View source for Takai olefination

{{Use dmy dates|date=February 2024}}
{{Reactionbox
| Name = Takai olefination
| Type = Carbon-carbon bond forming reaction
| NamedAfter = Kazuhiko Takai
}}
'''Takai olefination''' in [[organic chemistry]] describes the [[organic reaction]] of an [[aldehyde]] with a [[diorganochromium|diorganochromium compound]] to form an [[alkene]]. It is a [[name reaction]], named for [[Kazuhiko Takai]], who first reported it in 1986.<ref name=":0">{{Cite journal | doi=10.1021/ja00283a046| title=Simple and selective method for RCHO → (''E'')-RCH=CHX conversion by means of a CHX<sub>3</sub>–CrCL<sub>2</sub> system| year=1986| last1=Takai| first1=K.| last2=Nitta| first2=K.| last3=Utimoto| first3=K.| journal=Journal of the American Chemical Society| volume=108| issue=23| pages=7408–7410}}</ref>  In the original reaction, the organochromium species is generated from [[iodoform]] or [[bromoform]] and an excess of [[chromium(II) chloride]] and the product is a [[vinyl halide]].  One main advantage of this reaction is the ''[[entgegen|E]]''-configuration of the double bond that is formed.  According to the original report, existing alternatives such as the [[Wittig reaction]] only gave mixtures.

:[[Image:TakaiOlefination.svg|Takai olefination]]

In the [[reaction mechanism]] proposed by Takai, chromium(II) is oxidized to chromium(III) eliminating two equivalents of a halide. The geminal carbodianion complex thus formed (determined as [Cr<sub>2</sub>Cl<sub>4</sub>(CHI)(THF)<sub>4</sub>])<ref>{{Cite journal|last1=Werner|first1=Daniel|last2=Anwander|first2=Reiner|date=2018-09-28|title=Unveiling the Takai Olefination Reagent via Tris(''tert''-butoxy)siloxy Variants|journal=Journal of the American Chemical Society|volume=140|issue=43|pages=14334–14341|language=en|doi=10.1021/jacs.8b08739|pmid=30213182|s2cid=207194831|issn=0002-7863}}</ref><ref name="ReferenceA">{{Cite journal |doi = 10.1021/jacs.7b07487|title = Structural Characterization and Unique Catalytic Performance of Silyl-Group-Substituted Geminal Dichromiomethane Complexes Stabilized with a Diamine Ligand|year = 2017|last1 = Murai|first1 = Masahito|last2 = Taniguchi|first2 = Ryuji|last3 = Hosokawa|first3 = Naoki|last4 = Nishida|first4 = Yusuke|last5 = Mimachi|first5 = Hiroko|last6 = Oshiki|first6 = Toshiyuki|last7 = Takai|first7 = Kazuhiko|journal = Journal of the American Chemical Society|volume = 139|issue = 37|pages = 13184–13192|pmid = 28814078}}</ref> 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]]. In [[Newman projection]] it can be seen how the [[steric bulk]]s of chromium groups and the steric bulks of the alkyl and halogen groups drive this reaction towards anti elimination.<ref>{{cite book|last1=Kürti|first1=László|last2=Czakó|first2=Barbara|title=Strategic Applications of Named Reactions in Organic Synthesis|date=2005|publisher=Elsevier Academic Press|location=Burlington; San Diego; London|isbn=978-0-12-369483-6}}</ref>

:[[Image:TakaiMechanism.svg|Takai mechanism]]

==History==
Prior to the introduction of this chromium-based protocol, olefination reactions generally gave ''Z'' alkenes or mixtures of isomers.<ref name=":0"/> Similar olefination reactions had been performed using a variety of reagents such as zinc and lead chloride;<ref>{{Cite journal | doi=10.1021/jo00228a055| title=Alkylidenation of ester carbonyl groups by means of a reagent derived from RCHBr<sub>2</sub>, Zn, TiCl<sub>4</sub>, and TMEDA. Stereoselective preparation of (''Z'')-alkenyl ethers| year=1987| last1=Okazoe| first1=Takashi| last2=Takai| first2=Kazuhiko| last3=Oshima| first3=Koichiro| last4=Utimoto| first4=Kiitiro| journal=Journal of Organic Chemistry| volume=52| issue=19| pages=4410–4412}}</ref> however, these olefination reactions often lead to the formation of  diols—the [[McMurry reaction]]—rather than the methylenation or alkylidenation of aldehydes.<ref>{{Cite journal |doi = 10.1246/cl.1973.1041|title = Reductive coupling of carbonyl compounds to pinacols and olefins by using TiCl<sub>4</sub> and Zn|year = 1973|last1 = Mukaiyama|first1 = Teruaki|last2 = Sato|first2 = Toshio|last3 = Hanna|first3 = Junichi|journal = Chemistry Letters|volume = 2|issue = 10|pages = 1041–1044|doi-access = free}}</ref> 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.<ref name=":0" />
[[File:Takai olefination.png|center|frameless|500x500px|aldehyde specificity]]
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.<ref name="ReferenceA"/> Ways to limit the amount of chromium chloride exist, namely by utilization of zinc equivalent,<ref>{{Cite journal | doi=10.1055/s-1999-2829| title=A Practical Transformation of Aldehydes into (''E'')-Iodoalkenes with Geminal Dichromium Reagents| year=1999| last1=Takai| first1=Kazuhiko| last2=Ichiguchi| first2=Tetsuya| last3=Hikasa| first3=Shintaro| journal=Synlett| volume=1999| issue=8| pages=1268–1270}}</ref> but this method remains unpopular.

==Takai–Utimoto olefination==
In a second publication the scope of the reaction was extended to diorganochromium intermediates bearing [[alkyl group]]s instead of halogens:<ref>{{cite journal | title=(''E'')-Selective olefination of aldehydes by means of ''gem''-dichromium reagents derived by reduction of ''gem''-diiodoalkanes with chromium(II) chloride | first1=T. | last1= Okazoe | first2= Kazuhiko | last2= Takai | first3= K. | last3= Utimoto | journal= Journal of the American Chemical Society| year=1987 | volume= 109 | issue=3 | pages= 951–953 | doi= 10.1021/ja00237a081}}</ref>

:[[Image:TakaiVariation.svg|Takai reaction 1987]]

==References==
{{Reflist}}
{{Alkenes}}

[[Category:Olefination reactions]]
[[Category:Carbon-carbon bond forming reactions]]
[[Category:Name reactions]]