Solvation of alkyllithium compounds. Steric effects on heats of interaction of bases with hexameric versus tetrameric alkyllithiums

Heats of interaction of Lewis bases with hexameric and tetrameric alkyllithiums in hydrocarbon solution at 25° have been determined by high dilution solution calorimetry at low base to lithium atom ratios. The Lewis bases utilized include tetrahydrothiophene, tetrahydrofuran, triethylphosphine, triethylamine, and diethyl ether. The organolithiums investigated were $\text{n}$-butyllithium, ethyllithium, isopropyllithium, trimethylsilylmethyllithium, and $\text{t}$-butyllithium. The basicity order based on initial enthalpies of interaction is independent of the alkyllithium compound. Larger enthalpies of interaction were observed for the tetrameric $\text{versus}$ hexameric alkyllithiums with the exception of tetrameric $\text{t}$-butyllithium which does not interact significantly with these bases. The sensitivities of the enthalpies to the steric requirements of the base were probed by comparison of the enthalpies for tetrahydrofuran, 2-methyltetrahydrofuran, and 2,5-dimethyltetrahydrofuran. Base coordination to hexameric $\text{n}$-butyllithium is more sensitive to the steric requirements of the tetrahydrofuran bases than is coordination to tetrameric trimethylsilylmethyllithium or isopropyllithium. These results are interpreted in terms of coordination of tetrahydrofuran bases to the intact hexameric aggregate for $\text{n}$-butyllithium; however, it is concluded that the corresponding interaction with hexameric trimethylsilylmethyllithium leads directly to base-solvated tetramers.