Solution Structures of Lithium Amino Alkoxides Used in Highly Enantioselective 1,2-Additions

Lithium
ephedrates and norcarane-derived lithium amino alkoxides
used to effect highly enantioselective 1,2-additions on large scales
have been characterized in toluene and tetrahydrofuran. The method
of continuous variations in conjunction with 6Li NMR spectroscopy
reveals that the lithium amino alkoxides are tetrameric. In each case,
low-temperature 6Li NMR spectra show stereoisomerically
pure homoaggregates displaying resonances consistent with an S4-symmetric cubic core rather than the alternative D2d core. These assignments
are supported by density functional theory computations and conform
to X-ray crystal structures. Slow aggregate exchanges are discussed
in the context of amino alkoxides as chiral auxiliaries.