2,2′-Bipyridine: An Efficient Ligand in the Cobalt-Catalyzed Synthesis of Organozinc Reagents from Aryl Chlorides and Sulfonates
Igor Kazmierskia, Corinne Gosmini*a, Jean-Marc Parisb, Jacques Périchona a Laboratoire d’Electrochimie, Catalyse et Synthèse Organique UMR 7582, Université Paris 12-CNRS, 2 Rue Henri Dunant, 94320 Thiais, France b Rhodia, Research Center of Lyon, 85 rue des Frères Perret, BP 62, 69192 Saint-Fons Cedex, France Fax: +33(1)49781148; e-Mail: gosmini@glvt-cnrs.fr;
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25 January 2006 Publication Date: 14 March 2006 (online)
The synthesis of functionalized arylzinc reagents from aromatic chlorides, triflates, or mesylates, via cobalt catalysis, is reported using 2,2′-bipyridine as a ligand in a mixture of acetonitrile and pyridine. This procedure allows the synthesis of a variety of functionalized arylzinc species in good to excellent yields. Some of these arylzinc compounds have been coupled with aromatic bromides under palladium catalysis.
Functionalized Mesylates or Triflates; Typical Procedure
Phenol (5.3 g, 50 mmol) and pyridine (22 mL) were cooled to 0 °C in a three-way reaction vessel under an argon atmosphere. Tf2O (8.5 mL, 50 mmol) was added dropwise over 45 min to the magnetically stirred solution, giving rise to a strong red-orange color. Then the reaction mixture was stirred at r.t. until the starting phenol had been consumed (monitored by GC on hydrolyzed aliquots). H2O (40 mL) was added to the mixture, which was then extracted with CH2Cl2 (3 × 40 mL). The organic layer was treated with 10% HCl (40 mL), washed with a sat. solution of NaCl (40 mL), and dried over anhyd MgSO4. After removing the solvent under vacuum, the product was purified by flash chromatography on silica gel (pentane) to give phenyl trifluoromethanesulfonate in 95% yield as a colorless oil. The same procedure was applied to the synthesis of mesylates, by replacing Tf2O with MsCl.Coupling of Organozinc Chlorides or Triflates with 2-Bromopyridine; Typical Procedure (Table 1, entry 1) Preliminary Step: Zn powder (1.27 g, 1.95 equiv, <10 µm) and CoBr2 (219 mg, 0.1 equiv) were placed in a reaction vessel under an argon atmosphere. MeCN (6 mL), allyl chloride (0.25 mL, 0.3 equiv), dodecane (0.2 mL, internal reference for GC), and TFA (0.05 mL) were added in one batch to the vessel. The resulting reaction mixture was magnetically stirred under a flow of argon at r.t. for 3 min. Synthesis of the Organozinc Reagent: Pyridine (4 mL), 2,2′-bipyridine (0.156 g, 0.1 equiv), and methyl 4-chlorobenzo-ate (1.71 g, 10 mmol) were added to the reaction mixture, and stirred at 50 °C until the reaction was complete (aliquots were removed, monitored by GC after iodination). After 3 h, 4-carboxymethylzinc chloride was obtained in 86% yield (GC). Coupling Reaction: DMF or THF (5 mL), 2-bromopyridine (1 mL, 1 equiv) and PdCl2(PPh3)2 (70 mg, 0.01 equiv) were added to the reaction mixture obtained above, and the resulting solution was stirred for 3 h at 50 °C until 4-car-boxymethylzinc chloride had been completely consumed. The solution was hydrolyzed with NH4Cl (100 mL), extracted with Et2O, the organic layer was washed with brine, dried, and the solvent evaporated. The residue was isolated by flash column chromatography (silica gel; pentane-Et2O, 85:15) to give the Negishi coupled product in 73% yield. The structure was confirmed by 1H and 13C NMR spectroscopy and MS.
