Takai olefination

The Takai olefination is an organic reaction of aldehydes with Diorganochromverbindungen for the synthesis of alkenes (carbonyl olefination ). The reaction is named after its discoverer, the Japanese chemist Kazuhiko Takai. In 1986 he published the first time the reaction of benzaldehydes with organochromium compounds, which are made ​​of iodoform or bromoform with surpluses of chromium ( II ) chloride. It arises mainly E -olefins, which makes this reaction an alternative to the Wittig reaction, which is significantly unselective usually.

Mechanism

For the reaction mechanism suggests Takai oxidative insertion of the chromium (II) in two halogen -carbon bonds. The geminal Carbanionenkomplex attacks the aldehyde to nucleophilic and after the elimination of the olefin is obtained. In the Newman projection can be seen that the bulky chromium atoms and the steric demand of the alkyl and halogen atoms promotes the reaction in the direction of an anti-elimination.

Further development

In a further publication Takai provides olefination before in no vinyl halogens, but the alkyl olefins, such as in the Wittig reaction described as a product. We reacted, the chromium ( II) with a dihaloalkyl compound.

A chrome - free methylenation was known under the name Takai methylenation. This reaction is a zinc- organic reaction catalysed with the aid of lead (II ) chloride and is related to the Lombardo reagent.