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Wittig reagents are usually prepared from a phosphonium salt, which is in turn prepared by the quaternization of triphenylphosphine with an alkyl halide. Wittig reagents are usually derived from a primary alkyl halide. Quaternization of triphenylphosphine with secondary halides is typically inefficient.
The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide called a Wittig reagent.Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes.
This reagent reacts with a ketone or aldehyde in a Wittig reaction to give an enol ether, which can be converted to the aldehyde by acid-induced hydrolysis. The initial report of the reaction demonstrated its use on the steroid tigogenone. It was later used in the Wender Taxol total synthesis and the Stork quinine total synthesis.
Crystallographic characterization of the colourless ylide reveals that the phosphorus atom is approximately tetrahedral. The PCH 2 centre is planar and the P=CH 2 distance is 1.661 Å, which is much shorter than the P-Ph distances (1.823 Å). [5]
PPh 3 combines with alkyl halides to give phosphonium salts. This quaternization reaction is particularly fast for benzylic and allylic halides: PPh 3 + CH 3 I → [CH 3 PPh 3] + I −. These salts, which can often be isolated as crystalline solids, react with strong bases to form ylides, which are reagents in the Wittig reactions.
The methylation of triphenylphosphine is the first step in the preparation of the Wittig reagent. Illustrative phosphorus(V) compounds: the phosphonium ion P(CH 2 OH) 4 +, two resonance structures for the Wittig reagent Ph 3 PCH 2, and pentaphenylphosphorane, a rare pentaorganophophorus compound. The parent phosphorane (σ 5 λ 5) is PH 5 ...
Phosphonium ion Structure of PH + 4, the parent phosphonium cation. In chemistry, the term phosphonium (more obscurely: phosphinium) describes polyatomic cations with the chemical formula PR + 4 (where R is a hydrogen or an alkyl, aryl, or halide group). These cations have tetrahedral structures. The salts are generally colorless or take the ...
A reaction with triphenylphosphine then generated the phosphonium salt 142, and a Wittig reaction could then be performed to attach the silyl-protected cis-ene-yne function, which was then deprotected to yield the target (±)-HTX 283A. 25 Step Synthesis of HTX presented by the Kishi group in 1985.