Ruthenium-Catalyzed C–H Silylation of 1-Arylpyrazole Derivatives and Fluoride-Mediated Carboxylation: Use of Two Nitrogen Atoms of the Pyrazole Group

Abstract

Carboxylation of 1-arylpyrazole derivatives was developed using a ruthenium-catalyzed ortho silylation in conjunction with fluoride-mediated carboxylation with carbon dioxide. The two nitrogen atoms of pyrazole play crucial roles in promoting ortho silylation via the formation of a five-membered ruthenacycle and in accelerating aryl anion formation by lowering the electron density of the aromatic ring.

Key words

C–H activation - carbon dioxide - silylation - ruthenium - pyrazole

• References and Notes


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• 16 General Procedure for the One-Pot Carboxylation Into a 10 mL sealed tube was placed substrate 1j (47.5 mg, 0.3 mmol, 1.0 equiv), and then the tube was evacuated and backfilled with argon (3×). To the reaction tube were added toluene (0.15 mL, 2.0 M), norbornene (141.2 mg, 1.5 mmol, 5.0 equiv), RuH₂(CO)(PPh₃)₃ (5.6 mg, 0.006 mmol, 2 mol%), and HSiEt₃ (174.4 mg, 0.24 mL, 1.5 mmol, 5.0 equiv). The system was closed and stirred at 100 °C for 20 h. The reaction mixture was directly pumped up at r.t. to remove volatile materials such as toluene, HSiEt₃, and norbornene, followed by the introduction of CO₂ (balloon). To the residue were added DMF (3.0 mL, 0.1 M) and flame-dried CsF (136.7 mg, 0.9 mmol, 3.0 equiv). The system was closed again and stirred at 100 °C under 1 atm of CO₂ for 16 h. The reaction mixture was then cooled to r.t. and treated with Cs₂CO₃ (117.3 mg, 0.36 mmol, 1.2 equiv) and MeI (51.1 mg, 22.4 μL, 0.36 mmol, 1.2 equiv) followed by stirring at r.t. for 30 min. H₂O was added, and the mixture was extracted with EtOAc (3 × 10 mL). The combined organic layer was washed with H₂O (1×) and brine (1×) and then dried over Na₂SO₄. After removal of the solvent under reduced pressure, the residue was purified by silica gel column chromatography (hexane–EtOAc, 7:1) to afford the corresponding ester 3j (54.9 mg, 253.9 μmol, 85% yield); white solid; mp 64.3–66.0 °C. IR (Nujol): 2924, 2854, 1731, 1589, 1518, 1301, 1199, 1149, 1114, 747 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 7.74 (d, J = 7.7 Hz, 1 H), 7.71 (d, J = 1.7 Hz, 1 H), 7.52 (d, J = 2.3 Hz, 1 H), 7.46 (d, J = 8.3 Hz, 1 H), 7.40 (dd, J = 8.3, 7.7 Hz, 1 H), 6.44 (dd, J = 2.3, 1.7 Hz, 1 H), 3.63 (s, 3 H), 2.10 (s, 3 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 166.2, 140.1, 138.8, 136.9, 134.2, 131.3, 129.6, 128.7, 128.2, 106.0, 52.2, 17.3 ppm. HRMS (EI): m/z calcd for C₁₂H₁₂N₂O₂ [M]⁺: 216.0899; found: 216.0895.