Organolithium compounds configurational stability

Some heteroatom-substituted or chelate-stabilized organolithium compounds, on the other hand, can be sufficiently stable toward racemization to enable their use in stereoselective reactions with electrophiles [223, 225, 271, 531, 543, 552-554] (Scheme 5.75). This increased configurational stability of a-heteroatom-substituted carbanions might be due to the stronger pyramidalization of such carbanions [261,555] and fixation of the metal by chelate formation. 

Configurational stability has also been confirmed for various metalated carbamates by Hoppe and coworkers. Remarkably, carbamate-protected alcohols such as 20 are deprotonated enantioselectively, when treated with i-butyllithium in the presence of (—)-sparteine. The lithium carbenoids like 21 (R = alkyl) thus generated turn out to retain their configuration (equation 11). Similar results have been obtained for a-lithiated amines and carbamate protected amines " . As a rule, dipole stabilization of the organolithium compounds in general also enhances the configurational stability of a-oxygen-substituted lithium carbenoids. 

II. UNSTABILIZED ot-AMINO-ORGANOLITHIUM COMPOUNDS A. Synthesis and Configurational Stability... 

Brandt, P. Haeffner, F. A DFT-derived model predicts solvation-dependent configurational stability of organolithium compounds a case study of a chiral a-thioallyllithium compound. /. Am. Chem. Soc. 2003, 325, 48-A9. 

Boche, G., Opel, A., Marsch, M., Harms, K., Haller, F., Lohrenz, J. C. W., Thuemmler, C., Koch, W. a-Oxygen-substituted organolithium compounds calculations of the configurational stability and of LiCH20H model structures, crystal structure of... 

Benzyl-type organolithium compoimds are stabilized by the mesomeric interaction of the negative charge of the carbanionic center with the aromatic n-system. The thermodynamic acidity of the benzylic compounds is 10 to 15 pK units higher than of the corresponding methyl derivatives [127]. Mesomeric stabilization requires a considerable flattening of the carbanionic center towards sp -hy-bridization [128]. As a consequence, the configurational stability is decreased, and - with few exceptions - most of the benzyllithium derivatives suffer from rapid racemization [Eq. (56)],see Sect. 3.2. 

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