Grignard reaction

The Grignard reaction, named for the French chemist François Auguste Victor Grignard, is an boron and other carbon-heteroatom bonds.

The addition to the nucleophile is irreversible due to the high pK_a value of the alkyl component (pK_a = ~45). Grignard reagents react with acidic 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. To circumvent this issue, the reaction vessel is often flame-dried to evaporate all moisture, then sealed to prevent more from entering.

An example of the Grignard reaction is a key step in the industrial production of Tamoxifen:^[4]

Reaction mechanism

The addition of the Grignard reagent to the carbonyl typically proceeds through a six-membered ring transition state.^[5]

However, with hindered Grignard reagents, the reaction may proceed by single-electron transfer.

In a reaction 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 argon atmospheres, especially for smaller scales.

Synthesis of Grignard reagents

Grignard reagents are formed via the action of an alkyl or aryl halide on single electron transfer.

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 Rieke magnesium.

Many Grignard reagents such as diethyl ether solutions.

Via the Schlenk equilibrium, Grignard reagents form varying amounts of diorganomagnesium compounds (R = organic group, X = halide):

2 RMgX $\overrightarrow{\leftarrow}$ R₂Mg + MgX₂

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 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.

The addition of a small amount of amalgamates the surface of the metal, allowing it to react.

These methods weaken the MgO, thereby exposing highly reactive magnesium to the organic halide.

Variations

Grignard reagents will react with a variety of carbonyl derivatives.^[7]

In addition, Grignard reagents will react with other various electrophiles.

Also the Grignard reagent is very useful for forming carbon-heteroatom bonds.

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.^[8]

For the coupling of aryl halides with aryl Grignards, nickel chloride in Kumada-Corriu coupling gives access to styrenes.

Oxidation

The oxidation of a Grignard reagent with oxygen takes place through a 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 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.

Nucleophilic aliphatic substitution

Grignard reagents are Naproxen production:

Elimination

In the elimination reaction to the alkene. This reaction can limit the utility of Grignard reactions.

References

^ Grignard, V. (1900). "Sur quelques nouvelles combinaisons organométaliques du magnésium et leur applicatione à des synthèses d'alcools et d'hydrocabures". Compt. Rend. 130: 1322-1325.
^ 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. (doi:10.1002/poc.610020303)
^ 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)