Two aspects of the desymmetrization of selected prochiral aromatic or vinylic dihalides: enantioselective halogen-lithium exchange and prochiral recognition in chiral liquid crystals

Several classes of prochiral dihalides have been identified as potential candidates for asymmetric halogen-lithium exchange. Bis-aryl compounds I and 2, 2,3-dibromonorbornadiene 3, and 1,2-diiodoferrocene 4 were prepared and used as substrates in the asymmetric halogen-lithium exchange reaction. The enantioselective mono-lithiation was achieved in good yields by various combinations of n-BuLi and chiral ligands. Enantioselectivities in the range of 20-35% ee have been detected for this new type of asymmetric synthesis. The prochiral compounds 3 and 4 were also dissolved in a chiral liquid crystal based on organic solutions of poly-gamma-benzyl-L-glutamate and analyzed using natural abundance deuterium 2D NMR spectroscopy. The spectroscopic discrimination of enantiotopic sites in these solutes has been observed and discussed from the point of view of orientational order. (C) 2008 Elsevier Ltd. All rights reserved.