Giving phenyllithium a right-handed double-helical twist. Syntheses and crystal structures of enantiopure alkyl-, aryl-, and amidolithium aggregates

The influence of enantiopure nitrogen donor ligands on structure and aggregation in three lithium reagents has been investigated using single crystal X-ray diffraction methods. While the 1:1 complex between methyllithium and (−)-sparteine, [LiMe(spa)]2 (1), is dimeric, the 2:1 complex between phenyllithium and (−)-sparteine, [Li4Ph4(spa)2] (2), displays a tetranuclear Li4Ph4 core with one sparteine ligand in each end. The core in 2 has a laddered structure, which resembles a double-helix in the sense that it is twisted corresponding to approximately one ninth of a full rotation. The stereochemistry is predetermined to a right-handed double-helix by the terminal sparteine ligands. Synthesis and structural characterization of homoleptic [Li(pymp)]4 (3), pymp=(S)-2-(1-pyrrolidinylmethyl)pyrrolidido, displays a tetramer with a helical twist analogous to 2, which shows that this bidentate enantiopure amido ligand (pymp) also predetermines the chirality to a fragmented right-handed double-helix.