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Concept
Wittig reaction
Summary
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. Most often, the Wittig reaction is used to introduce a methylene group using methylenetriphenylphosphorane (Ph3P=CH2). Using this reagent, even a sterically hindered ketone such as camphor can be converted to its methylene derivative.
Mechanistic studies have focused on unstabilized ylides, because the intermediates can be followed by NMR spectroscopy. The existence and interconversion of the betaine (3a and 3b) is subject of ongoing research. For lithium-free Wittig reactions, studies support a concerted formation of the oxaphosphetane without intervention of a betaine. In particular, phosphonium ylides 1 react with carbonyl compounds 2 via a [2+2] cycloaddition that is sometimes described as having [π2s+π2a] topology to directly form the oxaphosphetanes 4a and 4b. Under lithium-free conditions, the stereochemistry of the product 5 is due to the kinetically controlled addition of the ylide 1 to the carbonyl 2. When lithium is present, there may be equilibration of the intermediates, possibly via betaine species 3a and 3b. Bruce E. Maryanoff and A. B. Reitz identified the issue about equilibration of Wittig intermediates and termed the process "stereochemical drift". For many years, the stereochemistry of the Wittig reaction, in terms of carbon-carbon bond formation, had been assumed to correspond directly with the Z/E stereochemistry of the alkene products. However, certain reactants do not follow this simple pattern. Lithium salts can also exert a profound effect on the stereochemical outcome.
Mechanisms differ for aliphatic and aromatic aldehydes and for aromatic and aliphatic phosphonium ylides. Evidence suggests that the Wittig reaction of unbranched aldehydes under lithium-salt-free conditions do not equilibrate and are therefore under kinetic reaction control. E.
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Peterson olefination
The Peterson olefination (also called the Peterson reaction) is the chemical reaction of α-silyl carbanions (1 in diagram below) with ketones (or aldehydes) to form a β-hydroxysilane (2) which eliminates to form alkenes (3). Several reviews have been published. One attractive feature of the Peterson olefination is that it can be used to prepare either cis- or trans-alkenes from the same β-hydroxysilane. Treatment of the β-hydroxysilane with acid will yield one alkene, while treatment of the same β-hydroxysilane with base will yield the alkene of opposite stereochemistry.
Tebbe's reagent
Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylidenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. It is a red solid that is pyrophoric in the air, and thus is typically handled with air-free techniques. It was originally synthesized by Fred Tebbe at DuPont Central Research. Tebbe's reagent contains two tetrahedral metal centers linked by a pair of bridging ligands.
Ylide
An ylide or ylid (ˈɪlɪd) is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. The result can be viewed as a structure in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X+–Y−. Ylides are thus 1,2-dipolar compounds, and a subclass of zwitterions.