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

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• 作者：Angela M. Bruneau ; Lara Liou ; David B. Collum
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：February 19, 2014
• 年：2014
• 卷：136
• 期：7
• 页码：2885-2891
• 全文大小：467K
• ISSN：1520-5126

文摘

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 Sb>4b>-symmetric cubic core rather than the alternative Db>2db> 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.