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1b
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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-.
5
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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.
For reviews on the chemistry of silyl enol ethers see:
8a
Brownbridge P.
Synthesis
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8b
Brownbridge P.
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