Metallation of Cyclohexene

The allylic protons in cyclohexene are less acidic than those in aliphatic 1- and 2-alkenes. Prolonged ( 24h) treatment at 25 °C of a 800 mol% excess of cyclohexene with a 1 1 molar mixture of sec.-BuLi and f-BuOK in isopentane, followed by addition of oxirane gave the corresponding alcohol in a good yield [1]. Our attempts to metallate cyclohexene with n-BuLi-TMEDA in hexane (20 °C or reflux) or with a 1 1 1 molar mixture of n-BuLi, f-BuOK, and TMEDA in hexane (—20 to + 10°C) gave poor results, presumably due to preferential attack of TMEDA by the base [2]. Reaction (at — 80 to 0 °C) of a five-fold molar excess of cyclohexene with n-BuLi-f-BuOKin a 1 1 mixture of THF and hexane, followed by reaction with oxirane gave the expected alcohol in only 35% yield. A much better result is obtained, however, if first the hexane is removed from the n-BuLi solution by evacuation. Yields in the region of 60% can be obtained, when cyclohexene is used in a four fold excess. 

A solution of 0.10 mol of n-BuLi in 66 ml of hexane is placed in the flask, which has been filled with inert gas. The flask is fitted with two stoppers and an evacuation device, after which it is placed in a water bath at 20 °C. The greater part of the hexane is removed in a water-pump vacuum (a tube filled with KOH pellets is placed between the flask and the water aspirator). The flask is then placed in a cooling bath ( — 50 °C) and inert gas is admitted. A mixture of 50 ml of THF and 0.5 mol (41 g) of cyclohexene (pre-cooled at — 40 °C under an atmosphere of inert gas) is then 

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