We have shown by the use of multinuclear and multidimensional NMR that
the reaction mixture of [Li-6]lithium methoxy-(R)-1-phenylethyl)((S)-
1-phenylethyl)amide (2) and cyclohexene oxide in DEE results in the fo
rmation of monomeric and dimeric complexes between 2 and cyclohexene o
xide at -80 degrees C. The dimeric complex of 2 exhibited a slow cyclo
hexene oxide substitution rate on the NMR time scale that was found to
be controlled by a dissociative mechanism. The Li-6,H-1-HOESY NMR spe
ctrum of the above reaction mixture showed NOE's between lithium and c
yclohexene oxide protons in both the monomer and dimer complexes of 2.
A single-crystal X-ray diffraction experiment revealed that the solid
state structure of a, crystallized from THP, is dimeric with a planar
Li2N2 core constructed from amido nitrogen bridges. Both methoxy grou
ps coordinate terminally to one lithium atom, Li(1), making it four-co
ordinated, while the other lithium, Li(2), binds to one THF and thus i
s three-coordinated. The dilithiated amide, [Li-6]lithium nylethyl)(2-
([Li-6]lithio)-(S)-1-phenylethyl)amide (4) was obtained by reaction of
1 equiv of [Li-6]-n-BuLi with 2 in DEE at 25 degrees C for 3-5 h. The
lithium amide 4 aggregates, giving a tetrameric lithium core consisti
ng of two molecules of 4. A titration study of 4 in DEE with THF also
showed that 4 exhibited slow ligand substitution rates on the NMR time
scale. The dimer 4 exhibited a slow cyclohexene oxide substitution ra
te that was fecund to be controlled by an associative mechanism. The l
ithium amide 2 has been used for the deprotonation and ring opening of
cyclohexene oxide, giving an ee of 47% of the (R)-cyclohexen-1-ol. Th
e deprotonation of cyclohexene oxide using 4 gave on the other hand th
e (S)-cyclohexen-1-ol with an ee of 41%. The rate of reaction was foun
d to be initially 1.5 times faster using 4 compared to using a in DEE.
This is probably due to the fact that the deprotonation of cyclohexen
e oxide is performed by the carbanionic carbon base and not by the lit
hium amide base in tt, The lithium salt )-1-phenylethyl)((R)-1-phenyl-
2-methylpropyl)amine (5) was prepared, as a potential reagent for the
deprotonation of cyclohexene oxide, from our NMR studies of lithium am
ide reagent-substrate complexes. The ee increased in the deprotonation
reaction from the previous 47% to 74% in the (R)-cyclohexen-1-ol.