Decarboxylation and Tandem Reduction/Decarboxylation Pathways to Substituted Phenols from Aromatic Carboxylic Acids Using Bimetallic Nanoparticles on Supported Ionic Liquid Phases as Multifunctional Catalysts

Abstract

Valuable substituted phenols are accessible via the selective decarboxylation of hydroxybenzoic acid derivatives using multifunctional catalysts composed of bimetallic iron-ruthenium nanoparticles immobilized on an amine-functionalized supported ionic liquid phase (Fe₂₅Ru₇₅@SILP+IL-NEt₂). The individual components of the catalytic system are assembled using a molecular approach to bring metal and amine sites into close contact on the support material, providing high stability and high decarboxylation activity. Operating under a hydrogen atmosphere was found to be essential to achieve high selectivity and yields. As the catalyst materials enable also the selective hydrogenation and hydrodeoxygenation of various additional functional groups (i.e., formyl, acyl, and nitro substituents), direct access to the corresponding phenols can be achieved via integrated tandem reactions. The approach opens versatile synthetic pathways for the production of valuable phenols from a wide range of readily available substrates, including compounds derived from lignocellulosic biomass.

Figure 1

General approach in this study. (a) Illustration of the bimetallic bifunctional Fe₂₅Ru₇₅@SILP+IL-NEt₂ catalyst, (b) proposed pathway for the synthesis of phenols through selective decarboxylation of hydroxybenzoic acid derivatives, and (c) examples of applications for phenol derivatives in pharmaceutical products.

Figure 2

Fe₂₅Ru₇₅@SILP+IL-NEt₂ bimetallic bifunctional catalytic system. (a) Illustration, (b) STEM-HAADF image, (c) STEM-HAADF-EDS elemental mapping (green = Ru, red = Fe).

Figure 3

Determination of the kinetic isotope effect for the decarboxylation of 1 using Fe₂₅Ru₇₅@SILP+IL-NEt₂. Reaction conditions: Catalyst (10 mg, metal content: 0.0034 mmol), 1 (0.221 mmol, 65 equiv compared to metal), heptane (0.5 mL), 175 °C, H₂ or D₂ (20 bar), 500 rpm; yield determined by GC-FID using tetradecane as internal standard. The selectivity is >99% in all cases, so conversion = yield. Data points represent average values, and error bars correspond to standard deviations.

Figure 4

Time profile of the reduction and decarboxylation of 3-nitro-4-hydroxybenzoic acid (12) using Fe₂₅Ru₇₅@SILP+IL-NEt₂. Conditions: Catalyst (10 mg), substrate = 24.2 mg (30 equiv related to amine, 65 equiv related to total metal loading), 0.5 mL of DMF, 50 bar of H₂, 150 °C, 500 rpm. Y = Yield, determined by GD-FID using tetradecane as internal standard. Error bars represent the standard deviation for two experiments.

Figure 5

Catalyst recycling, decarboxylation of 4-hydroxybenzoic acid (1). Conditions: Fe₂₅Ru₇₅@SILP+IL-NEt₂ (20 mg), substrate = 30.6 mg (30 equiv related to metal), 1 mL of mesitylene, 50 bar of H₂, 200 °C, 6 h, 500 rpm. Product yields were determined by GD-FID using tetradecane as internal standard. Selectivity toward 1a >99%. Error bars represent the standard deviation for three experiments.