What is the modified Wittig reaction?
What is the modified Wittig reaction?
The Schlosser Modification of the Wittig Reaction allows the selective formation of E-alkenes through the use of excess lithium salts during the addition step of the ylide and subsequent deprotonation/protonation steps.
What is stabilized ylide?
ii) Stabilized ylides: The ylides with electron withdrawing groups adjacent to the negatively charged carbon are more stable. These are usually stabilized by conjugation. * It is generally observed that the geometry of the final alkene depends on the stability of the ylide.
What is Wittig reaction used for?
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.
How is ylide formed?
Ylides can be synthesized from an alkyl halide and a trialkyl phosphine. Typically triphenyl phosphine is used to synthesize ylides. Because a SN2 reaction is used in the ylide synthesis methyl and primary halides perform the best. Secondary halides can also be used but the yields are generally lower.
What is a stabilized ylide?
What is the Horner–Wadsworth–Emmons reaction?
The Horner–Wadsworth–Emmons (HWE) reaction is a chemical reaction used in organic chemistry of stabilized phosphonate carbanions with aldehydes (or ketones) to produce predominantly E- alkenes. In 1958, Leopold Horner published a modified Wittig reaction using phosphonate-stabilized carbanions.
What is the rate limiting step in the Horner-Wadsworth-Emmons reaction?
The Horner-Wadsworth-Emmons reaction begins with the deprotonation of the phosphonate to give the phosphonate carbanion 1. Nucleophilic addition of the carbanion onto the aldehyde 2 (or ketone) producing 3a or 3b is the rate-limiting step.
What is the stereoselectivity of the Horner-Wadsworth-Emmons reaction of ketones?
The stereoselectivity of the Horner-Wadsworth-Emmons reaction of ketones is poor to modest. Since many substrates are not stable to sodium hydride, several procedures have been developed using milder bases. Masamune and Roush have developed mild conditions using lithium chloride and DBU.
What is the mechanism of the Horner-Wadsworth-Emmons reaction of lithium enolate?
[Pg.298] The mechanism of the Horner-Wadsworth-Emmons reaction of the lithium enolate derived from trimethyl phosphonoacetate with acetaldehyde has been investigated by ab initio calculations. Oxaphosphetane formation is rate determining, both in the gas phase and with one ether moleeule solvating.