α‐Oxygen‐Substituted Organolithium Compounds and Their Carbenoid Nature: Calculations of the Configurational Stability and of LiCH₂OH Model Structures, Crystal Structure of Diphenyl(trimethylsilyloxy)methyllithium · 3 THF, and the Stereochemistry of the (Reverse) Brook Rearrangement

The comparatively high configurational stability of α‐oxygen‐substituted organolithium compounds is confirmed by ab initio calculations [MP2/6–311 + + G(d,p)]. Model calculations of the LiCH 2 OH structures 21A–D (MP2/6–31G(d)//3–21G and MP2/6–311 + + G(d,p)//MP2/6–311 + + G(d,p)) are in agreement with recently determined crystal structures of α‐oxygen‐sub‐stituted organolithium compounds. They also suggested the preparation of a new structural type ( 21C ) which has been verified by means of diphenyl(trimethylsilyloxy)methyllithium · 3 THF ( 24 · 3 THF) as shown by X‐ray crystallography. The lengthening of the CO bonds in all four α‐oxygen‐sub‐stituted organolithium compounds known to date contrasts strongly with the wealth of structural data from organolithium compounds α‐substituted with “typical” acceptor substituents: on lithiation the carbon‐acceptor bond length in these compounds is always shortened . This difference is in agreement with a carbenoid character of α‐oxygen‐substituted organolithium species. Finally, the crystal structure of 24 · 3 THF serves as a model for the surprising stereochemistry of the (reverse) Brook rearrangement.