Godenschwager Peter F, Collum David B
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca, New York 14853-1301, USA.
J Am Chem Soc. 2007 Oct 3;129(39):12023-31. doi: 10.1021/ja074018m. Epub 2007 Sep 12.
Enolizations of 2-methylcyclohexanone by lithium hexamethyldisilazide (LiHMDS) in the presence of three chelating ligands--trans-N,N,N',N'-tetramethylcyclohexanediamine, N,N,N',N'-tetramethylethylenediamine, and dimethoxyethane--reveal an approximate 40-fold range of rates. NMR spectroscopic analyses and rate studies reveal isostructural transition structures based on monomeric LiHMDS for the diamines. Rate studies of LiHMDS/dimethoxyethane-mediated enolizations implicate a substantial number of monomer- and dimer-based mechanisms. The rate laws vary for the three ligands because of ligand-dependent structural differences in both the reactants and the transition structures. The importance of LiHMDS-ketone complexes and the role of hydrocarbon cosolvents are discussed.
在三种螯合配体——反式-N,N,N',N'-四甲基环己二胺、N,N,N',N'-四甲基乙二胺和二甲氧基乙烷存在的情况下，用六甲基二硅基锂胺（LiHMDS）对2-甲基环己酮进行烯醇化反应，反应速率范围约为40倍。核磁共振光谱分析和速率研究表明，二胺类反应基于单体LiHMDS具有同构的过渡结构。对LiHMDS/二甲氧基乙烷介导的烯醇化反应的速率研究表明，存在大量基于单体和二聚体的反应机制。由于反应物和过渡结构中存在依赖配体的结构差异，三种配体的速率定律各不相同。文中讨论了LiHMDS-酮配合物的重要性以及烃类共溶剂的作用。
