Anchoring of a Copper(II)–Schiff Base Complex onto Silica-Coated Ferrite Nanoparticles: A Magnetically Separable Catalyst for Oxidative C–O Coupling by Direct C(sp²)–H and C(sp³)–H Bond Activation

Abstract

A novel catalyst consisting of a Schiff base–copper complex on surface-modified silica-coated ferrite nanoparticles was been prepared and used for oxidative C–O cross-coupling reactions of 1,3-dicarbonyl compounds with formamides for the synthesis of enol carbamates. The new catalyst has also been used for esterification of alkylbenzenes with cyclic ethers.

Key words

catalysis - bond activations - Schiff bases - oxidative coupling - green chemistry - carbamates

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• 15 Enol Carbamates 2a–g; General Procedure A 70% aq solution of TBHP (0.193 g, 1.5 equiv) was added dropwise to a mixture of the appropriate 1,3-dicarbonyl compound 1 (1 mmol), the magnetic catalyst (10 mg), and DMF (2 mL). The mixture was heated to 80 °C, stirred for 20 min, and then cooled to r.t. The catalyst was removed by using an external magnet, and the residue was extracted with EtOAc (3 × 10 mL), dried (MgSO₄), filtered, and concentrated under vacuum. The crude product was purified by column chromatography. Allyl (2Z)-3-{[(Dimethylamino)carbonyl]oxy}but-2-enoate (2g) Yellow oil; yield: 160 mg (75%); IR (KBr): 1725, 1669, 1382, 1320, 1271, 1211, 1145, 1035, 750 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ = 2.06 (s, 3 H, CH₃), 2.97 (s, 3 H, CH₃N), 3.02 (s, 3 H, CH₃N), 4.57 (dd, ³ J _HH = 5.6 Hz, ⁴ J _HH = 1.2 Hz, 2 H, CH₂O), 5.18–5.34 (m, 2 H, CH₂=CH), 5.58 (s, 1 H, CHCO₂), 5.82–5.97 (m, 1 H, CH₂=CH). ¹³C NMR (100 MHz, CDCl₃) δ = 22.21, 36.59, 64.62, 107.61, 118.03, 132.31, 152.99, 161.33, 163.66.
• 16 Aryl Esters 5a–5h; General Procedure In a round-bottomed flask, a mixture of alkylbenzene 3 (1 mmol), magnetic catalyst (15 mg), and 70% aq TBHP (0.167 g, 1.3 equiv) in 1,4-dioxane (2 mL) was stirred at 80 °C for 10 h, then cooled to r.t. The catalyst was removed by using an external magnet and the residue was extracted with EtOAc (2 × 10 mL). The extracts were washed with 5% aq NaHCO₃ (15 mL), dried (MgSO₄), filtered, and concentrated under vacuum. The crude product was purified by column chromatography. 1,4-Dioxan-2-yl Benzoate (5a) Pale yellow oil; yield: 156 mg (75%); IR (KBr): 3071, 2970, 2861, 1718, 1529, 1455, 1402, 1276, 1014, 911, 879, 715 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 3.64−3.69 (m, 1 H), 3.80−3.83 (m, 2 H), 3.85−3.89 (m, 2 H), 4.16−4.23 (m, 1 H), 6.08 (t, ³ J _HH = 2.1 Hz, 1 H), 7.42−7.46 (m, 2 H), 7.55−7.57 (m, 1 H), 8.09−8.11 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃) δ = 61.9, 66.4, 68.1, 90.0, 128.6, 129.9, 130.2, 133.56, 165.4.