Alkyllithium- -sparteine complexes asymmetric deprotonation

The crucial reagents for the synthesis of the enantioemiched lithium compound (iS)-2 are (-)-sparteine complexes of simple alkyllithium bases. P. Beak and cowokers showed that f-BuLi and i-PrLi complexed by (-)-sparteine could be efficiently used for an asymmetric deprotonation reaction, whereas complexes of the chiral amine and t-BuLi or n-BuLi showed hardly any stereoselectivity or reactivity (Scheme 3) [2]. 

In solution [7] and solid state (-)-sparteine-coordinated i-PrLi exists as an unsymmetric aggregate containing two lithium alkyls [5] this dissociates during the reaction under formation of a precoordinated complex. Apart from j-BuLi, j-PrLl is the only known simple alkyllithium compound which can, in combination with (-)-sparteine, efiSciently be used for asymmetric deprotonation of iV-Boc-pyrrolidine [2]. 

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