Metallation of Propene and Isobutene

Subsequent reactions with n-octyl bromide, c-hexyl bromide, bromoacetaldehyde diethylacetal, benzyl chloride, oxirane, carbon dioxide, carbon disulfide (followed by reaction with methyl iodide), and methyl isothiocyanate. 

Apparatus p. 24, 500 ml. The carboxylation reaction is carried out in a 1-1 round-bottomed, four-necked flask, equipped with a gas-inlet tube, dipping in the solution, a mechanical stirrer, a thermometer and a gas-outlet. The latter is removed when the solution of isobutenyllithium is introduced by means of a syringe (internal diameter of the needle 1 mm). 

For the metallation of propene and homologues the following base-solvent systems are in principle available BuLi TMEDA-hexane [1], BuLi t-BuOK-hexane [2], BuLi t-BuOK-THF + hexane [3], BuLi t-BuOK TMEDA-hexane [4]. In order to maintain a sufficiently high concentration, the metallation of gaseous alkenes (propene, 1- and 2-butene, and isobutene) has to be carried out at relatively low temperatures. As a consequence, reaction times may be long [1]. With the third and fourth base-solvent system the gaseous alkenes can be metallated quickly, while reactions with a number of electrophiles indicate that the metallations have proceeded successfully. 

A solution of 0.10 mol of BuLi in 66 ml of hexane is placed in the flask. After cooling to — 80 °C, liquified propene (0.4 mol, 16.8g) or isobutene (0.3 mol, 17g) is added in one portion. A solution of 0.10 mol (11.3 g) of t-BuOK in 80 ml of THF is added dropwise over 15 min, while the temperature is kept between —70 and — 80 °C. After an additional 15 min, the cooling bath is removed and the temperature allowed to rise to 0 °C. To prepare the lithium compounds, a solution of 0.10 mol (8.7 g) of anhydrous lithium bromide (the commercial anhydrous salt is heated for 30 min at 150°C in a vacuum of 15mmHg or less) in 40 ml of THF is added over a few seconds. A fine white suspension is formed. 

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