Takai olefination

Takai olefination in

In the

History

Prior to the introduction of this chromium-based protocol, olefination reactions generally gave Z alkenes or mixtures of isomers.^[1] Similar olefination reactions had been performed using a variety of reagents such as zinc and lead chloride;^[5] however, these olefination reactions often lead to the formation of diols—the McMurry reaction—rather than the methylenation or alkylidenation of aldehydes.^[6] To circumvent this issue, the Takai group examined the synthetic potential of chromium(II) salts.

The reaction primarily employs the use of aldehydes, but ketones may be used.  However, ketones do not react as well as aldehydes; thus, for a compound with both aldehyde and ketone groups, the reaction can target just the aldehyde group and leave the ketone group intact.^[1]

The drawbacks to the reaction include the fact that stoichiometrically, four equivalents of chromium chloride must be used, since there is a reduction of two halogen atoms.^[3] Ways to limit the amount of chromium chloride exist, namely by utilization of zinc equivalent,^[7] but this method remains unpopular.

Takai–Utimoto olefination

In a second publication the scope of the reaction was extended to diorganochromium intermediates bearing alkyl groups instead of halogens:^[8]

References