A Highly Active Cobalt Catalyst System for Kumada Biaryl Cross-Coupling

Abstract

A highly active cobalt catalyst system has been developed for the cross-coupling reactions of arylmagnesium reagents and aryl bromides. In the presence of 1 mol% CoCl₂, 2 mol% IPr·HCl and 2 mol% NaO ^t Bu, a wide range of (hetero)biaryls are prepared in 51–99% yields at room temperature within a short reaction time.

Key words

carbenes - catalysis - cross-coupling - Grignard reaction - ­halides

• References and Notes

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• 8 General Experimental Procedure In a 25 mL Schlenk flask, a mixture of CoCl₂ (2 mg, 0.015 mmol, 1 mol%), NaO ^t Bu (3 mg, 0.03 mmol, 2 mol%) and IPr·HCl (13 mg, 0.03 mmol, 2 mol%) in THF (1.0 mL) was stirred under argon at rt for 1 h. A solution of bromobenzene (1a, 236 mg, 1.5 mmol, 1.0 equiv) in 0.5 mL THF was added followed by a solution of p-tolylmagnesium bromide (2a, 2.3 mL, 0.78 M in THF, 1.8 mmol, 1.2 equiv). The reaction progress was monitored by GC using dodecane as an internal standard. Once completed, the reaction mixture was quenched with saturated aqueous NH₄Cl solution (10 mL) and extracted with ethyl acetate (20 mL) three times. The combined organic layers were dried over anhydrous magnesium sulfate and concentrated in vacuo. The resulting crude mixture was purified by silica gel column chromatography to give 3a (237 mg, 94%) as a white solid. Analytical Data for Compound 3a ¹H NMR (400 MHz, CDCl₃): δ = 7.68–7.59 (m, 2 H), 7.58–7.51 (m, 2 H), 7.47 (dd, J = 8.5, 6.9 Hz, 2 H), 7.38 (d, J = 7.3 Hz, 1 H), 7.30 (d, J = 7.9 Hz, 2 H), 2.45 (s, 3 H). ¹³C{1 H} NMR (101 MHz, CDCl₃): δ = 141.3, 138.5, 137.1, 129.6, 128.8, 127.2, 127.1, 127.0, 126.9, 21.2.
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