Cobalt(I)-catalyzed Neutral Diels-Alder Reactions of Oxygen-functionalized Acyclic 1,3-Dienes with Alkynes

Abstract

The cobalt-catalyzed Diels-Alder reaction of alkoxy-substituted 1,3-butadienes with terminal and internal alkynes is described. While the reaction of 1-alkoxy derivatives gave the aromatic hydrocarbons upon elimination of alcohol from the alkoxy-substituted dihydroaromatic intermediates, the reactions with 2-alkoxy derivatives generated stable dihydroaromatic enol ethers in good to excellent chemical yields and good to high regioselectivities for unsymmetrical starting materials. The enol ethers can be easily hydrolysed to the corresponding β,γ-unsaturated ketones in a one pot reaction sequence or used in cyclopropanation or other subsequent chemical transformations.

Key words

cobalt - neutral Diels-Alder reaction - alkyne - 1,3-diene - enol ether

References

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4

The active catalyst (presumably [(dppe)Co⁺]) is generated in situ upon one electron reduction of (dppe)CoBr₂ towards the corresponding Co(I)Br-complex and further abstraction of the remaining halide by the ZnI₂ to form the reactive species and ZnI₂Br^-.

6

Reactions of terminal alkynes were complete within 0.5 h, reactions of internal alkynes needed 2-4 h, while for steri-cally hindered substrates, such as 1-trimethylsilyl-2-phenyl-acetylene (Table [2] , entry 9) the reaction were complete after 16 h (40 °C) with higher catalyst loadings (10 mol%).

7

Synthesis of 4-Phenyl-1,4-cyclohexadien-1-yl Trimethylsilyl Ether: (Table [2] , entry 2):
To a suspension of CoBr₂(dppe) (32 mg, 0.05 mmol, 5 mol%) and ZnI₂ (100 mg, 0.3 mmol) in dry dichloromethane (3 mL) under a nitrogen atmosphere, phenylacetylene (102 mg, 1 mmol), 2-trimethylsilyloxy-1,3-butadiene (170 mg, 1.2 mmol) and Bu₄NBH₄ (20 mg) were added. The mixture immediately turned dark brown. In the case of larger scale preparation (more than 5 mmol) it is advisable to cool the reaction mixture with a water bath. After 30 min, the mixture was diluted with pentane (70 mL) and filtered through Kieselgur. Evaporation of the solvent in vacuo gave 229 mg (94%) of a yellow-brown material (>95% purity by GC and ¹H NMR), which was suitable for the further transformation.
Analytically pure samples were obtained by purification by column chromatography on silica gel (pentane:diethyl ether 100:1).
¹H NMR (300 MHz, C₆D₆): δ = 0.17 [s, 9 H, Si(CH ₃)₃], 2.77-2.89 (m, 2 H, CH _{2 ,CHD}), 3.00-3.10 (m, 2 H, CH _2,CHD), 4.90-4.96 (m, 1 H, =CH), 5.82-5.88 (m, 1 H, =CH), 7.05-7.30 (m, 5 H, H _Ar).
¹³C NMR (75 MHz, C₆D₆): δ = 0.4, 29.2, 32.1, 100.8, 121.3, 125.5, 127.2, 128.5, 134.4, 141.5, 148.1.
IR (KBr): 2959(m), 1494(m), 1444(m), 1348(m), 1251(s), 1197(s), 747(m), 689(m) cm^-1.
MS (EI): m/z (%) = 244(100) [M⁺], 227(18), 211(17), 153(34), 128(11), 73(48).
HRMS: Calcd for C₁₅H₂₀OSi: 244.1283. Found: 244.1264.