Grignard reagents or Grignard compounds are chemical compounds with the general formula R−Mg−X, where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride Cl−Mg−CH₃ and phenylmagnesium bromide (C₆H₅)−Mg−Br. They are a subclass of the organomagnesium compounds.

Grignard compounds are popular reagents in organic synthesis for creating new carbon–carbon bonds. For example, when reacted with another halogenated compound R'−X' in the presence of a suitable catalyst, they typically yield R−R' and the magnesium halide MgXX' as a byproduct; and the latter is insoluble in the solvents normally used. In this aspect, they are similar to organolithium reagents.

Grignard reagents are rarely isolated as solids. Instead, they are normally handled as solutions in solvents such as diethyl ether or tetrahydrofuran using air-free techniques. Grignard reagents are complex with the magnesium atom bonded to two ether ligands as well as the halide and organyl ligands.

The discovery of the Grignard reaction in 1900 was recognized with the Nobel Prize awarded to Victor Grignard in 1912.

Synthesis

From Mg metal

Traditionally Grignard reagents are prepared by treating an organic halide (normally organobromine) with magnesium metal. Ethers are required to stabilize the organomagnesium compound. Water and air, which rapidly destroy the reagent by protonolysis or oxidation, are excluded.^[1] 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.^[2]

As is common for reactions involving solids and solution, the formation of Grignard reagents is often subject to an induction period. During this stage, the passivating oxide on the magnesium is removed. After this induction period, the reactions can be highly exothermic. This exothermicity must be considered when a reaction is scaled-up from laboratory to production plant.^[3] Most organohalides will work, but carbon-fluorine bonds are generally unreactive, except with specially activated magnesium (through Rieke metals).

Magnesium

Typically the reaction to form Grignard reagents involves the use of magnesium ribbon. All magnesium is coated with a passivating layer of magnesium oxide, which inhibits reactions with the organic halide. Many methods have been developed to weaken this passivating layer, thereby exposing highly reactive magnesium to the organic halide. Mechanical methods include crushing of the Mg pieces in situ, rapid stirring, and sonication.^[4] Iodine, methyl iodide, and 1,2-dibromoethane are common activating agents. The use of 1,2-dibromoethane is advantageous as its action can be monitored by the observation of bubbles of ethylene. Furthermore, the side-products are innocuous:

Mg + BrC₂H₄Br → C₂H₄ + MgBr₂

The amount of Mg consumed by these activating agents is usually insignificant. A small amount of mercuric chloride will amalgamate the surface of the metal, enhancing its reactivity. Addition of preformed Grignard reagent is often used as the initiator.

Specially activated magnesium, such as Rieke magnesium, circumvents this problem.^[5]The oxide layer can also be broken up using ultrasound, using a stirring rod to scratch the oxidized layer off,^[6] or by adding a few drops of iodine or 1,2-Diiodoethane. Another option is to use sublimed magnesium or magnesium anthracene.^[7]

"Rieke magnesium" is prepared by a reduction of an anhydrous magnesium chloride with an potassium:

MgCl₂ + 2 K → Mg + 2 KCl

Mechanism

In terms of mechanism, the reaction proceeds through single electron transfer:^[8][9][10]

$${\displaystyle {\begin{aligned}{\ce {R-X{}+Mg}}&\longrightarrow {\ce {^{-}{}++}}\\{\ce {-}}&\longrightarrow {\ce {R^{\bullet }{}+X-}}\\{\ce {R^{\bullet }{}++}}&\longrightarrow {\ce {R-Mg+}}\\{\ce {R-Mg+{}+X-}}&\longrightarrow {\ce {R-MgX}}\end{aligned}}}$$

Mg transfer reaction (halogen–Mg exchange)

An alternative preparation of Grignard reagents involves transfer of Mg from a preformed Grignard reagent to an organic halide. Other organomagnesium reagents are used as well.^[11] This method offers the advantage that the Mg transfer tolerates many functional groups. An illustrative reaction involves isopropylmagnesium chloride and aryl bromide or iodides:^[12]

i-PrMgCl + ArCl → i-PrCl + ArMgCl

From alkylzinc compounds (reductive transmetalation)

A further method to synthesize Grignard reagents involves reaction of Mg with an organozinc compound. This method has been used to make adamantane-based Grignard reagents, which are, due to C-C coupling side reactions, difficult to make by the conventional method from the alkyl halide and Mg. The reductive transmetalation achieves:^[13]

AdZnBr + Mg → AdMgBr + Zn

Testing Grignard reagents

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'-biquinoline causes a color change.^[14]

Reactions of Grignard reagents

With carbonyl compoundsedit

Grignard reagents react with a variety of carbonyl derivatives.^[15]

The most common application of Grignard reagents is the alkylation of aldehydes and ketones, i.e. the Grignard reaction:^[16]

Note that the acetal functional group (a protected carbonyl) does not react.

Such reactions usually involve an aqueous acidic workup, though this step is rarely shown in reaction schemes. In cases where the Grignard reagent is adding to an aldehyde or a prochiral ketone, the Felkin-Anh model or Cram's Rule can usually predict which stereoisomer will be formed. With easily deprotonated 1,3-diketones and related acidic substrates, the Grignard reagent RMgX functions merely as a base, giving the enolate anion and liberating the alkane RH.

Grignard reagents are nucleophiles in nucleophilic aliphatic substitutions for instance with alkyl halides in a key step in industrial Naproxen production:

Grignard reagents also react with many "carbonyl-like" compounds and other electrophiles:

Reactions as a baseedit

Grignard reagents serve as a base for non-protic substrates (this scheme does not show workup conditions, which typically includes water). Grignard reagents are basic and react with alcohols, phenols, etc. to give alkoxides (ROMgBr). The phenoxide derivative is susceptible to formylation by paraformaldehyde to give salicylaldehyde.^[17]

Alkylation of metals and metalloidsedit

Like organolithium compounds, Grignard reagents are useful for forming carbon–heteroatom bonds.

$${\displaystyle {\begin{matrix}{\ce {R4B-}}\\{\color {White}\scriptstyle {\ce {Et2O.BF3\ or\ NaBF4}}}{\Bigg \uparrow }\scriptstyle {\ce {Et2O.BF3\ or\ NaBF4}}\\{\ce {Ph2PR<-{\ce {Ph2PCl}}RMgX->{\ce {Bu3SnCl}}Bu3SnR}}\\{\color {White}\scriptstyle {\ce {B(OMe)3}}}{\Bigg \downarrow }\scriptstyle {\ce {B(OMe)3}}\\{\ce {RB(OMe)2}}\end{matrix}}}$$

2 RMgX + CdCl₂ → R₂Cd + 2 Mg(X)Cl

Schlenk equilibriumedit

Most Grignard reactions are conducted in ethereal solvents, especially diethyl ether and THF. Grignard reagents react with 1,4-dioxane to give the diorganomagnesium compounds and insoluble coordination polymer MgX₂(dioxane)₂ and (R = organic group, X = halide):

2 RMgX + dioxane ⇌ R₂Mg + MgX₂(dioxane)₂

This reaction exploits the Schlenk equilibrium, driving it toward the right.

Precursors to magnesiatesedit

Grignard reagents react with organolithium compounds to give ate complexes (Bu = butyl):^[19]

BuMgBr + 3 BuLi → LiMgBu₃ + BuBr

Coupling with organic halidesedit

Grignard reagents do not typically react with organic halides, in contrast with their high reactivity with other main group halides. In the presence of metal catalysts, however, Grignard reagents participate in C-C 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.^[20]

For the coupling of aryl halides with aryl Grignard reagents, nickel chloride in tetrahydrofuran (THF) is also a good catalyst. Additionally, an effective catalyst for the couplings of alkyl halides is the Gilman catalyst lithium tetrachlorocuprate (Li₂CuCl₄), prepared by mixing lithium chloride (LiCl) and copper(II) chloride (CuCl₂) in THF. The Kumada-Corriu coupling gives access to substituted styrenes.

Oxidationedit

Treatment of a Grignard reagent with oxygen gives the magnesium organoperoxide. Hydrolysis of this material yields hydroperoxides or alcohol. These reactions involve radical intermediates.