Carbon–Oxygen Homocoupling of 2-Naphthols through Electrochemical Oxidative Dearomatization

Abstract

A homocoupling reaction of 2-naphthols with formation of a C–O bond through electrochemical oxidative dearomatization in the presence of catalytic amounts of ferrocene and a ruthenium complex was developed. Mechanistic studies revealed that the reaction might proceed through coupling between two identical radical species. Moreover, a gram-scale experiment was performed to illustrate the potential practicability of this methodology in organic synthesis.

Key words

electrochemistry - homocoupling - C–O bond formation - naphthols - dearomatization - naphthyloxynaphthalenones

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• 21 Electrochemical Oxidative Homocoupling of 2-Naphthols; General Procedure A 25 mL four-necked flask equipped with stirrer bar, a reflux condenser, an RVC anode (100 pores/inch, 10 × 10 × 12 mm; Goodfellow Cambridge Ltd.), and a Pt plate cathode (10 × 12 × 0.1 mm; Beijing Global International Science and Technology Co., Ltd) was charged with the appropriate substituted 2-naphthol, [RuCl₂(p-cymene)]₂ (2.5 mol%), Cp₂Fe (0.2 equiv), KPF₆ (2.0 equiv), H₂O (12.0 mL), and 1,4-dioxane (3.0 mL). Constant-current electrolysis (10 mA) was performed at 90 °C with magnetic stirring for 4 h. The resulting solution was then cooled to r.t., the layers was separated, and the aqueous phase was extracted with EtOAc (3 × 20 mL). The organic phases were combined, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography. 1-Phenyl-1-[(1-phenyl-2-naphthyl)oxy]naphthalen-2(1H)-one (2a) Yellow foam solid; yield: 40.1 mg (92%); TLC: R_f = 0.45 (EtOAc–PE, 1:5). IR (neat): 2973, 1923, 1734, 1684, 1507, 1053, 697 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.85 (d, J = 7.4 Hz, 1 H), 7.71–7.64 (m, 2 H), 7.62 (d, J = 10.0 Hz, 1 H), 7.57–7.47 (m, 6 H), 7.39 (td, J = 7.2, 1.8 Hz, 1 H), 7.35–7.27 (m, 4 H), 7.16–7.11 (m, 1 H), 7.06 (dd, J = 10.1, 4.9 Hz, 2 H), 7.01–6.97 (m, 2 H), 6.42 (d, J = 9.1 Hz, 1 H), 6.25 (d, J = 10.0 Hz, 1 H).¹³C NMR (101 MHz, CDCl₃): δ = 196.78, 150.02, 145.09, 143.89, 139.57, 137.11, 133.97, 131.81, 131.37, 130.79, 130.56, 129.71, 128.91, 128.87, 128.44, 128.21, 128.16, 127.82, 127.19, 126.94, 126.37, 125.93, 125.75, 125.48, 123.78, 116.41, 85.58, 29.84. HRMS (ESI): m/z [M + Na]⁺ calcd for C₃₂H₂₂NaO₂: 461.1512; found: 461.1508.