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DOI: 10.1055/s-2002-32592
Cobalt(I)-catalyzed Neutral Diels-Alder Reactions of Oxygen-functionalized Acyclic 1,3-Dienes with Alkynes
Publication History
Publication Date:
07 February 2007 (online)

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
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References
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-.
6Reactions 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.