Palladium–Polypyrrole Nanocomposites Pd@PPy for Direct C–H Functionalization of Pyrroles and Imidazoles with Bromoarenes

Dedicated to the memory of Dr. Guy Lavigne

Abstract

Palladium–polypyrrole nanocomposites (Pd@PPy) with unique combination of high palladium dispersion (nanoparticle size 2.4 nm) and high palladium content (35 wt%) are efficient catalysts for the selective arylation of substituted pyrroles and imidazoles with either activated or deactivated aryl bromides. The performances of the recoverable supported palladium catalyst matches the best performances of homogeneous systems based on Pd(OAc)₂ at 0.5–0.2 mol%, and largely overwhelm the classical Pd/C catalyst.

Key words

palladium - polypyrrole - nanocomposite - C–H arylation - azoles

• References and Notes

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• 14 Typical Procedure All reactions were run under argon in Schlenk tubes using vacuum lines. DMAc analytical grade was not distilled before use. KOAc (99%) was used. Commercial aryl bromides, pyrroles, and imidazoles were used without purification. The reactions were followed by GC and NMR spectroscopy. ¹H NMR spectra were recorded with a Bruker 300 MHz spectrometer in CDCl₃ solutions. Chemical shifts are reported in ppm relative to CDCl₃ (7.25 for ¹H NMR). Flash chromatography was performed on silica gel (230–400 mesh). In a typical procedure, the aryl bromide (1 mmol), pyrrole (2 mmol), and KOAc (2 mmol) were introduced in a Schlenk tube, equipped with a magnetic stirring bar. The catalyst [either Pd@PPy,⁵ or Pd(OAc)₂ at 0.02–2.0 mol%] and DMAc (2.5 ml) were added, and the Schlenk tube was purged several times using vacuum/argon flow. The Schlenk tube was placed in a preheated oil bath at 150 °C, and reactants were allowed to stir for 17 h. The reaction mixture was analyzed by GC and NMR to determine the conversion of aryl bromide. The solvent was then removed by heating the reaction vessel under vacuum, and the residue formed was charged directly onto a silica gel column. The products were eluted, using an appropriate ratio of EtOAc and heptane. Recycling procedure were based on simple Pd@PPy powder paper filtration, rinsing with a small portion of organic solvent, and drying under vacuum at 60 °C for 4 h. Subsequent catalytic tests were conducted using DMAc at 150 °C in the presence of KOAc and sufficient Pd@PPy collected from several experiments.
• 15 1-[4-(5-Acetyl-1-methyl-1H-pyrrol-2-yl)phenyl]ethanone (6a) The reaction of 4-bromoacetophenone (0.100 g, 1 mmol), 1-methyl-2-acetylpyrrole (0.120 mL, 2 mmol), and KOAc (0.098 g, 2 mmol) with Pd@PPy (0.003 g, 2% mol) affords the corresponding product 6a in 28% isolated yield. ¹H NMR (200 MHz, CDCl₃): δ = 8.02 (d, J = 8.4 Hz, 2 H), 7.51 (d, J = 8.4 Hz, 2 H), 7.03 (d, J = 4.0 Hz, 1 H), 6.29 (d, J = 4.0 Hz, 1 H), 3.91 (s, 3 H), 2.64 (s, 3 H), 2.48 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 197.4, 188.7, 141.4, 136.4, 136.3, 132.7, 129.3, 128.6, 119.7, 110.1, 35.4, 27.5, 26.7. Anal. Calcd (%) for C₁₅H₁₅NO₂: C, 74.67; H, 6.27; 5.81. Found: C, 73.92; H, 6.33; N, 5.40. HRMS (ESI+): m/z [M + H⁺] calcd for C₁₅H₁₅NO₂: 242.118; Found: 242.172.
• 16 1-Methyl-5-(m-tolyl)-1H-pyrrole-2-carbaldehyde (7d) The reaction of 3-bromotoluene (0.060 mL, 1 mmol), 1-methyl-2-formylpyrrole (0.100 ml, 2 mmol), and KOAc (0.098 g, 2 mmol) with Pd@PPy (0.003 g, 2 mol%) affords the corresponding product 7d in 41% isolated yield. ¹H NMR (300 MHz, CDCl₃): δ = 9.57 (s, 1 H), 7.37–7.32 (m, 1 H), 7.26–7.20 (m, 3 H), 6.96 (dd, J = 4.1 Hz, 1 H), 6.29 (d, J = 4.1 Hz, 1 H), 3.93 (s, 3 H), 2.41 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 179.5, 144.6, 138.4, 133.0, 131.1, 129.9, 129.4, 128.5, 126.3, 124.5, 110.7, 34.4, 21.5. Anal. Calcd (%) for C₁₃H₁₃NO: C, 78.36; H, 6.58; N, 7.03. Found: C, 77.93; H, 6.53; N, 6.70. HRMS (ESI+): m/z [M + H⁺] calcd for C₁₃H₁₃NO: 200.107; found: 200.190.