Alkyl and Aryl 4,5-Dichloro-6-oxopyridazin-1(6H)-carboxylates: A Practical Alternative to Chloroformates for the Synthesis of Symmetric and Asymmetric Carbonates

Abstract

Symmetric and asymmetric carbonates were synthesized by using alkyl or aryl 4,5-dichloro-6-oxopyridazin-1(6H)-carboxylates. Five aryl 4,5-dichloro-6-oxopyridazin-1(6H)-carboxylates were converted into the corresponding diaryl carbonates in good to excellent yields by treatment with potassium carbonate in refluxing THF. When the 4,5-dichloro-6-oxopyridazin-1(6H)-carboxylates were treated with aliphatic or aromatic alcohols in the presence of potassium tert-butoxide in toluene at room temperature, they gave the corresponding symmetric or asymmetric carbonates in moderate to excellent yields. Alkyl and aryl 4,5-dichloro-6-oxopyridazin-1(6H)-carboxylates are therefore efficient, stable, and ecofriendly alternatives to chloroformates.

Key words

pyridazines - alcohols - phenols - carbonates - alkoxycarbonylations - acyl-transfer agents

• References and Notes

• 1 These authors contributed equally to this work.
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• 8 Symmetric Carbonates 2a–e ; General Procedure Using Potassium Carbonate K₂CO₃ (1.0 mmol) was added to a solution of the appropriate pyridazine 1 (1.0 mmol) in THF (20 mL), and the mixture was allowed to reflux until pyridazine 1 was consumed (TLC). The solvent was evaporated under reduced pressure, and the symmetric carbonates 2a–e were isolated by column chromatography [silica gel (2.5 × 3 cm), CH₂Cl₂]. Once the desired product had been isolated, the column was eluted with EtOAc to isolate 4,5-dichloropyridazin-3(2H)-one, which was obtained quantitatively and reused. Diphenyl Carbonate (2) White solid; yield: 193 mg (80%); mp 75–76 °C. IR (KBr): 3058, 1773, 1592, 1490, 1255, 1233, 1182, 1071, 1016, 996, 751, 685, 501 cm^–1. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.30–7.35 (m, 2 H), 7.40–7.51 (m, 8 H). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 121.2, 126.4, 129.7, 150.7, 151.7. HRMS (EI): m/z [M⁺] calcd for C₁₃H₁₀O₃: 214.0630; found: 214.0634. Symmetric (2) or Asymmetric (4 and 6) Carbonates; General Procedure Using Potassium tert-Butoxide Alcohol 3 or 5 (0.84 mmol) was added to a solution of the appropriate pyridazine 1 (0.7 mmol) and t-BuOK (0.84 mmol) in toluene (10 mL), and the mixture was stirred at r.t. until pyridazine 1 was consumed (TLC). 10% aq NaOH (50 mL) and CH₂Cl₂ (30 mL) were added to the mixture with stirring. The organic layer was separated, washed with H₂O (50 mL), dried (MgSO₄), and concentrated under reduced pressure. The residue was transferred to an open-bed column of silica gel (2.5 × 4 cm), which was eluted with hexane–EtOAc (3:1) to give the symmetric carbonates 2, or the asymmetric carbonates 4 or 6, and then eluted with EtOAc to isolate 4,5-dichloropyridazin-3(2H)-one quantitatively for reuse. Phenyl 2-Phenylethyl Carbonate (4) White solid; yield: 148 mg (87%); mp 83–85 °C. IR (KBr): 3109, 3081, 3058, 3033, 2969, 2938, 2895, 2868, 1753, 1492, 1260, 1210, 1077, 967, 778, 753, 699, 499 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 3.00 (t, J = 7.0 Hz, 2 H), 4.38–4.43 (m, 2 H), 7.10–7.35 (m, 10 H). ¹³C NMR (75 MHz, CDCl₃): δ = 35.1, 69.1, 121.0, 121.2, 126.1, 126.4, 126.9, 128.7, 129.1, 129.6, 129.7, 137.1, 151.1, 151.2, 153.7. HRMS (EI): m/z [M⁺] calcd for C₁₅H₁₄O₃: 242.0943; found: 242.0941.