The Grignard reaction (pronounced /ɡriɲar/) is an organometallic chemical reaction in which alkyl- or aryl-magnesium halides (Grignard reagents) add to a carbonyl group in an aldehyde or ketone. This reaction is an important tool for the formation of carbon–carbon bonds. The reaction of an organic halide with magnesium is not a Grignard reaction, but provides a Grignard reagent.
Grignard reactions and reagents were discovered by and are named after the French chemist François Auguste Victor Grignard (University of Nancy, France), who was awarded the 1912 Nobel Prize in Chemistry for this work. Grignard reagents are similar to organolithium reagents because both are strong nucleophiles that can form new carbon-carbon bonds.
The Grignard reagent functions as nucleophiles attacking electrophilic carbon atoms that are present within the polar bond of the carbonyl group. The addition of the Grignard reagent to the carbonyl typically proceeds through a six-membered ring transition state.
However, with hindered Grignard reagents, the reaction may proceed by single-electron transfer. Similar pathways are assumed for other reactions of Grignard reagents, e.g., in the formation of carbon–phosphorus, carbon–tin, carbon–silicon, carbon–boron and other carbon–heteroatom bonds.
Grignard reagents form via the reaction of an alkyl or aryl halide with magnesium metal. The reaction is conducted by adding the organic halide to a suspension of magnesium in an etherial solvent, which provides ligands required to stabilize the organomagnesium compound. Empirical evidence suggests that the reaction takes place on the surface of the metal. The reaction proceeds through single electron transfer: In the Grignard formation reaction, radicals may be converted into carbanions through a second electron transfer.
A limitation of Grignard reagents is that they do not readily react with alkyl halides via an SN2 mechanism. On the other hand, they readily participate in transmetalation reactions:
For this purpose, commercially available Grignard reagents are especially useful because this route avoids the problem with initiation.
In reactions involving Grignard reagents, it is important to exclude water and air, which rapidly destroy the reagent by protonolysis or oxidation. Since most Grignard reactions are conducted in anhydrous diethyl ether or tetrahydrofuran, side-reactions with air are limited by the protective blanket provided by solvent vapors. Small-scale or quantitative preparations should be conducted under nitrogen or argon atmospheres, using air-free techniques. Although the reagents still need to be dry, ultrasound can allow Grignard reagents to form in wet solvents by activating the magnesium such that it consumes the water.
Grignard reactions often start slowly. As is common for reactions involving solids and solution, initiation follows an induction period during which reactive magnesium becomes exposed to the organic reagents. After this induction period, the reactions can be highly exothermic. Alkyl and aryl bromides and iodides are common substrates. Chlorides are also used, but fluorides are generally unreactive, except with specially activated magnesium.
Typical Grignard reactions involve the use of magnesium ribbon. All magnesium is coated with a passivating layer of magnesium oxide, which inhibits reactions with the organic halide. Specially activated magnesium, such as Rieke magnesium, circumvents this problem.
Most Grignard reactions are conducted in ethereal solvents, especially diethyl ether and THF. With the chelating diether dioxane, some Grignard reagents undergo a redistribution reaction to give diorganomagnesium compounds (R = organic group, X = halide):
This reaction is known as the Schlenk equilibrium.
Because Grignard reagents are so sensitive to moisture and oxygen, many methods have been developed to test the quality of a batch. Typical tests involve titrations with weighable, anhydrous protic reagents, e.g. menthol in the presence of a color-indicator. The interaction of the Grignard reagent with phenanthroline or 2,2'-bipyridine causes a color change.
Many methods have been developed to initiate sluggish Grignard reactions. These methods weaken the passivating layer of MgO, thereby exposing highly reactive magnesium to the organic halide. Mechanical methods include crushing of the Mg pieces in situ, rapid stirring, and sonication of the suspension. Iodine, methyl iodide, and 1,2-dibromoethane are common activating agents. The use of 1,2-dibromoethane is particularly advantageous as its action can be monitored by the observation of bubbles of ethylene. Furthermore, the side-products are innocuous:
Grignard reagents are produced in industry for use in situ, or for sale. As with at bench-scale, the main problem is that of initiation; a portion of a previous batch of Grignard reagent is often used as the initiator. Grignard reactions are exothermic, and this exothermicity must be considered when a reaction is scaled-up from laboratory to production plant.
The most common application is for alkylation of aldehydes and ketones, as in this example:
Note that the acetal function (a masked carbonyl) does not react.
Such reactions usually involve an aqueous acidic workup, though this is rarely shown in reaction schemes. In cases where the Grignard reagent is adding to a prochiral aldehyde or ketone, the Felkin-Anh model or Cram's Rule can usually predict which stereoisomer will be formed. With easily 1,3-diketones and related substrates, the Grignard reagent RMgX functions merely as a base, giving the enolate anion and liberating the alkane RH.
Grignard reagents will react with other various electrophiles, serving both as a nucleophile for many and as a base for protic substrates.
Not shown, the reaction of bromoethane and Mg in ether followed by the addition of phenol in THF converts the phenol into C6H5-OMgBr. In the presence of paraformaldehyde powder and triethylamine after the addition of benzene and distillation of the latter solvents, salicylaldehyde will be the major product after the addition of 10% HCl. The reaction works also with iodoethane instead of bromoethane.
Like organolithium compounds, Grignard reagents are useful for forming carbon–heteroatom bonds.
Dialkylcadmium reagents are used for preparation of ketones from acyl halides:
A Grignard reagent can also participate in coupling reactions. For example, nonylmagnesium bromide reacts with methyl p-chlorobenzoate to give p-nonylbenzoic acid, in the presence of Tris(acetylacetonato)iron(III) (Fe(acac)3), after workup with NaOH to hydrolyze the ester, shown as follows. Without the Fe(acac)3, the Grignard reagent would attack the ester group over the aryl halide.
For the coupling of aryl halides with aryl Grignards, nickel chloride in tetrahydrofuran (THF) is also a good catalyst. Additionally, an effective catalyst for the couplings of alkyl halides is dilithium tetrachlorocuprate (Li2CuCl4), prepared by mixing lithium chloride (LiCl) and copper(II) chloride (CuCl2) in THF. The Kumada-Corriu coupling gives access to [substituted] styrenes.
The simple oxidation of Grignard reagents to give alcohols is of little practical import as yields are generally poor. In contrast, two-step sequence via a borane (vide supra) that is subsequently oxidized to the alcohol with hydrogen peroxide is of synthetic utility.
The synthetic utility of Grignard oxidations can be increased by a reaction of Grignard reagents with oxygen in presence of an alkene to an ethylene extended alcohol. This modification requires aryl or vinyl Grignards. Adding just the Grignard and the alkene does not result in a reaction demonstrating that the presence of oxygen is essential. Only drawback is the requirement of at least two equivalents of Grignard although this can partly be circumvented by the use of a dual Grignard system with a cheap reducing Grignard such as n-butylmagnesium bromide.
At one time, the formation and hydrolysis of Grignard reagents was used in the determination of the number of halogen atoms in an organic compound. In modern usage Grignard degradation is used in the chemical analysis of certain triacylglycerols.
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