For example, according to molecular weight determinations, allyllithium is associated to the extent of twelvefold in diethyl ether depending on the concentration of the solution,"" the degree of association increasing with increasing concentration. Other structures are also conceivable, especially cyclic structures, as was recently demonstrated in the case of crystalline [{(PhCH,)Na(tmeda)),] (tmeda=
N . N , N', N'-tetramethylethylenediamine)."31
Experimental
Preparation of 1 and crystal growing: A solution of butyllithium (16.4 mmol) in hexane (10 mL) was treated at -78ยฐC firstly with T H F (1.42 g, 19.7 mmol; 20ยฐ% excess) and then with 3,3-dimethyl-l-butyne (1.39 g, 17.0 mmol). After 30 min at -78ยฐC the crystals that had formed were recovered by suction filtration and dried under vacuum until they no longer appeared moist. Yield ca. 1.8 g (70%). In order to avoid efflorescence, the crystals were treated with two drops of T H F and prepared in a THF-saturated argon atmosphere.
Preparation of 2 and crystal growing: A solution of 3,3-dimethyl-l-butyne (1.7 g, 20.7 mmol) in cyclohexane (10 mL) was treated dropwise at room temperature with a 1 . 6 ~ solution of n-butyllithium in hexane until the initially precipitated tert-butylethynyllithium had re-dissolved (ca. 18 mL, 29 mmol). The solution was then evaporated down to 6 m L and treated with I mL of THF. After storage for several days at room temperature crystals of 2 separated out. Yield ca. 0.8 g (35%). To avoid cleavage of T H F they were prepared in a THF-saturated argon atmosphere.
The T H F content of 1 and 2 after hydrolytic decomposition was determined gas-chromatographicall y.