Organic Chemistry
Name Reactions
Search
The Wittig ReactionTutorial link: Organo-Sulphur and Organo-Phosphorous ChemistryThis is the formation of double bonds from carbonyl compounds and phosphorous ylids.
Stereoselectivity in the Wittig reaction depends on the ylid. Those with anion-stabilizing substituents adjacent to negative charge produce predominantly E- alkenes (thermodynamic control). Those without stabilising groups give predominantly Z- alkenes (kinetic control). The Schlosser Modification of the Wittig ReactionAs has been shown earlier, reactive ylides will predominantly afford E-alkenes if equilibration of the erythro and threo betaines can be accelerated. However, it has been shown that addition of another equivalent of the organolithium reagent (used commonly to form the betaine) into the reaction mixture leads to the formation of E -alkenes in a selective fashion. This procedure involves a-metallation of the betaine that is initially formed to afford a ß-oxido phosphorous ylides (betaine ylides). These diastereoisomeric betaine ylides can undergo inversion, even at low temperatures (-78 ºC) and equilibration occurs so as to produce the most energetically favourable stereoisomer. Consequently, quenching with a proton donor to the betaine ylide leads to stereospecific protonation to afford the most stable threo ylide.
Biographical Detail: Georg Wittig(b. June 16, 1897, Berlin--d. Aug. 26, 1987, Heidelberg, W.Ger.), German chemist whose studies of organic phosphorus compounds won him a share (with Herbert C. Brown) of the Nobel Prize for Chemistry in 1979. Wittig graduated from the University of Marburg in 1923, received his doctorate there in 1926, and remained as a lecturer in chemistry until 1932. He taught at the Technical University in Braunschweig and at the universities of Braunschweig, Freiburg, and Tübingen before joining the faculty of the University of Heidelberg in 1956, where he became emeritus in 1965 but continued to pursue research. In investigating reactions involving carbanions, negatively charged organic species, Wittig discovered a class of compounds called ylides mediating a particular type of reaction that became known as Wittig reactions and that proved of great value in the synthesis of numerous organic compounds. Copyright 1994-1999 Encyclopædia Britannica |
| |
