Stereochemical aspects of enantiopure and racemic organolithium aggregates Li4Ar4 {Ar = C6H4[CH(Me)NMe2]-2}

The compounds (R)- and rac-[1-(dimethylamino)ethyl]benzene, (R)-1 and rac-1, respectively, react quantitatively with tBuLi in a 1: 1 molar ratio in pentane to give the corresponding (R)- and rac-[Li-4{C6H4[CH(Me)NMe2]-2}(4)], (R)-2 and rac-2, respectively. The same reaction with nBuLi afforded a 1:1:1 aggregate of nBuLi, 2, and 1 according to the results of quench experiments with H2O and D2O. The H-1, C-13, and Li-7 NMR spectra, as well as the single-crystal X-ray structure, reveal that (R)-2 has a unique self-assembled tetranuclear structure of two diastereoisomeric dimeric units both in solution, in non-coordinating hydrocarbon solvents, and in the solid state. Detailed information about rac-2, which, in contrast to the high solubility of (R)-2, is essentially insoluble in hydrocarbon solvents, was provided by C-13 CP/MAS NMR spectra. We conclude that rac-2 is a self-assembled tetra-nuclear aggregate comprising a combination of two dimeric units, but in this case they are derived from the (R)- and (S)-enantiomers, respectively. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).