A 1,2-Wittig rearrangement is a categorization of

chemical reactions A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products ...

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, and consists of a

1,2-rearrangement A 1,2-rearrangement or 1,2-migration or 1,2-shift or Frank C. Whitmore, Whitmore 1,2-shift is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atom ...

of an

ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R� ...

with an alkyllithium compound. The reaction is named for

Nobel Prize The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...

winning chemist

Georg Wittig Georg Wittig (; 16 June 1897 – 26 August 1987) was a German chemist who reported a method for synthesis of alkenes from aldehydes and ketones using compounds called phosphonium ylides in the Wittig reaction. He shared the Nobel Prize i ...

. The intermediate is an

alkoxy In chemistry, the alkoxy group is an alkyl group which is Single bond, singularly bonded to oxygen; thus . Denoted usually with apostrophe('). The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the ...

lithium salt, and the final product an

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

. When R" is a good

leaving group In organic chemistry, a leaving group typically means a Chemical species, molecular fragment that departs with an electron, electron pair during a reaction step with heterolysis (chemistry), heterolytic bond cleavage. In this usage, a ''leaving gr ...

and

electron withdrawing group The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up and down quarks. E ...

such as a

cyanide In chemistry, cyanide () is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. Ionic cyanides contain the cyanide anion . This a ...

(CN) group,''Preparation of aryl benzyl ketones by ,2Wittig rearrangement'' Alan R. Katritzky, Yuming Zhang, Sandeep K. Singh

Arkivoc ''Arkivoc'' (''Archive for Organic Chemistry'') is a peer-reviewed open access scientific journal covering all aspects of organic chemistry. It is published by the non-profit organization Arkat USA, which was established in 2000 through a personal ...

pp. 146–50 2002 (vii
link
this group is eliminated and the corresponding

ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

is formed.

Reaction mechanism

The

reaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage ...

centers on the formation of a

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...

pair with lithium migrating from the carbon atom to the oxygen atom. The R radical then recombines with the

ketyl A ketyl group in organic chemistry is an anion radical that contains a group R2C−O•. It is the product of the 1-electron reduction of a ketone. Another mesomeric structure has the radical position on carbon and the negative charge on oxyge ...

.''Wittig Rearrangement of Lithiated Allyl Aryl Ethers: A Mechanistic Study'' Sven Strunk, Manfred Schlosser

European Journal of Organic Chemistry The ''European Journal of Organic Chemistry'' is a weekly peer-reviewed scientific journal covering organic chemistry. It is published by Wiley-VCH on behalf of Chemistry Europe. The journal, along with the ''European Journal of Inorganic Chemistr ...

Volume 2006, Issue 19, pp. 4393–97 The alkyl group migrates in the order of thermodynamical stability

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...

< primary alkyl < secondary alkyl < tertiary alkyl, this is in line with the radical mechanism. The radical-ketyl pair is short lived and due to a solvent cage effect some isomerizations take place with

retention of configuration Walden inversion is the inversion of a stereogenic center in a chiral molecule in a chemical reaction. Since a molecule can form two enantiomers around a stereogenic center, the Walden inversion converts the configuration of the molecule from ...

. With certain allyl aryl ethers a competing reaction takes place. The reaction of ''allyl phenyl ether'' 1 with ''sec-butyllithium'' at −78 °C gives the lithiated intermediate 2 which on heating to −25 °C only shows the rearranged product 5 but not 4 after trapping the lithium alkoxide with

trimethylsilyl chloride Trimethylsilyl chloride, also known as chlorotrimethylsilane is an organosilicon compound ( silyl halide), with the formula , often abbreviated or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. It is widely u ...

. This result rules out a radical-ketyl intermediate 3a in favor of the

Meisenheimer complex A Meisenheimer complex or Jackson–Meisenheimer complex in organic chemistry is a 1:1 reaction adduct between an arene compound, arene carrying electron withdrawing groups and a nucleophile. These complexes are found as reactive intermediates in nu ...

3b. Additional evidence for this mechanism is provided by the finding that with a para tert-butyl substituent the reaction is retarded. The reaction is a formal

dyotropic reaction A dyotropic reaction (from the Ancient Greek, Greek ''dyo'', meaning two) in organic chemistry is a type of organic reaction and more specifically a pericyclic valence isomerization in which two sigma bonds simultaneously migrate Intramolecular re ...

References

{{DEFAULTSORT:1, 2-Wittig Rearrangement Rearrangement reactions Name reactions Reactions of ethers

Reaction mechanism

See also

References