Crystal Structures of <i>n</i>-BuLi Adducts with (<i>R</i>,<i>R</i>)-TMCDA and the Consequences for the Deprotonation of Benzene

Combinations of organolithium compounds and diamine bases have become a powerful tool in synthetic chemistry. Because of the structure−reactivity relationship, the elucidation of reaction mechanisms of these reagents is strongly connected with the structural determination of intermediate species. In mixtures of the diamine TMCDA (N,N,N′,N′-tetramethylcyclohexane-1,2-diamine) and n-butyllithium, two different structures, the dimeric [n-BuLi·(R,R)-TMCDA]2 and the aggregate [(n-BuLi)2·(R,R)-TMCDA]2, can be isolated, depending on the n-BuLi/TMCDA ratio. Thereby, [(n-BuLi)2·(R,R)-TMCDA]2 is a rare example of an organolithium compound with a ladder arrangement of the central four-membered Li−C−Li−C rings. Two isomers of the ladder structure are formed in the crystal by changing from the enantiomerically pure to racemic TMCDA. As n-BuLi/TMCDA mixtures are also able to deprotonate benzene, these structures give hint to possible mechanisms. Supported by theoretical studies, transition states based on the dimer, the ladder structure, and a hypothetical monomer are discussed.