[(thf)Li2{H2CS(NtBu)2}]2: Synthesis, Polymorphism, and Experimental Charge Density to Elucidate the Bonding Properties of a Lithium Sulfur Ylide

Sulfur ylides (R2S+−−CR2) are widely used in organic synthesis for stereoselective epoxidations, cyclopropane formations, and ring expansion reactions. Nevertheless, their electronic properties are still under debate, because their ylenic textbook formulation (R2S═CR2) contradicts the reactivity. In order to elucidate the electronic situation in a sulfur ylide, we present an experimental charge density study via multipole refinement and subsequent topological analysis based on high-resolution X-ray data of [(thf)Li2{H2CS(NtBu)2}]2 (1). The title compound is of special interest, since additionally the formal hypervalency can be investigated along with the controversial interaction between a carbanion with a Li3 triangle. As a prerequisite for these studies, the polymorphism and synthesis of 1 were investigated. The findings clearly support the ylidic, non-hypervalent description of the molecule. The ylidic carbanion was determined to be 6-fold coordinated with three single bonds to the sulfur atom and both...