Takai olefination in

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, J ...

describes the

organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical ...

of an

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...

with a diorganochromium compound to form an

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...

. It is a name reaction, referencing Kazuhiko Takai, who first reported it in 1986. In the original reaction, the organochromium species is generated from

iodoform Iodoform (also known as triiodomethane and, inaccurately, as carbon triiodide) is the organoiodine compound with the chemical formula C H I3. A pale yellow, crystalline, volatile substance, it has a penetrating and distinctive odor (in older ch ...

bromoform Bromoform (CHBr3) is a brominated organic solvent, colorless liquid at room temperature, with a high refractive index, very high density, and sweet odor is similar to that of chloroform. It is one of the four haloforms, the others being fluoroform, ...

and an excess of chromium(II) chloride and the product is a

vinyl halide In organic chemistry, a vinyl halide is a compound with the formula CH2=CHX (X = halide). The term vinyl is often used to describe any alkenyl group. For this reason, alkenyl halides with the formula RCH=CHX are sometimes called vinyl halides. Fr ...

. One main advantage of this reaction is the '' E''-configuration of the double bond that is formed. According to the original report, existing alternatives such as the

Wittig reaction The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide called a Wittig reagent. Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes. Mos ...

only gave mixtures. : In the

reaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage o ...

proposed by Takai, chromium(II) is oxidized to chromium(III) eliminating two equivalents of a halide. The geminal carbodianion complex thus formed (determined as r₂Cl₄(CHI)(THF)₄ reacts with the aldehyde in a 1,2-addition along one of the carbon to chromium bonds and in the next step both chromium bearing groups engage in an

elimination reaction An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one- or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 r ...

. In

newman projection A Newman projection is a drawing that helps visualize the 3-dimensional structure of a molecule. This projection most commonly sights down a carbon-carbon bond, making it a very useful way to visualize the stereochemistry of alkanes. A Newman pro ...

it can be seen how the

steric bulk Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

s of chromium groups and the steric bulks of the alkyl and halogen groups drive this reaction towards anti elimination. : :

History of the reaction

Prior to the introduction of this chromium-based protocol, olefination reactions generally gave ''Z'' alkenes or mixtures of isomers. Similar olefination reactions had been performed using a variety of reagents such as zinc and lead chloride; however, these olefination reactions often lead to the formation of diols—the McMurry reaction—rather than the methylenation or alkylidenation of aldehydes. 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. Takai olefination

The drawbacks to the reaction include the fact that stoichiometrically, 4 equivalents of chromium chloride must be used, since there is a reduction of two halogen atoms. Ways to limit the amount of chromium chloride exist, namely by utilization of zinc equivalent, 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: :

References

{{Alkenes Olefination reactions Carbon-carbon bond forming reactions Name reactions