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editThe Grignard reaction (French: [ɡʁiɲaʁ]) is an organometallic chemical reaction in which, according to the classical definition, carbon alkyl, allyl, vinyl, or aryl magnesium halides (Grignard reagent) are added to the carbonyl groups of either an aldehyde or ketone under anhydrous conditions. This reaction is important for the formation of carbon–carbon bonds .
(R2 or R3 could be a hydrogen)
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Grignard reactions and reagents were discovered by and are named after the French chemist François Auguste Victor Grignard (University of Nancy, France), who published it in 1900 and was awarded the 1912 Nobel Prize in Chemistry for this work. The reaction of an organic halide with magnesium is not a Grignard reaction, but provides a Grignard reagent. Other variations of the Grignard reagent have been discovered to improve the chemoselectivity of the Grignard reaction which include but are not limited to: Turbo-Grignards, Organocerium reagents, and Organocuprate (Gilman) reagents.
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editGrignard Reaction Conditions:
editThe Grignard reaction must be run under anhydrous conditions.[1] Otherwise, the reaction will fail because the Grignard reagent will act as a base rather than a nucleophile to pick up a labile proton rather than attacking the electrophilic site. This will result in no formation of the desired product as the R-group of the Grignard reagent will become protonated while the MgX portion will stabilize the species that was deprotonated.
To prevent this, Grignard reactions are completed under an inert atmosphere to remove all water from the reaction flask and ensure that the desired product is formed.[2] Additionally, if there are acidic protons in the starting material as shown in the figure above, one can overcome this by protecting the acidic site of the reactant by turning it into an ether, or a silyl ether to eliminate the labile proton from solution prior to the Grignard reaction.
Grignard-like reactions using more chemoselective reagents:
editThe classical Grignard reaction typically refers only to the reaction between a ketone or aldehyde group and a Grignard reagent to form a primary or tertiary alcohol. However, some chemists understand the definition to mean all reactions of electrophilic molecules with Grignard reagents.
Therefore, there is some dispute about the modern definition of the Grignard reaction.
March's Advanced Organic Chemistry, a reputable graduate level text, defines it as "The addition of Grignard reagents to aldehydes and ketones...". The IUPAC Goldbook, a similarly reputable text, does not address either side. In the Merck Index, published online by the Royal Society of Chemistry, the classical definition is acknowledged, followed by "A more modern interpretation extends the scope of the reaction to include the addition of Grignard reagents to a wide variety of electrophilic substrates."
Shown below are some reactions involving Grignard reagents, but themselves are not classically understood as Grignard reactions.
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Turbo-Grignards:
editTurbo-Grignards can be used to form complex aryl and heteroaryl Grignard reagents[3]. The formation of Turbo-Grignards are chemoselective. Esters, amides, and nitriles don’t react with the Turbo-Grignard reagent[3].
Organoceruim Reagents:
editOrganocerium reagents are a variation of the classic Grignard reagent which selectively adds 1,2 to conjugated Ketones and Aldehydes.[4] Organocerium reagents favour direct addition to α,β-unsaturated carbonyl groups.[4]
Organoceruim reagents are usually made by transmetalation.
Organocuprate Reagents (Gilman Reagents):
editOrganocuprates are a type of Grignard reagent that preferentially does 1,4 addition. Since conjugate additions are slower than 1,2 addition, Lewis Acids can be used to speed up a reaction.
References
edit- ^ a b Ouellette, Robert J.; Rawn, J. David (2014-01-01), Ouellette, Robert J.; Rawn, J. David (eds.), "15 - Alcohols: Reactions and Synthesis", Organic Chemistry, Boston: Elsevier, pp. 491–534, doi:10.1016/b978-0-12-800780-8.00015-2, ISBN 978-0-12-800780-8, retrieved 2023-11-06
- ^ Carey, Francis A. "Grignard reagent". Britannica.
- ^ a b Ziegler, Dorothée S.; Wei, Baosheng; Knochel, Paul (2019-02-21). "Improving the Halogen–Magnesium Exchange by using New Turbo‐Grignard Reagents". Chemistry – A European Journal. 25 (11): 2695–2703. doi:10.1002/chem.201803904. ISSN 0947-6539.
- ^ a b Imamoto, Tsuneo; Sugiura, Yasushi (1985-04-16). "Selective 1,2-addition of organocerium(III) reagents to α,β-unsaturated carbonyl compounds". Journal of Organometallic Chemistry. 285 (1): C21–C23. doi:10.1016/0022-328X(85)87395-2. ISSN 0022-328X.