Structure
Organolithium reagents can be aggregated, with lithium coordinating to more than one carbon atom and carbon coordinating to more than one lithium atom. Three general factors affect aggregation: the electrostatic interaction between opposite charges, the coordination sphere of lithium which can be either solvent molecules or Lewis base and the steric hindrance of the hydrocarbon part. The lithium atoms tend to form triangles and higher aggregates.
In the solid state of methyllithium, 4 lithium atoms form a tetrahedron with each face capped by a methyl group which bonds simultaneously to three Li atoms (η3 hapticity). Long-range interactions between (MeLi)4 units are based on η3-Li-CH3-η1-Li bonding. Butyllithium forms a hexameric Li6 octahedron without any long-range interactions.
The addition of Lewis bases such as solvents diethyl ether or THF or nitrogen ligands TMEDA, PMDTA or sparteine tend to deaggregate organolithium compounds, making them more soluble and more reactive. The solid-state complex of MeLi with (-)-sparteine is a dimer. The complex of butyllithium with PMDTA comes the closest to monomeric BuLi.
In solution, methyllithium in THF at 1M is a tetramer, n-butyllithium in benzene at 3M is a hexamer and in THF at 1M a tetramer. t-BuLi in THF is a dimer. Isopropyllithium in cyclopentane is a mixture of the hexamer, octamer, and nonamer.
Different organolithium aggregation states are encountered in the simple deprotonation of the terminal alkyne (phenylthio)acetylene by n-butyllithium in THF at -135°C, a process that can be followed by 7Li NMR spectroscopy:
The cubane-like tetramer A is hardly reactive compared to the dimer B which forms first mixed-dimer species C and ultimately homodimer D. In fact the dimer is more reactive than the tetramer by a factor 3.2x108.
Read more about this topic: Organolithium Reagent
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