CHIRAL ORGANOMETALLIC REAGENTS .17. FORMATION OF DIASTEREOISOMERIC COMPLEXES BETWEEN ALPHA-PHENYLSELANYLALKYLLITHIUM COMPOUNDS AND CHIRAL DIAMINES

The racemic alpha-phenylselanylalkyllithium compound 6 is monomeric in diethyl ether and forms diastereoisomeric complexes with a variety of chiral diamines. Diastereoisomer ratios were determined from Se-77 NMR spectroscopy to lie around 60:40 for most examples, but reached 90:10 with N,N,N',N'-tetramethylcyclopentane-1,2-diamine (22). The complexation constants for the formation of the diastereoisomeric complexes 24a and 24b formed from 6 with the latter ligand were estimated by NMR titration to be >800 dm(3) mol(-1) and >90 dm(3) mol(-1). The diastereoisomeric complexes 24 epimerize at the lithium bearing stereocentre with a barrier of Delta G(double dagger) = 12.1 +/- 0.3 kcal mol(-1) at -4 degrees C. As this epimerization process is not slower than the racemization of the uncomplexed alkyllithium compound 6, the complexes 24 equilibrate directly and do not have to dissociate into 6 in order to equilibrate.