Copper-Catalyzed Twofold Silylmetalation of Alkynes

Abstract

The first twofold silylmetalation across a C≡C triple bond was achieved. In the presence of a catalytic amount of copper cyanide, diarylacetylenes were converted into 1,2-dimetalated 1,2-disilyl-1,2-diarylethanes on treatment with silylpotassium species generated in situ from disilane and t-BuOK. The dimetalated species were subsequently protonated to yield a series of 1,2-disilyl-1,2-diarylethanes.

Key words

silylmetalation - disilane - copper catalysis - ate complex - disilyldiarylethanes

• References and Notes

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• 5 1,2-Bis[dimethyl(phenyl)silyl]-1,2-diphenylethane (3a); Typical Procedure An oven-dried Schlenk tube was charged with diphenylacetylene (1a; 53.5 mg, 0.300 mmol), CuCN (5.4 mg, 0.060 mmol), and t-BuOK (108 mg, 0.960 mmol). DME (2.0 mL) and 1,1,2,2-tetramethyl-1,2-diphenyldisilane (2, 244 mg, 0.900 mmol) were added sequentially to the mixture, which was then stirred at 25 °C for 1.5 h. H₂O (30 mL) was added at 0 °C, and the resulting biphasic solution was extracted with Et₂O (3 × 15 mL). The combined organic layer was washed with brine, dried (Na₂SO₄), and concentrated under reduced pressure. The resulting residue was purified by column chromatography (silica gel, hexane) and then by GPC (CHCl₃ eluent) to give a white solid; yield: 89.2 mg (0.198 mmol, 66%) (minor/major = 1:1.2 as an inseparable mixture). ¹H NMR (CDCl₃): δ = 7.38–7.36 (m, 2 H × 0.55), 7.34–7.30 (m, 8 H × 0.55), 7.22 (ddd, J = 7.2, 7.2, 1.2 Hz, 2 H × 0.45), 7.14–7.11 (m, 6 H × 0.55 + 4 H × 0.45), 7.07–7.01 (m, 6 H × 0.45), 6.98 (dd, J = 7.2, 1.2 Hz, 4 H × 0.45), 6.92 (d, J = 7.2 Hz, 4 H × 0.45), 6.84–6.83 (m, 4 H × 0.55), 2.88 (s, 2 H × 0.45), 2.75 (s, 2 H × 0.55), 0.10 (s, 6 H × 0.55), –0.17 (s, 6 H), –0.22 (s, 6 H × 0.45). ¹³C NMR (CDCl₃): δ = 142.5, 141.6, 138.5, 138.4, 134.1, 134.0, 131.5, 129.5, 128.8, 128.2, 127.7, 127.6, 127.3, 127.0, 125.1, 125.1, 38.5, 36.8, –2.36, –2.42, –3.36, –4.04. HRMS: m/z [M⁺] calcd for C₃₀H₃₄Si₂: 450.2194; found: 450.2197.
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• 10 Even though a free silylpotassium species seems to function in the reaction, the formation of a silicate species generated from disilane and tert-butoxide cannot be excluded.
• 11 Ref. 3b reports an example of monosilylcupration by (R₃Si)₂Cu(CN)Li₂ of a C≡C triple bond however, no report of any type of addition of (R₃Si)₂Cu(CN)K₂ is known.
• 12 In the presence of smaller amounts of disilane (1.0 equiv) and t-BuOK (1.2 equiv), we obtained recovered alkyne 1a (32%) and disilylated product 3a (19%), as well as the monosilylated alkene (25%), after protic workup. This suggests that a monosilylated copper species is formed just before the the silylpotassium species is consumed. The monosilylated copper species might also add to the alkyne to provide the monosilylated alkene eventually.