Controlled Aerobic Oxidation of Primary Benzylic Alcohols to Aldehydes Catalyzed by Polymer-Supported Triazine-Based Dendrimer–Copper Composites

Abstract

A controlled aerobic oxidation of primary benzylic alcohols to the corresponding benzaldehydes by using polystyrene–poly­(ethylene glycol) (PS–PEG) resin-supported triazine-based polyethyleneamine dendrimer–copper complexes [PS–PEG-TD2–Cu(II)] was developed. In particular, PS–PEG-TD2–Cu(OAc)₂ efficiently catalyzed the aerobic oxidation of benzylic alcohols in the presence of a catalytic amount of TEMPO under atmospheric conditions to give the corresponding aldehydes in up to quantitative yield. The catalyst was readily recovered by simple filtration and reused four times without significant loss of its catalytic activity.

Key words

dendrimer - aldehydes - aerobic oxidation - polymer support - copper catalysis

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• 18 Synthesis of PS–PEG-TD2–Cu(II) Complexes A–D A mixture of PS–PED-TD2 (1.0 g) and the appropriate copper salt (1.0 mmol) in MeOH (10 mL) was stirred at r.t. for 6 h. The mixture was then filtered, and the resulting resin beads were washed with MeOH (10 × 10 mL) and dried in vacuo overnight. The copper loadings of the catalysts were determined by ICP–AES analysis.
• 19 We investigated the solvent effect (toluene, H₂O, CH₃CN, and THF) on the aerobic oxidation of benzyl alcohol under the standard conditions. The aerobic oxidation in toluene and H₂O proceeded well to give benzaldehyde (2a) in 100 and 83% GC yield, respectively, while the reactions in CH₃CN and THF were sluggish (56 and 9% GC yield, respectively). In addition, ICP analysis for the recovered catalyst from the reaction in H₂O showed that 32% of the copper species leached into the solution during the reaction. On the basis of these results, we selected heptane for the further investigation.
• 20 We monitored the formation of benzaldehyde in the aerobic oxidation of benzyl alcohol under O₂ and air. Similar reaction rates were observed under O₂ and air. Under O₂: 64% GC yield (2 h), 83% GC yield (4 h), 89% GC yield (8 h), 90 % GC yield (12 h). Under air: 52% GC yield (2 h), 79% GC yield (4 h), 87% GC yield (8 h), 96% GC yield (12 h).
• 21 We also tested various solvents for washing the recovered catalyst. MTBE was an effective solvent to extract the organic materials and keep the copper content in the polymer matrix.
• 22 Synthesis of Aldehydes 2a–y; General Procedure A mixture of catalyst PS–PEG-TD2–Cu(OAc)₂ (100 mg, 0.05 mmol Cu), BnOH (1a, 27.0 mg, 0.25 mmol), and TEMPO (7.8 mg, 0.05 mmol) in heptane (2.0 mL) was stirred at 80 °C for 24 h under air (1 atm). The mixture was then cooled and filtered, and the resulting solid material was washed with Et₂O (3 × 2 mL). The organic phases were combined, concentrated to a volume of 2 mL, and the internal standard was added to determine the GC yield. The crude product was purified by column chromatography [silica gel, hexane–Et₂O (99:1)]. In the formation of some benzaldehydes, low isolated yields were observed because of instability of the benzaldehydes on silica gel. Benzaldehyde (2a) Colorless oil; yield: 10.8 mg (41%). ¹H NMR (400 MHz, CDCl₃): δ = 10.03 (s, 1 H), 7.89 (dd, J = 8.4, 1.2 Hz, 2 H), 7.64 (t, J = 7.6 Hz, 1 H), 7.56–7.52 (m, 2 H); ¹³C NMR (100 MHz, CDCl₃): δ = 192.4, 136.4, 134.5, 129.7, 129.0.