Grignard reaction
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The Grignard reaction is an organometallic chemical reaction involving alkyl- or aryl-magnesium halides, also called Grignard reagents, with electrophiles. The Grignard reaction is an important tool in the formation of carbon-carbon bonds and for the formation of carbon-phosphorus, carbon-tin, carbon-silicon, carbon-boron and other carbon-heteroatom bonds.
Such reactions are irreversible due to the high pKa value of the alkyl component (pka = ~60). Grignard reagents react with electrophilic chemical compounds. It should be noted that such reactions are not ionic; the Grignard reagent exists as an organometallic cluster (in ether). Victor Grignard (University Of Nancy, France) was awarded the 1912 Nobel Prize in Chemistry for the discovery of such reagents. The disadvantage of the Grignard reagent is that they readily react with protic solvents (such as water), or functional groups with protons, such as alcohols and amines. In fact, atmospheric humidity in the lab can dictate one's success when trying to synthesize a grignard reagent from magnesium turnings and an alkyl halide.
An example of the Grignard reaction is a key step in the industrial production of Tamoxifen:
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[edit] Reaction mechanism
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.
In reactions involving Grignard reagents, it is important to ensure that no water is present, which would otherwise cause the reagent to rapidly decompose. Thus, most Grignard reactions occur in solvents such as anhydrous diethyl ether or tetrahydrofuran, because the oxygen of these solvents stabilizes the magnesium reagent. The reagent may also react with oxygen present in the atmosphere, inserting an oxygen atom between the carbon base and the magnesium halide group. Thus, many of these reactions are carried out in nitrogen or argon atmospheres.
[edit] Formation of Grignard reagents
Grignard reagents are formed via the action of an alkyl or aryl halide on magnesium metal. The reaction is conducted by adding the organic halide to a suspension of magnesium in an ether, which provides ligands required to stabilize the organomagnesium compound. Typical solvents are diethyl ether and tetrahydrofuran. Oxygen and protic solvents such as water or alcohols are not compatible with Grignard reagents.
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, such as Rieke magnesium.
Many Grignard reagents such as phenylmagnesium bromide are available commercially in tetrahydrofuran or diethyl ether solutions.
Via the Schlenk equilibrium, Grignard reagents form varying amounts of diorganomagnesium compounds (R = organic group, X = halide):
- 2 RMgX
R2Mg + MgX2
[edit] Practical tips
Many methods have been developed to initiate sluggish Grignard reactions. Mechanical methods include crushing of the Mg pieces in situ; rapid stirring and sonication of the suspension is also effective. Iodine, methyl iodide, and 1,2-dibromoethane are commonly employed 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:
- Mg + BrC2H4Br → C2H4 + MgBr2
The amount of Mg consumed by these activating agents is usually insignificant.
These methods weaken the passivating layer of MgO, thereby exposing highly reactive magnesium to the organic halide.
[edit] Variations
Grignard reagents will react with a variety of carbonyl derivatives.
In addition, Grignard reagents will react with other various electrophiles.
Also the grignard reagent is very useful for forming carbon-heteroatom bonds.
[edit] Coupling reactions
A Grignard reagent can also be involved in coupling reactions. For example, nonylmagnesium bromide reacts with an aryl chloride to a nonyl benzoic acid.
For the coupling of aryl halides with aryl grignards, nickel chloride in 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 (CuCl) in THF.
[edit] Oxidation
The oxidation of a Grignard reagent with oxygen takes place through a radical intermediate to a magnesium hydroperoxide. Hydrolysis of this complex yields hydroperoxides and reduction with an additional equivalent of Grignard reagent gives an alcohol.
The synthetic utility of Grignard oxidations can be increased by a reaction of Grignards 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.
[edit] Nucleophilic aliphatic substitution
Grignard reagents are nucleophiles in nucleophilic aliphatic substitutions for instance with alkyl halides in a key step in industrial Naproxen production:
[edit] References
- ↑ Grignard, V. Compt. Rend. 1900, 130, 1322.
- ↑ Shirley, D. A. Org. React. 1954, 8, 28-58. (Review)
- ↑ Huryn, D. M. Comp. Org. Syn. 1991, 1, 49-75. (Review)
- ↑ Grignard Reagents: New Developments H. G. Richey (Editor) ISBN 0-471-99908-3
- ↑ Maruyama, K.; Katagiri, T. J. Phys. Org. Chem. 1989, 2, 205.
- ↑ Lai, Y. H. Synthesis 1981, 585-604. (Review)
- ↑ Butyric acid, α-methyl- Henry Gilman and R. H. Kirby Organic Syntheses, Coll. Vol. 1, p.361 (1941); Vol. 5, p.75 (1925). (Article)
- ↑ 4-Nonylbenzoic Acid A. Fürstner, A. Leitner, G. Seidel. Org. Syn. 2004, 81, 33-42. (Article)
- ↑ Air-Assisted Addition of Grignard Reagents to Olefins. A Simple Protocol for a Three-Component Coupling Process Yielding Alcohols Youhei Nobe, Kyohei Arayama, and Hirokazu Urabe J. Am. Chem. Soc. 2005, 127(51), 18006 - 18007. (DOI:10.1021/ja055732b)
