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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 <sup>6</sup>Li NMR spectroscopy
reveals that the lithium amino alkoxides are tetrameric. In each case,
low-temperature <sup>6</sup>Li NMR spectra show stereoisomerically
pure homoaggregates displaying resonances consistent with an <i>S</i><sub>4</sub>-symmetric cubic core rather than the alternative <i>D</i><sub>2<i>d</i></sub> 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