Microwave-Assisted Palladium-Catalyzed Allylation of β-Enaminones

Abstract

A new palladium-catalyzed approach for the C-allylation of β-enaminones under microwave irradiation is reported. This methodology provides an easy access to a variety of α-allylated enaminones. The reaction takes place with the preservation of the enamine function, which is poised for further transformations towards nitrogen-containing heterocycles.

Key words

enaminone - palladium - allylation - allyl acetate - microwave irradiation

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• 12 In order to avoid the hydrolysis of the unreacted starting material and/or the product, the yields were determined by ¹H NMR spectroscopy of the crude mixture (obtained after rapid filtration on silica gel and evaporation) using 1,3,5-trimethoxybenzene as internal standard.
• 13 In general, the allylated product 2a was obtained with an average 1:1 E/Z ratio (determined by ¹H NMR spectroscopy of the crude mixture). This ratio is variable and can change during silica gel purification.
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• 15 See Supporting Information.
• 16 When the reaction was carried out without either the precatalyst/ligand system or the precatalyst, only unreacted enaminone 1a was recovered with traces of the corresponding imino–enol tautomer 1a′ (see Supporting Information).
• 17 General Procedure To a suspension of Pd(OAc)₂ (13 mg, 0.057 mmol, 10 mol%), dppf (35 mg, 0.063 mmol, 11 mol%), and proton sponge (0.12 g, 0.57 mmol, 1 equiv) in THF (0.5 mL) in a Schlenk flask equipped with a septum, under argon atmosphere, was added allyl acetate (0.12 mL, 1.14 mmol, 2.0 equiv). After 5 min stirring, a solution of enaminone 1a (100 mg, 0.57 mmol, 1 equiv) in THF (0.5 mL) was added, the flask was sealed, and the mixture was stirred during 1 h under microwave irradiation at 100 °C. The resulting crude was filtered on a plug of silica gel. The solvent was removed, and the mixture was purified by flash chromatography on silica gel (EtOAc–cyclohexane, 20:80) to afford 86 mg of the allylated enaminone 2a as a mixture of Z and E isomers. Analytical Data for Compound 2a Yield 60%; yellow oil; Z/E ratio = 1.7:1 (analysis of the crude ¹H NMR spectrum showed a Z/E ratio of 1:1). IR (film): 3272, 3030, 2920, 1638 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 10.24 [br s, 1 H, NH_(Z)], 7.42–7.25 [m, 11 H, =CHNH_(E) + CH _Ar(Z+E)], 6.66 [d, J = 12.4 Hz, 1 H, =CHNH_(Z)], 5.94–5.71 [m, 2 H, HC=CH_2(Z+E)], 5.10–4.99 (m, 4 H, HC=CH_2(Z+E)], 4.43 (d, J = 5.9 Hz, 2 H, CH ₂ Ph_(E)], 4.39 (d, J = 6.1 Hz, 2 H, CH ₂ Ph_(Z)], 3.12 [dt, J = 6.0, 1.6 Hz, 2 H, CH ₂ CH=CH_2(E)], 2.94 [dt, J = 5.8, 1.6 Hz, 2 H, CH ₂ CH=CH_2(Z)], 2.22 (s, 3 H, CH₃CO_(E)], 2.13 (s, 3 H, CH ₃ CO_(Z)]. ¹³C NMR (75 MHz, CDCl₃): δ = 198.2, 194.2, 153.0, 149.5, 138.6, 138.5, 138.4, 136.0, 128.9, 128.8, 127.8, 127.6, 127.0, 126.9, 114.8, 114.6, 102.8, 52.5, 52.2, 35.6, 28.0, 27.6, 24.4. HRMS: m/z calcd for C₁₄H₁₇NONa [M + Na]⁺: 238.1208; found: 238.1204.
• 18 Formation of the linear product, when using cinnamyl acetate, is a further proof of the direct C-allylation mechanism.