Direct Cobalt-Catalyzed Cross-Coupling Between Aryl and Alkyl Halides

Abstract

An operationally simple cross-coupling reaction between aryl halides and alkyl halides with high selectivity has been developed. The underlying domino process utilizes CoCl₂/Me₄-DACH as a catalyst system. The methodology exhibits high sustainability as it obviates the need for the pre-formation and handling of stoichiometric amounts of hazardous Grignard compounds.

cobalt - cross-coupling - aryl halides - domino reactions

References

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12b

The single electron transfer (SET) into the π* orbital of the ArBr is reversible, and the π*-σ* transition required for dissociation of the C-Br bond is slow.

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14a

For further details, see Supporting Information.

14b

General procedure: A 10 mL flask was placed in a water bath (r.t.), charged with Mg turnings (63 mg, 2.6 mmol), fitted with a rubber septum, and purged with argon (1 min). A solution of CoCl₂ (13 mg, 0.1 mmol, 5 mol%) and Me₄-DACH (35 µL, 0.2 mmol, 10 mol%) in anhydrous THF (4 mL) was added via syringe. The mixture was stirred at r.t. for 15 min, then the reaction was cooled to 0 ˚C and aryl bromide (2.4 mmol) and alkyl bromide (2.0 mmol) were added. After 6 h at 0 ˚C, the reaction was quenched with saturated aqueous NH₄Cl (5 mL) and aqueous HCl (10%,
2 mL) and extracted with ethyl acetate (3 × 5 mL). The combined organic phases were dried over Na₂SO₄, concentrated in vacuo, and subjected to flash chromatography (SiO₂; cyclohexane-ethyl acetate).

15

The rate of Grignard formation is not significantly accelerated by the presence of CoCl₂. The presence of amines slows down the Grignard formation from organohalides and Mg, probably by blocking the metal surface.

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18b

However, reaction of alkyl-MgBr with ArBr under identical conditions gave only minimal amounts of cross-coupling product testifying to a far less active catalyst species being formed. See also Supporting Information.

18c We also observed a beneficial effect on the yield of the cross-coupling product by the employment of an excess of ArBr. The observed formation of 5-7% of biaryl from a 5 mol% catalyst loading mirrors the stoichiometry of the CoCl2 ® I reduction as shown in Scheme 2. A small portion of biaryl might also result from a cobalt-catalyzed oxidative dimerization of ArMgBr in the presence of ArBr, see: Kharasch MS. Fields EK. J. Am. Chem. Soc.  1941,  63:  2316 

19 Racemic 3-bromobutylbenzene was prepared from 3-hydroxybutylbenzene and PBr₃ (CH₂Cl₂, 20 ˚C, 16 h, 80%). See also: Khan AT. Parvin T. Choudhury LH. Ghosh S. Tetrahedron Lett.  2007,  48:  2271 

20

Reactions with (-)-sparteine and quinine as chiral ligands instead of Me₄-DACH each afforded 3q in <7% yield (ee was not determined)