General, Mild and Efficient Synthesis of β-Enaminones Catalyzed by Ceric Ammonium Nitrate

Abstract

Ceric ammonium nitrate catalyzes the reaction between aromatic or aliphatic primary amines and a variety of β-dicarbonyl compounds, including β-ketoesters, β-ketothioesters and β-diketones. The reaction proceeds smoothly at room temperature in short reaction times and gives β-enaminones in good to excellent yields.

Key words

enaminones - ceric ammonium nitrate - lanthanides - Lewis acids - β-dicarbonyl compounds

References and Notes

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31

General Experimental Procedure for β-Enaminone Synthesis To a stirred solution of 3 mmol of amine 1 and 3 mmol of β-dicarbonyl compound 2 in 3 mL of EtOH was added 5 mol% of CAN. The mixture was stirred at r.t. for the time periods specified in Tables [1] and [2] . After completion of the reaction the mixture was dissolved in CH₂Cl₂, washed with H₂O, dried, and evaporated. Analytically pure compounds 3 were obtained by a rapid column chromatography on Et₃N-pretreated silica gel, eluting with PE-EtOAc mixtures. Characterization data for two representative, previously unknown enamines follow.
Ethyl 2-(4-methylphenylamino)cyclohex-1-enecarboxylate (3b): viscous liquid. IR (neat): 3228.5, 2935.4, 2858.6, 1650.6, 1603.4, 1515.9, 1236.9, 1175.8, 1077.8 cm^-1. ¹H NMR (250 MHz, CDCl₃): δ 1.33 (t, J = 7.1 Hz, 3 H), 1.61-1.63 (m, 4 H), 2.26-2.40 (m, 7 H), 4.21 (q, J = 7.1 Hz, 2 H), 6.98 (d, J = 8.2 Hz, 2 H), 7.13 (d, J = 8.2 Hz, 2 H), 10.69 (br s, 1 H). ¹³C NMR (62.9 MHz, CDCl₃, DEPT-135): δ = 15.1 (CH₃), 21.3 (CH₃), 22.7 (CH₂), 23.1 (CH₂), 24.3 (CH₂), 28.5 (CH₂), 59.5 (CH₂), 92.7 (CH), 125.7 (CH), 129.8 (CH), 134.6 (C), 137.5 (C), 157.5 (C), 171.3 (C). Anal. Calcd for C₁₆H₂₁NO₂: C, 74.10; H, 8.16; N, 5.40. Found: C, 74.06; H, 8.32; N, 5.71.
tert-Butyl 3-(butylamino)but-2-enethioate (3ac): viscous liquid. IR (neat): 3253.3, 2960.1, 2928.2, 1606.0, 1582.8, 1499.8, 1360.6, 1266.3, 1084.5 cm^-1. ¹H NMR (250 MHz, CDCl₃): δ = 0.94 (t, J = 7.3 Hz, 3 H), 1.37-1.56 (m, 4 H), 1.50 (s, 9 H), 1.87 (s, 3 H), 3.20 (q, J = 7.3 Hz, 2 H), 4.86 (s, 1 H), 9.34 (br s, 1 H). ¹³C NMR (62.9 MHz, CDCl₃, DEPT-135): δ = 14.2 (CH₃), 19.2 (CH₃), 20.4 (CH₂), 31.0 (CH₃), 32.8 (CH₂), 43.3 (CH₂), 46.8 (C), 94.2 (CH), 160.1 (C), 188.9 (C). Anal. Calcd for C₁₂H₂₃NOS: C, 62.83; H, 10.11; N, 6.11; S, 13.98. Found: C, 62.76; H, 9.96; N, 5.86; S, 13.74.