Copper-Catalyzed Oxidative Self-Coupling of α-Amino Carbonyl Compounds for the Synthesis of Tetrasubstituted 1,4-Enediones

Abstract

A protocol for the copper-catalyzed oxidative self-coupling of α-amino carbonyl compounds has been developed for the synthesis of tetrasubstituted 1,4-enediones (Z-isomers) in moderate to good yields through the cleavage of four sp³C–H bonds and the simultaneous formation of one C=C double bond in the α-amino carbonyl compound. The strategy has the advantages of using readily available starting ­materials and of high stereoselectivity.

Key words

enediones - copper catalysis - oxidative coupling - amino carbonyl compounds - diaminobutenediones

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• 14 (2Z)-2,3-Diamino-1,4-diarylbut-2-ene-1,4-diones (2a–s);General Procedure A pressure tube was charged with the α-amino carbonyl compound 1 (0.5 mmol), Cu(acac)₂ (0.05 mmol, 13 mg), DBU (1 mmol, 152 mg), DTBP (1.5 mmol, 219 mg), and CH₃CN (2 mL). The tube was then sealed and the mixture was stirred at 100 °C for 12 h. When reaction was complete, the mixture was diluted with EtOAc (15 mL) and washed with H₂O. The separated aqueous phase was extracted with EtOAc (2 × 15 mL), and the combined organic layers were washed with brine, dried (­Na₂SO₄), filtered, and concentrated in vacuo. The crude mixture was purified by column chromatography (silica gel, ­­PE–EtOAc). (2Z)-2,3-Bis[methyl(phenyl)amino]-1,4-diphenylbut-2-ene-1,4-dione (2a) Orange solid; yield: 89 mg (80%); mp = 166–168 °C. ¹H NMR (400 MHz, CDCl₃): δ = 7.79 (d, J = 7.2 Hz, 2 H), 7.43 (t, J = 7.2 Hz, 1 H), 7.32 (t, J = 7.2 Hz, 2 H), 7.11 (t, J = 7.6 Hz, 2 H), 6.83 (t, J = 7.2 Hz, 1 H), 6.76 (d, J = 8.0 Hz, 2 H), 3.02 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ = 194.2, 145.1, 137.8, 132.2, 128.7, 128.5, 128.1, 121.1, 117.2, 38.3. HRMS (EI): m/z [M]⁺ calcd for C₃₀H₂₆N₂O₂: 446.1989; found: 446.2005.