“1,2-Wittig rearrangement”的意思、由来-开放百科全书

A 1,2-Wittig rearrangement is a categorization of chemical reactions in organic chemistry, and consists of a 1,2-rearrangement of an ether with an alkyllithium compound.^[1]^[2] The reaction is named for Nobel Prize winning chemist Georg Wittig.

The intermediate product is an alkoxy lithium salt and the final product an alcohol. When R" is a good leaving group and electron withdrawing functional group such as a cyanide (CN) group,^[3] this group is eliminated and the corresponding ketone is formed.

Reaction mechanism

The reaction mechanism centers on the formation of a free radical pair with lithium migrating from the carbon atom to the oxygen atom. The R radical then recombines with the ketyl.^[4]

The alkyl group migrates in the order of thermodynamical stability methyl < primary alkyl < secondary alkyl < tertiary alkyl in this is 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 mechanism takes place.^[4] 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. This result rules out a radical-ketyl intermediate 3a in favor of the Meisenheimer complex 3b. Additional evidence for this mechanism is provided by the finding that with a para tert-butyl substituent the reaction is retarded.

See also

References

1. ^Georg Wittig, L. Löhmann, Ann. 550, 260 (1942)
2. ^G. Wittig, Experientia 14, 389 (1958).
3. ^Preparation of aryl benzyl ketones by [1,2]-Wittig rearrangement Alan R. Katritzky, Yuming Zhang, Sandeep K. Singh Arkivoc pp. 146–50 2002 (vii) link {{Webarchive|url=https://web.archive.org/web/20060928190900/http://www.arkat-usa.org/ARKIVOC/JOURNAL_CONTENT/manuscripts/2002/SK-533EP%20as%20published%20mainmanuscript.pdf |date=28 September 2006 }}
4. ^¹Wittig Rearrangement of Lithiated Allyl Aryl Ethers: A Mechanistic Study Sven Strunk, Manfred Schlosser European Journal of Organic Chemistry Volume 2006, Issue 19 , pp. 4393–97 {{DOI|10.1002/ejoc.200600304}}