Lithium carbenoids configurational stability

The question of configurational stability has been investigated first for vinylidene carbenoids and, more recently, for alkylcarbenoids. Vinyl anions are usually considered to be configurationally stable" ° the calculated inversion barrier of the ethenyl anion 10 (R = H) is about 35 kcal mol (equation 4)" . Concerning lithioalkenes, this configurational stability has been confirmed experimentally for a-hydrogen, a-alkyl and a-aryl substituted derivatives . The inversion of vinylidene lithium carbenoids was already... 

SCHEME 7. Test of the configurational stability of alkyl hthium carbenoids (a) fast equilibrium between enantiomeric lithium carbenoids (b) configurational stable hthium carbenoid 13... 

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. 

Hammerschmidt, F. Hanninger, A. Enantioselective deproto nation of benzyl phosphates by homochiral lithium amide bases. Configurational stability of benzyl carbanions with a dialkoxyphosphoryloxy substituent and their rearrangement to optically active a-hydroxy phosphonates. Chem. Ber. 1995, 328, 823-830. Avolio, S. Malan, C. Marek, I. Knochel, P. Preparation and reactions of functionalized magnesium carbenoids. Synlett 1999, 1820-1822. 

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