NMR studies of chiral lithium amides with phosphine chelating groups reveal strong Li-P-interactions in ethereal solvents

Chiral lithium amides with a chelating diphenylphosphine group have previously been found to mediate excellent enantioselectivity in the asymmetric addition of alkyllithiums to benzaldehyde. NMR studies reveal for the first time that chiral lithium amides can form chelating dimeric complexes with P-Li interactions in both diethyl ether and THF. The two lithium atoms in the dimer are found to be non-equivalent, one of them is coordinated to two phosphines, detected by Li,P-couplings, while the other lithium is only solvent bound. Slow ethereal ligand exchange on the NMR time scale below –50 °C has been detected by C NMR. Excess of n-butyllithium generates mixed dimeric complexes in both diethyl ether and THF.