Niobium Pentachloride Mediated Direct Bradsher-Type Reaction: A Rapid and Efficient Strategy to Synthesize Novel Substituted 9-Anthraldehydes

Abstract

Dichloromethyl methyl ether in the presence of niobium pentachloride and activated triarylmethanes leads to an efficient, mild, and rapid one-pot synthesis of several 9-anthraldehyde derivatives via a direct Bradsher-type reaction. This practical methodology offers several advantages, such as a broad substrate scope, simple workup process, and excellent yields.

Key words

anthracene - Bradsher reaction - anthraldehyde - niobium catalysis - triarylmethanes

• References and Notes

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• 20 9-Anthraldehydes 7a–o; General Procedure Cl₂CHOMe (0.5 mmol) and NbCl₅ (0.4 mmol) were added to a stirred suspension of the appropriate triarylmethane (0.1 mmol) in CH₂Cl₂ (2 mL) at 0 °C, and the resulting mixture was then stirred for 5 min until the triarylmethane was completely consumed (TLC). The reaction was quenched with sat. aq Na₂CO₃, and the mixture was extracted with CH₂Cl₂ (×2). The organic layer was washed with brine, dried (MgSO₄), filtered, and concentrated. The residue was purified by flash column chromatography (silica gel, CH₂Cl₂). 2,3,6,7-Tetramethoxy-10-(4-nitrophenyl)anthracene-9-carbaldehyde (7i) Orange solid; yield: 41.6 mg (93%); mp 274–275 °C. ¹H NMR (400 MHz, CDCl₃): δ = 11.36 (s, 1 H), 8.44 (d, J = 8.69 Hz, 2 H), 8.34 (s, 2 H), 7.55 (d, J = 8.69 Hz, 2 H), 6.50 (s, 2 H), 4.04 (s, 6 H), 3.65 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 55.5 (2 CH₃O), 56.0 (2 CH₃O), 101.2 (2 CH), 103.8 (2 CH), 121.5 (C), 124.1 (2 CH), 125.6 (2 C), 128.7 (2 C), 131.6 (2 CH), 138.1 (C), 146.5 (C), 147.8 (C), 149.4 (2 C), 152.0 (2 C), 192.3 (CHO). APCI-MS: m/z [M + H]⁺ calcd: 448.1; found: 447.9.