Cyclopropanation of silyl enol ethers with diethyl zinc and diiodomethane; cyclopropanes

Helen Jenkins ([email protected]),
A contribution from the Booker-Milburn Group, Bristol

Chemicals Used
3-(But-3-enyl)-1-trimethylsiloxycyclohex-1-ene ([SyntheticPage 35] ) diethyl zinc (Aldrich, 1M solution in hexane) diiodomethane (Lancaster) diethyl ether (anyhydrous)

Procedure
To a stirred solution of S-(But-3-enyl)-1-trimethylsiloxycyclohex-1-ene ([SyntheticPage 35] ) (3.76 g, 16.76 mmol) in diethyl ether (34 ml) under an argon atmosphere was added diethylzinc (33.4 ml, 33.4 mmol, 1M in hexane) and diiodomethane (2.69 ml, 33.4 mmol). The solution was heated to reflux for 17 hours after which a further 0.3 eq. of both diethylzinc and diiodomethane were added. The reaction mixture was heated to reflux for a further 2.5 hours at which point the cloudy white solution was allowed to cool before the addition of pyridine (5.35 ml, 66.8 mmol). After stirring for 15 min petrol (200 ml) was added and the resulting precipitate filtered, the filtrate was evaporated to give a yellow liquid. Kugelrohr distillation (86-90 °C, 0.1 mmHg) gave a clear colourless liquid (3.24 g, 13.6 mmol, 81%)

Author's Comments
On quenching the reaction a solid residue of zinc-pyridine salts often forms as a solid cake at the bottom of the flask (especially on larger scale). It is advisable to break up this cake with a spatula or a glass rod and stir/wash thoroughly with several portions of petrol in order to maximise the yields. This reaction also works well for 5 and 7- membered enol ethers but fails for acyclic versions. The resulting siloxy cyclopropanes can survive chromatography and prolonged storage, although are very sensitive to acid.

Data
IR(thin film) 2926 and 1641 cm^-1; ¹H(400 MHz; CDCl₃) 5.66 (1H, m, 3�-H), 4.89 (2H, m, 4�-H), 2.10-1.89 (2H, m), 1.79-1.70 (1H, m), 1.50-1.05 (6H, m), 0.86 (2H, m), 0.65 (2H, m), 0.14 (1H, t, J 4.5, endo-CH), 0.07 (9H, s, SiMe₃); äC(100 MHz; CDCl₃) 139.3 (C-3�), 114.8 (C-4�), 57.5 (quat.), 38.4 (CH), 38.1 (CH₂), 32.9 (CH₂), 31.9 (CH₂), 30.5 (CH₂), 25.5 (CH), 21.9 (CH₂), 19.4 (CH₂), 1.5 (SiMe₃). m/z (EI) 238 ([M]+, 15%), 195 (55), 73 (100).

Lead Reference
K. I. Booker-Milburn and D. F. Thompson, J. Chem. Soc. Perkin Trans. 1, 1995, 2315.

Other References

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