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. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

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

, and consists of a 1,2-rearrangement of an

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...

with an

alkyllithium In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...

compound. The reaction is named for

Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...

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 in Che ...

. :

\mathsf

The intermediate product is an

alkoxy In chemistry, the alkoxy group is an alkyl group which is singularly bonded to oxygen; thus . The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the organic compound ethyl phenyl ether (, also ...

lithium salt and the final product an alcohol. When R" is a good

leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited ...

and

electron withdrawing group In chemistry, an electron-withdrawing group (EWG) is a substituent that has some of the following kinetic and thermodynamic implications: *with regards to electron transfer, electron-withdrawing groups enhance the oxidizing power tendency of ...

such as a

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...

(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 is formed. :

\mathsf

Reaction mechanism

The reaction mechanism centers on the formation of a

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...

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 Chemi ...

Volume 2006, Issue 19 , pp. 4393–97 The alkyl group migrates in the order of thermodynamical stability methyl < 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. 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 (CH3)3SiCl, often abbreviated Me3SiCl or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. I ...

. 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 carrying electron withdrawing groups and a nucleophile. These complexes are found as reactive intermediates in nucleophilic aroma ...

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 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 intramolecularly. The reactio ...

References

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

Reaction mechanism

See also

References