The [(DABCO)₇·(LiCH₂SiMe₃)₈] Octamer: More Aggregated than the Parent Starting Material [LiCH₂SiMe₃]₆ but Also Higher in Reactivity

Abstract

Herein, we report on the reaggregation of hexameric trimethylsilylmethyllithium [LiCH₂SiMe₃]₆ with the donor base DABCO (1,4-diazabicyclo[2.2.2]­octane) to give the unprecedented octamer [(DABCO)₇·(LiCH₂SiMe₃)₈] (<b>1</b>). The structure consists of four dimers, forming Li₂C₂ four-membered rings, connected to two [(DABCO)₃·{(LiCH₂SiMe₃)₂}₂] chain fractions, interconnected by a single DABCO molecule. Interestingly, two different conformers of (LiCH₂SiMe₃)₂ dimers are present, caused by different steric demand. Higher steric strain in the center of the molecule causes an ecliptic arrangement of the Me₃Si group along the Si–C_α bond, while at the periphery the more relaxed staggered conformation is enabled. The reactivity of trimethylsilylmethyllithium coordinated by DABCO was tested in the benchmark reaction with toluene. Although the aggregation of <b>1</b> is much higher than that of the parent [LiCH₂SiMe₃]₆, the reactivity of the first is higher than that of the starting material, provided the octameric aggregation found in the solid state is maintained in nondonating solvents. While the hexamer would not react with toluene, the octamer gives benzyllithium, coordinated by DABCO. The reaction was monitored by ¹H NMR spectroscopy. Revisiting that known structure with modern technology revealed that [(DABCO)·(LiCH₂Ph)]_∞ (<b>2</b>) crystallizes in the space group <i>P</i>2₁. <b>2</b> still is the only benzyllithium compound featuring the η³-coordination mode to the C_<i>ortho</i> atom of the phenyl ring, presumably triggered by the singly donating DABCO molecule. More donor centers supersede this extra coordination to the carbanion