X-Ray Structural Analyses of Organolithium Compounds

Abstract

This chapter discusses the X-ray structural analyses of organolithium compounds. Lithium compounds, perhaps the most widely used organometallic synthetic intermediates, have fascinating structures. Nevertheless, organic chemistry textbooks often treat organolithium compounds merely as “carbanions” or as monomers. The chemical behavior of lithium compounds cannot be understood without the knowledge of the structures, the degree of aggregation, and the extent and nature of solvation. “Terminally bonded” organolithium compounds afford, in principle, the simplest examples of lithium–carbon σ–bonds. This class comprises lithium compounds with a single contact between lithium and a carbon atom. The dimer of lithiobicyclo butane, and of lithiodimethylsulfide, illustrates the bridging between sp 3 -hybridized carbon atoms while phenyllithium exemplifies bridging between sp 2 carbon atoms. A few organolithium compounds crystallize in aggregates with more than four subunits. Such compounds, derived from substituted acetylenic substrates (excluding simple acetylides), form oligomeric structures involving bridging lithium atoms and possible interactions of the lithium atoms with acetylenic π system. The benzophenonedilithium compound 50, formed by the reduction of benzophenone with lithium metal, crystallizes as a dimer. A large number of mixed-metal organometallic compounds have been prepared in which the hydrocarbon ligands bridge the lithium and the other metal atoms. A number of lithium/transition-metal compounds involving metal–metal multiple bonds as well as lithium–alkyl group interactions have been structurally characterized. The structure of lithium compounds without lithium–carbon bonds summarizes the structures of some lithium compounds that do not involve Li–C contacts.