Chiral organometallic reagents. Part XVII. Formation of diastereoisomeric complexes between α-phenylselanylalkyllithium compounds and chiral diamines

Abstract

The racemic α-phenylselanylalkyllithium compound 6 is monomeric in diethyl ether and forms diastereoisomeric complexes with a variety of chiral diamines. Diastereoisomer ratios were determined from ⁷⁷Se 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 dm³ mol^–1 and > 90 dm³ mol^–1. The diastereoisomeric complexes 24 epimerize at the lithium bearing stereocentre with a barrier of ΔG^‡= 12.1 ± 0.3 kcal mol^–1 at –4 °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.