Arenechromium tricarbonyls directed lithiation

Some headway has been made using sulphoxides to direct the lithiation of arenechromium tricarbonyls in the manner of Kagan s work with ferrocenes . Diastereoselectiv-ities in the lithiation-quench of 392 are excellent, though yields are poor with most electrophiles. Diastereoselectivity reverses on double lithiation, because the last-formed anionic site in 394 is the most reactive (Scheme 163). 

A problem with (—)-sparteine 362 is its lack of availability in both enantiomeric forms. Reversed selectivity in the generation of planar chirality has been achieved by second lithiations (see Schemes 163 and 171) and a remarkable modification of this strategy works with arenechromium tricarbonyls. By using excess BuLi (sometimes f-BuLi is required) in the presence of sparteine 362, a doubly lithiated species 450 may be formed from 448. The formation of the doubly lithiated species may be confirmed by double deuteriation with excess D2O. However, other electrophiles react selectively only once and give products of opposite absolute stereochemistry from those formed after monolithiation, if in rather low yield. Presumably, the first lithiation, which is directed by (—)-sparteine, produce an organolithium 448 whose complexation with (—)-sparteine remains favourable. The second lithiation must produce a less stable organolithium—one which cannot form a... 

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