Aggregation in Alkyllithiums

This is a view of the methyllithium tetramer.  Note how the lithium atoms in the center form a tetrahedron, and the methyl groups coordinate to each face of the tetrahedron..

An important aspect of the aggregation phenomenon is that the substrate interacts with the oligomer. Look at the structure of acetone as it approaches the (MeLi)₄ tetramer:

The electostatic potential map (right) shows several salient features. The methyl groups are, as expected, quite negative (yellow-orange to red). You can see at the upper left the positive (blue) character of the lithium atom. The acetone (aqua; bottom) has a significant positive potential at the carbonyl carbon. This view shows the alignment of one methyl group with the p system of acetone; this is poised to migrate readily.

Nonpolar solvents such as hexane promote aggregation of lithiated carbanions. Switching to a highly coordinating solvent (e.g., THF) or addition of coordinating reagents such as TMEDA (N, N, N', N' tetramethyl ethylenediamine) or HMPA (hexamethyl phosphoramide) can break up these aggregates. Dimers and monomers are more reactive than higher aggregates, and quite often display different reactivity due to the different steric environment at the alkyl group.

The structure of the TMEDA-phenyllithium dimer is shown below.

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Last updated 09/21/00
Comments to K. Gable