Lithium-Olefin π-Complexes and the Mechanism of Carbolithiation: Synthesis, Solution Behavior, and Crystal Structure of (2,2-Dimethylpent-4-en-1-yl)lithium

We describe the first crystallographically characterized example of a nonconjugated olefin bound in a simple dihapto fashion to a lithium center, as part of a study of two alkyllithium compounds that contain C=C double bonds at the alkyl chain terminus: (2,2-dimethylbut-3-en-1-yl)lithium (1) and the related pentenyl compound (2,2-dimethylpent-4-en-1-yl)lithium (2). The Li-olefin interactions in the crystal structure of 2 serve as a model for those proposed to be present in the [RLi···olefin] intermediate in olefin carbolithiation reactions. As seen in other systems, the Li-olefin interaction is correlated with deshielding of the ¹H NMR resonances of the olefinic hydrogen atoms. DOSY and NOE measurements show that 1 and 2 remain tetrameric in cyclohexane and that the lithium-olefin interactions persist in solution. Addition of a Lewis base such as THF to these ω-alkenyllithium species has two effects: the THF displaces the lithium-olefin interactions while accelerating the rate of carbolithiation. A deuteration experiment shows that compound 2 undergoes reversible carbolithiation to the corresponding cyclobutylmethyllithium species in the presence of Lewis bases, but this transformation is thermodynamically uphill owing to ring strain. In comparison, the longer chain hexenyl species (2,2-dimethylhex-5-en-1-yl)lithium is thermodynamically unstable with respect to the intramolecular carbolithiation product [(3,3-dimethylcyclopentyl)methyl]lithium (3). We suggest that rate-determining step in carbolithiation reactions may not always be formation of the C-C bond, as is often assumed, but in some cases may be formation of the lithium-olefin complex; the coordination of the olefin to lithium may occur in a concerted fashion with disaggregation of lithium clusters. Finally, we point out that activation enthalpies can be obtained solely from NMR line shapes above the coalescence point.