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DOI: 10.1055/s-0029-1218012
Direct Cobalt-Catalyzed Cross-Coupling Between Aryl and Alkyl Halides
Publication History
Publication Date:
02 October 2009 (online)
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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.
Keywords
cobalt - cross-coupling - aryl halides - domino reactions
- Supporting Information for this article is available online:
- Supporting Information
-
1a
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12a C(sp3)-Br
and C(sp2)-Br bond strengths at 298 K: EtBr (68 kcal mol-1);
PhBr (80 kcal mol-1). Taken from:
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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). -
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Lee J.-s.Velarde-Ortiz R.Guijarro A.Wurst JR.Rieke RD. J. Org. Chem. 2000, 65: 5428 -
18a We
cannot exclude reduction of CoCl2 by the alkyl-MgX, which
would result in similar Co(MgX) complexes, see:
Jonas K.Koepe G.Krüger C. Angew. Chem. Int. Ed. Engl. 1986, 25: 923 ; Angew. Chem. 1986, 98, 901 -
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
References
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15The 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.
20Reactions with (-)-sparteine and quinine as chiral ligands instead of Me4-DACH each afforded 3q in <7% yield (ee was not determined)