Electrophilic substitution 1 - -sparteine complexes

Beak and coworkers found the (—)-sparteine-complex of iV-Boc-Af-(p-methoxyphe-nyl)benzyllithium 244, obtained from 243 by deprotonation with n-BuLi/(—)-sparteine (11) in toluene, to be configurationally stable (equation 57) . On trapping 244 with different electrophiles, the substitution products 245 are formed with high ee. Efficient addition reactions with imines and aldehydes have also been reported. The p-methoxyphenyl residue is conveniently removed by treatment with cerinm ammoninm nitrate (CAN). 

The formation of diastereoisomerically pure complexes of 90 with (-)-sparteine is also controlled by crystallisation. Treatment of the indene 89 with BuLi and (-)-sparteine in ether gives, on warming, a yellow precipitate which reacts with carbonyl electrophiles to provide the products 91 typically with good regioselectivity and >95% ee.52 An X-ray crystal structure proved the stereochemistry of the intermediate complex to be that shown as 90b, and hence proved the stereochemical course of the substitution (see section 6.1). The complex is readily decomposed by THF, in the presence of which it rearranges to a racemic V allyllithium. 

Although early studies by Nozaki examined (-)-sparteine 2 in the asymmetric lithiation of isopropylferrocene (as noted in Sect. 1.1 above), the first enantioselective generation of planar chirality in good ees using an organolithium (Clay-don, in this volume) was reported by Uemura in 1994 in the lithiation of tricar-bonyl(q -phenyl carbamate)chromium complexes using chiral diamines. After quenching with electrophiles, enantioenriched (o-substituted phenyl car-bamate)chromium complexes were obtained in up to 82% ee (Scheme 19) [61]. 

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