Takai olefination
The Takai olefination is an organic reaction of aldehydes with diorganochrome compounds for the synthesis of alkenes (carbonyl olefination). The reaction is named after its discoverer, the Japanese chemist Kazuhiko Takai . In 1986 he published for the first time the reaction of benzaldehydes with organochromium compounds, which are made from iodoform or bromoform with excess chromium (II) chloride. Mainly ( E ) -olefins are formed, which makes this reaction an alternative to the Wittig reaction , which is usually significantly less selective.
mechanism
For the reaction mechanism, Takai suggests oxidative insertion of chromium (II) into two halogen-carbon bonds. The geminal carbanion complex attacks the aldehyde nucleophilically and after elimination the olefin is obtained. In the Newman projection it can be seen that the sterically demanding chromium atoms and the steric demands of the alkyl and halogen atoms promote the reaction in the direction of anti-elimination.
Further development
In another publication, Takai presents an olefination in which no vinyl halogens, but the alkyl olefins, as in the Wittig reaction, are described as a product. Here the chromium (II) is reacted with a dihaloalkyl compound.
Chromium-free methylenation became known as Takai methylenation . This reaction is an organic zinc reaction which is catalyzed with the help of lead (II) chloride and is related to the Lombardo reagent , see Takai-Lombardo reaction .
Individual evidence
- ↑ K. Takai, K. Nitta, K. Utimoto: In 'Simple and selective method for aldehydes (RCHO) → (E) -haloalkenes (RCH: CHX) conversion by means of a haloform-chromous chloride system' J. Am. Chem. Soc. 1986 , 108 , 7408-7410.
- ↑ T. Okazoe, Kazuhiko Takai, K. Utimoto: In ( E ) -Selective olefination of aldehydes by means of gem-dichromium reagents derived by reduction of gem-diiodoalkanes with chromium (II) chloride J. Am. Chem. Soc. 1987 , 109 , 951-953.
- ↑ `` A Novel Catalytic Effect of Lead on the Reduction of a Zinc Carbenoid with Zinc Metal Leading to a Geminal Dizinc Compound. Acceleration of the Wittig-Type Olefination with the RCHX2-TiCld-Zn Systems by Addition of Lead " J. Org. Chem. 1994 , 59 , 2668-2670.