(4-bromophenoxy)(tert-butyl)dimethylsilane Page 429

1,2-Dibromoethane

Magnesium

Selenium

A mixture of (4-bromophenoxy)-tert-butyldimethylsilane (23.8 g, 0.083 mol) and 1,2-dibromoethane (9.0 g, 0.048 mol) in diethyl ether (50 cm³) was added dropwise to a stir­red suspension of magnesium metal turnings (7.0 g, 0.29 mol) in diethyl ether (50 cm³) under nitrogen. When the reaction commenced more diethyl ether (100 cm³) was added and the rate of addition regulated to maintain gentle reflux. After the end of the addition the mixture was refluxed for 5 h. After cooling, the solution was decanted from excess magnesium metal. Precipitated selenium (7.0 g, 0.091 mol) added over a period of 15 min. Stirring was continued for 12 h, then, with ice-cooling, HCl (50 cm³, 2 M) was added and the suspension filtered. The phases of the filtrate were separated and the diethyl ether phases evaporated to give a viscous orange oil which was dissolved in ethanol (200 cm³), NaOH (2.0 g) added and air was bubbled through for 24 h. The dark solution was filtered with char­coal and Celite and evaporated to give a deep yellow oil (22.3 g) which was used directly for the next step.

A Grignard reaction with added 1,2-dibromoethane (which ends up as ethylene) to promote a steady reaction and with excess Mg to ensure conversion of all the starting material was used. The product was not purified or characterised and was used directly for further syntheses.