Palladium-Catalyzed Cross-Coupling Reaction of Potassium Diaryldifluoroborates with Aryl Halides

Abstract

The catalytic cross-coupling reaction of potassium di­aryldifluoroborates with aryl halides proceeds to afford the biphenyls in good yields. These salts on an arylation reagent were readily prepared via a protocol of the Grignard method and transformation to potassium salts. Two aryl groups of these salts were efficiently transferred in the Suzuki-Miyaura reaction.

Grignard reactions - cross-coupling - organoborate - biaryls - arylations

References

For review see:

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10 Raction of bis(3,5-dimethylphenyl)borinic acid with vinyl triflate was reported. See: Winkle DD. Schaab KM. Org. Proc. Res. Dev.  2001,  5:  450 

11

The Representative Procedure for the Preparation of Potassium Diaryldifluoroborates is as follows: Under an argon atmosphere, to a solution of phenylmagnesium bromide prepared from magnesium turnings (2.67 g, 110 mmol) and bromobenzene (18.1 g, 115 mmol) in anhyd THF (150 mL) was slowly added triisopropyl borate (9.4 g, 50 mmol) and anhyd THF (30 mL) at 0 °C over 30 min. After addition of triisopropyl borate, the reaction mixture was allowed to warm slowly to r.t. and stirred at the same temperature for 16 h. The reaction was then hydrolyzed with 1 M HCl (100 mL). After the usual work-up and a short column chromatography of the resulting oil, 10.2 g of crude diphenylborinic acid was obtained. To the solution of diphenylborinic acid in MeOH (200 mL) at 0 °C, KHF₂ (11.7 g, 150 mmol) was added and stirred for 1 h. The solvent was removed in vacuo and the residual solid was extracted with an appropriate amount of acetone, and the organic layer was concentrated. The amount of 100 mL of Et₂O was added to the residue. The product was collected and dried in vacuo. Potassium diphenyldifluoroborate (1a, 9.94 g) was obtained as a white powder (82% yield). Mp 211 °C (dec.), ref. [9] 230 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.32 (br d, 2 H, J = 6.6 Hz), 7.01 (t, 2 H, J = 7.2 Hz), 6.95-6.80 (m, 1 H). ¹³C NMR (75.5 MHz, DMSO-d ₆): δ = 131.34 (t, J = 3.7 Hz), 125.87, 123.72. ¹⁹F NMR¹² (254 MHz, DMSO-d ₆): δ = -157.2 (q, J = 73.3 Hz). ¹¹B NMR¹² (86.6 MHz, DMSO-d ₆): δ = 7.0 (t, J = 73.2 Hz). Elemental analysis: Anal. Calcd for C₁₂H₁₀BF₂K: C, 59.53; H, 4.16. Found: C, 59.71; H, 4.12.

12

The B-F coupling on NMR measurement was observed at 80 °C.

13

The Representative Procedure for the Cross-Coupling Reaction of Potassium Diaryldifluoroborates is as follows: Under Ar atmosphere, a solution of potassium diphenyldifluoroborate (1a) (133 mg, 0.55 mmol), Pd(OAc)₂ (6 mg, 0.03 mmol), PPh₃ (31 mg, 0.12 mmol), 1-bromo-2-nitorobenzene (2f) (202 mg, 1.0 mmol), and Na₂CO₃ (159 mg, 1.5 mmol) in 2-methoxyethanol (5 mL) and H₂O (0.5 mL) was heated under reflux. The reaction mixture was stirred at the same temperature for 6 h and then cooled to r.t., and acidified with 1 M HCl. The mixture was extracted with MTBE (30 mL × 3). The organic layer was washed with brine (30 mL) and dried over MgSO₄. The solvent was removed in vacuo, and the crude product was purified by silica gel column chromatography to afford 2-nitrobiphenyl (3d) (187 mg, 0.94 mmol) in 94% yield.

14

Reaction of potassium bis(4-biphenyl)difluoroborates (1d) and 4-chloroacetophenone (2a) afforded the coupled product and biphenyl which resulted from potassium salt in 66% and 31% yields, respectively.

15 A similar decomposition was reported for Ph₂BOH. See: Abel EW. Gerrard W. Lappert MF. J. Chem. Soc.  1958,  1451