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W. M. Czaplik et al.
LETTER
Coord. Chem. Rev. 2004, 248, 623. (b) For transition-metal
catalysis, see for example: Bogdanović, B.; Schwickardi, M.
Angew. Chem. Int. Ed. 2000, 39, 4610; Angew. Chem. 2000,
112, 4788.
Acknowledgment
This work was financed by the Saltigo GmbH, Leverkusen. We
thank Prof. H.-G. Schmalz (University of Cologne) for excellent
technical support. W.M.C. received a Max Buchner stipend of the
DECHEMA. M.M. is a fellow of the Deutsche Bundesstiftung Um-
welt (DBU). A.J.v.W. is an Emmy Noether fellow of the Deutsche
Forschungsgemeinschaft (DFG) and recipient of the 2009 Science
Award of the Industrieclub Düsseldorf e.V.
(11) For related direct cobalt-catalyzed sp2–sp2 cross-coupling
reactions, see: (a) Gomes, P.; Gosmini, C.; Perichon, J. Org.
Lett. 2003, 5, 1043. (b) Amatore, M.; Gosmini, C. Angew.
Chem. Int. Ed. 2008, 47, 2089; Angew. Chem. 2008, 120,
2119.
(12) (a) C(sp3)–Br and C(sp2)–Br bond strengths at 298 K: EtBr
(68 kcal mol–1); PhBr (80 kcal mol–1). Taken from: CRC
Handbook of Chemistry and Physics; Weast, R. C.; Astle,
M. J., Eds.; CRC Press: Boca Raton, 1981. (b) The single
electron transfer (SET) into the p* orbital of the ArBr is
reversible, and the p*–s* transition required for dissociation
of the C–Br bond is slow.
(13) For related iron catalysis, see: Czaplik, W. M.; Mayer, M.;
Jacobi von Wangelin, A. Angew. Chem. Int. Ed. 2009, 48,
607; Angew. Chem. 2009, 121, 616.
(14) (a) For further details, see Supporting Information.
(b) 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 mL, 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.
References
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(18) (a) 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.
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(b) 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.
(c) 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, M. S.; Fields, E. K. 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, A. T.; Parvin, T.; Choudhury, L. H.; 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)
(g) Shirakawa, E.; Sato, T.; Imazaki, Y.; Kimura, T.;
Hayashi, T. Chem. Commun. 2007, 4513.
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(10) For a general explanation of promoting effects of
(magnesium) salts, see: (a) Garst, J. F.; Soriaga, M. P.
Synlett 2009, No. 18, 2931–2934 © Thieme Stuttgart · New York