Why don't Alkyl Fluorides form Grignard Reagents

Question

The reactivity of forming Grignards reagent were given as RI > RBr > RCl > RF and was noted that Grignard reagent with alkyl fluorides cannot be formed. What is the reason for this?

Answer 1

As suggested by Waylander, the greater strength of the carbon-fluorine bond versus other carbon-halogen bonds raises the activation energy for obtaining a Grignard reagent with fluorine. A secondary issue is that if you do run a Grignard-reagent synthesis using a fluoride, you may complicate the workup by forming magnesium fluoride precipitate or (especially hazardous and glass-corrosive) hydrofluoric acid. For both reasons it is more feasible in most applications to use a halogen that is easier to work with.

Fluoride Grignard reagents can be made by first using dioxane and heating to promote the disproportionation of a heavier-halide Grignard reagent forming the dialkyl magnesium compound ($\ce{2RMgX <=> R2Mg + MgX2}$; see Cope[1]) followed by reaction of the dialkylmagnesium compound with appropriate fluorine compounds (see Ashby and Nackashi[2]).

1. Arthur C. Cope (1935). "The Preparation of Dialkylmagnesium Compounds from Grignard Reagents". J. Am. Chem. Soc. 57, 11, 2238–2240. https://doi.org/10.1021/ja01314a059.

2. E.C. Ashby, John Nackashi (1974). "The preparation of organomagnesium fluorides by organometallic exchange reactions". Journal of Organometallic Chemistry, 72, 1, 11-20. ISSN 0022-328X, https://doi.org/10.1016/S0022-328X(00)82011-2.

Answer 2

Grignard reagents where the halogen is fluorine was previously not taken into interest owing to the unreactive nature of the carbon-fluorine bond and the preparation of such species was very time and resource consuming. Until recently, chemists looked for ways to create this species:

1. Using specially activated magnesium (Rieke metals)
2. Organometallic exchange reactions (See @OscarLanzi's answer especially the 2nd paper)
3. Selective direct fluorination (Deyoung et.al., 1992)
4. Mechanochemical activation (Speight et. al, 2020)