ALPHA-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 alpha-oxygen-substituted organolithium compounds is confirmed by ab initio calculations [MP2/6-311 + + G(d,p)]. Model calculations of the LiCH2OH structures 21 A - D(MP2/6-31G(d)//3-21 G and MP2/6-311 + + G (d,p)//MP2/6-311 + + G (d,p)) are in agreement with recently determined crystal structures of alpha-oxygen-substituted organolithium compounds. They also suggested the preparation of a new structural type (21 C) 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 ( - 0 bonds in all four alpha-oxygen-substituted organolithium compounds known to date contrasts strongly with the wealth of structural data from organolithium compounds alpha-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 alpha-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.