Arenechromium tricarbonyls diastereoselective

Diastereoselective lithiation of chiral arenechromium tricarbonyl complexes... 

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). 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

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