Solution structures of chiral lithium amides with internal sulfide coordination: sulfide versus ether coordination in chiral lithium amides

Abstract

A series of chiral lithium amides with chelating sulfide groups has been investigated by NMR spectroscopy. These chiral lithium amides have previously been found to mediate higher enantioselectivity in the asymmetric addition of alkyllithium reagents to aldehydes than the corresponding lithium amides containing ether groups. The chiral lithium amido sulfide chelates form homodimers in both diethyl ether and tetrahydrofuran. In diethyl ether both sulfur atoms coordinate to the same lithium while the other lithium in the dimer is solvent coordinated. Tetrahydrofuran favours equivalently solvated homodimers. Mixed dimeric complexes are formed with excess n-butyllithium in both diethyl ether and tetrahydrofuran. The structural information was obtained from observed 6Li, 15N couplings and cross peaks in the 6Li, 1H HOESY spectrum. DFT calculations at B3LYP/6-31G(d) level of theory indicate that the Li–S chelate is much less stable than the Li–O chelate. However the calculations indicate that the stability of the dimeric chelates are in agreement with the NMR results of the diethyl ether and tetrahydrofuran solutions.