In the Wittig reaction

Table 12. Fluorinated Esters and Amides in the Wittig Reaction to Form Enol Ethers [47 and Enamines 48 ...

In the Wittig reaction, a phosphorus ylide, R2C—P(C6H03, also called a phosphoreme and sometimes written in the resonance form R2C=P(C6H5)3, adds to an aldehyde or ketone to yield a dipolar intermediate called a betaine. (An ylide—pronounced ill-id—is a neutral, dipolar compound with adjacent plus and minus charges. A betaine—pronounced bay-ta-een—is a neutral, dipolar compound with nonadjacent charges.)... 

The present preparation illustrates a general and convenient method for the fnms-iodopropenylation of an alkyl halide.4 The iodopropenyl-ated material is not usually stable but is a useful synthetic intermediate. For example, it forms a stable crystalline triphenylphosphonium salt for use in the Wittig reaction, and under Kornblum reaction conditions (DMS0-NaHC03, 130°, 3 minutes) it gives an (E)-a,/9-unsaturated aldehyde.4 In addition to the phosphonium salt described in Note 15, the following have been prepared (4-p-methoxyphenyl-2-butenyl)-triphenylphosphonium iodide [Phosphonium, [4-(4-methoxyphenyl)-2-butenyl]triphenyl-, iodide], m.p. 123-127° (2-octenyl)triphenyl-phosphonium iodide [Phosphonium, 2-octenyltriphenyl-, iodide], m.p. 98° and (2-octadecenyl)triphenylphosphonium iodide [Phosphonium, 2-octadecenyltriphenyl-, iodide], m.p. 50°. 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

Betaines formed in this way can then be converted to the alkene, and this is one reason why betaine intermediates were long accepted in the Wittig reaction. 

At room temperature the chemical and radiochemical yields of 155 were different. The chemical yields were in the 30-40% range, while the radiochemical, not very reproducible yields were in the 6-15% range. Cattel and coworkers122 assigned these differences to tritium isotope effect in the Wittig reaction. No correlation between the specific activity of 155 and the degree of chemical conversion of 157 into 155 has been presented. The temperature dependence of the observed secondary tritium isotope effect has also not been... 

Phenoxy derivatives (31) are formed in substantial amounts, particularly after long reaction times, in the Wittig reaction of pregnenolone using triphenylphos-phonium salts.33... 

Tab. 1 Yields and stereochemistry of stilbene as a function of the EGB and the reaction conditions in the Wittig reaction of Ph3P CH2Ph with PhCHO ...

Wang, C.H. Huang, M.-W. Lee, C.-Y. Chei, H.-L. Huang, J.P. Shiea, J. Detection of a Thermally Unstable Intermediate in the Wittig Reaction Using Low-Temperature Liquid Secondary Ion and Atmospheric Pressure lonization-MS. J. Am. Soc. Mass Spectrom. 1998, 9, 1168-1174. 

For the synthesis of alkenes, the Wittig and Horner-Wadsworth-Emmons reactions have become important tools. Triphenylphosphine that is used in the Wittig reaction can be immobilized either on the polymer or can be used in solution for solid-phase chemistry (Scheme 3.18). The Horner-Wadsworth-Emmons reaction for example is used for the synthesis of aldehyde building blocks [261]. 

Baeyer-Villiger oxidation afforded the desired lactone 28 accompanied with hydro-xycarboxylic acid 29 as a byproduct. After removal of the acid 29, the lactone was reduced with DIBAL-H and employed in the Wittig reaction to form the PG structure 31 (Scheme 4). 

Recent detailed discussions229,283,284,286,289,290 of the bonding in phosphoranes has usually taken for granted the three-centre, four-electron bond model described above and have concentrated on other related issues of importance in these systems, viz. apicophilicity, pseudorotation and the oxaphosphetanes involved in the Wittig reaction. 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

C KIE in the Wittig reaction of benzophenone(carbonyl- 4C) with iso-propy/idenetriphenylphosphorane... 

Carbonyl-uCKIE in the Wittig reaction of non-stabilizedylides with benzaldehyde and benzophenone... 

A QSAR (quantitative structure-activity relationship) approach has been taken to predicting stereoselectivity in the Wittig reaction.239... 

In the Wittig reaction a phosphoroti ketone to yield an alkene. s ylide reacts with a... 

DMA followed by sodium methoxide furnished ylide (479) which was immediately condensed with diethyl JV-(p-formylbenzoyl)-L-glutamate to afford (480). Catalytic hydrogenation gave the 5,6,7,8-tetrahydro derivative oxidation with hydrogen peroxide followed by hydrolysis of the ester groups then gave (478). This is the first report of the preparation and use of a pteridine ylide in the Wittig reaction. 

Addition of the ylide to the carbonyl is postulated to lead first to the zwitterionic intermediate betaine, which would then close to form a four-membered cyclic intermediate, an oxaphosphetane. The existence of the betaine hasn t been fully established, although its intermediacy plays an important role in the Schlosser Modification. Betaines may be stabilized by lithium salts leading to side products therefore, suitable bases in the Wittig Reaction are for example NaH, NaOMe, NEt3). 

In the final product there is scrambling of label between the 17 and 15 positions. The strongly basic conditions required in the Wittig reaction necessitate the protection of base-labile groups. Thus the 3- and 13-hydroxy groups can be converted to trimethylsilyl ethers (38) or tetrahydropyranyl ethers which are then easily removed by mild acid treatment. 

The simplest sulfur ylids are formed from sulfonium salts 69 by deprotonation in base. These ylids react with carbonyl compounds to give epoxides.18 Nucleophilic attack on the carbonyl group 70 is followed by elimination 71 of dimethylsulfide 72 and formation of the epoxide 73. You should compare diagram 71 with diagram 23 in chapter 15. The phosphonium ylid reacted by formation of a P-0 bond and an alkene in the Wittig reaction. The sulfonium compound reacts by formation of a C-O bond 71 as the S-O bond is much weaker than the P-0 bond. The sulfonium salt 69 can be reformed by reaction of 72 with Mel. 

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