THE CONFIGURATIONAL STABILITY OF CHIRAL LITHIO ALPHA-AMINO CARBANIONS - THE EFFECT OF LI-O VS LI-N COMPLEXATION

alpha-Lithio pyrrolidine, as its N-t-BOC or formamidine derivative, has been generated in high enantiomeric purity via Sn-Li exchange of the enantiomerically enriched (88-95%) alpha-tributylstannane derivative, 4. The alkylation of these lithio carbanions gave 2-methyl pyrrolidines as well as providing a measure of their configurational stability. It was found that the alpha-lithio formamidines were configurationally more stable than the t-BOC derivatives and this is attributed to the stronger O-Li bond which weakens the adjacent C-Li bond, thus allowing configurational decay to occur more readily. This is the first report wherein a difference is observed between O-Li-C and N-Li-C configurational stability.