silylated primary alcohols in preference to the more hindered silylated secondary alcohols, as described for oxidation of the prostaglandin intermediate 2963 to the rather labile aldehyde 2964, which is immediately subjected to a Horner-Wittig-reaction to introduce the lower side chain [206] (Scheme 12.61). 

The use of water-soluble reagents and catalysts allows reactions to be performed in aqueous buffered solutions. PEG-supported triarylphos-phine has been used in a Wittig reaction under mildly basic aqueous conditions (Eq. 8.115). The PEG-supported phosphine oxide byproduct can be easily recovered and reduced by alane to regenerate the starting reagent for reuse.312 The aqueous Wittig reaction has also been used in... 

The acidity of such R2B—CH2—R systems has been utilized as the basis for developing the so-called boron-Wittig reaction for organic synthesis.52-54 One of the distinct advantages of such reagents, the direct conversion of an aliphatic aldehyde to a ketone, is illustrated in Eq. (19).55... 

The olefin metathesis of 3-hydroxy-4-vinyl-l,2,5-thiadiazole 112 and a McMurry coupling reaction (Ti3+ under reductive conditions) of the aldehyde 114 were both unsuccessful <2004TL5441>. An alternative approach via a Wittig reaction was successful. With the use of the mild heterogenous oxidant 4-acetylamino-2,2,6,6-tetramethyl-piperidine-l-oxoammonium perfluoroborate (Bobbitt s reagent), the alcohol 113 was converted into the aldehyde 114. The phosphonium salt 115 also obtained from the alcohol 113 was treated with the aldehyde 114 to give the symmetrical alkene 116 (Scheme 16) <2004TL5441>. 

A useful application of the silver-mediated additions is 1,3 -diene synthesis by three-carbon elongation of aldehydes [48,51,53]. The bimetallic reagent 3-trimethylsilyl-l-propenylzirco-nocene chloride (A Scheme 8.23) reacts with aldehydes under the influence of a catalytic amount of Ag+ to give the intermediate zirconocene-alkoxide B, which then undergoes a Peterson-type 1,4-elimination of TMS alkoxide to stereoselectively afford ( )-dienes (fc/Z > 96 4) (Scheme 8.23). A Wittig reaction yields the same products without stereoselectivity (ca. 1 1 mixtures of E- and Z-isomers). 

The Wittig reaction consists in the replacement of carbonyl oxygen of aldehydes and ketones by a methylene group with the aid of phosphine-methylenes resulting in the formation of cis or trans olefines. The reaction proceeds through the nucleophilic addition of Wittig reagent (phosphine methylene) across the > C = O bond and formation of an intermediate cyclic. 

The Peterson reaction utilizes a metal derivative of chloromethyltrimethylsilane and the yields are better than the Wittig reaction. The intermediate P-hydroxysilanes can undergo either cis or trans elimination depending on reagent choice. 

Reagents i, Cl COCl ii, Ph2PLi iii, H202 iv, Wittig reaction... 

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. 

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