Abstract
A variety of aggregation motifs dominate the structural chemistry of organolithium compounds. At least four distinct modes of dimerization exist: the common 4-center dimers,1 6-center dimers as in 2-lithio-2-methyldithiane,2 solvent-bridged dimers as in (LiBr)2‚(HMPA)3, and triple ions (lithium ate complexes) of type R-Li-R-//Li+.5,6a,7 The reactivity of these various dimers compared to monomers or higher aggregates is usually unknown. Triple ions possess an intriguing combination of features of both a dimer and a solvent-separated ion pair. Several lithium amides,8,9a â-keto10 and â-imino9b enolates, and a few other lithium reagents7b have been rigorously shown through crystal structures or NMR data to form triple ions. Delocalized, lithiocene-type sandwich structures are also known;6a,7a however, there is only a single report of a localized carbanion triple ion: a crystal structure was obtained of the triple ion of tris(trimethylsilyl)methyllithium.5 Here, we present NMR evidence that triple ions form in THF/HMPA solution4b for a variety of localized lithiated carbanions, such as aryllithiums and sulfurand silicon-substituted alkyllithiums.
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