Titanium Ate Enolate Complexes: An NMR Study
Titanium Ate Enolate Complexes: An NMR Study
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DOI：
10.1021/jo00092a033
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发表时间：
1994-07
影响因子：
3.6
通讯作者：
A. Bernardi;M. Cavicchioli;C. Marchionni;D. Potenza;C. Scolastico
中科院分区：
文献类型：
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作者：
A. Bernardi;M. Cavicchioli;C. Marchionni;D. Potenza;C. Scolastico
据报道，锂烯醇化物与Ti(OiPr) 4 在THF中于40℃下反应产生所谓的钛酸烯醇化物络合物。 1 人们发现这些配合物表现出与母体烯醇锂明显不同的反应性。 Reetz 和 Thornton 描述了它们在立体选择性羟醛缩合中的应用。 1· 2 最近，我们已经证明它们以 1, 4 的方式向不饱和羰基化合物进行区域选择性和立体选择性添加，并比其锂对应物具有多种优势。 3 特别是，酮烯醇化物 Ti 配合物被证明比母体 Li 烯醇化物具有更高的区域选择性，甚至可以对不受阻碍的 9-烯酮进行高度选择性的 1, 4 加成。从立体化学的角度来看，反应结果取决于起始材料的构型：当添加到 E 烯酮时，E 烯醇化物产生顺式化合物，Z 烯醇化物产生反异构体（方案 1）。母体锂烯醇化物也表现出相同的趋势，但其立体选择性较低。尽管钛酸烯醇化物配合物的合成价值已得到证明，但对其结构知之甚少。大多数研究人员一致认为，锂烯醇化物与 Ti (OiPr) 4 的反应不太可能导致其中一个烷氧基配体被烯醇化物取代，而是形成五配位钛物质。然而，文献中没有关于反应产物的结构数据。第二个重要问题涉及烯醇化物的构型以及它在与 Ti (OiPr) 4 反应期间是否保守。对于 ClTi-(OiPr) s 的相关案例，Thornton 提供了一些证据，表明钛化过程中烯醇化物异构化的可能性不大。 4 然而，Ti ate 烯醇化物与氯硅烷的极低反应性阻碍了通过相应的烯醇硅烷通常测定其立体结构。当试图合理化羟醛缩合和迈克尔加成反应中获得的立体化学结果时，这显然是一个主要关注点。更好地了解这些配合物的结构也将有助于设计有效的手性
Reaction of lithium enolates with Ti (OiPr) 4 in THF at-40 C is reported to give rise to the so-called titanium ate enolate complexes. 1 These complexes have been found to display a reactivity markedly different from that of the parent lithium enolates. Their use in stereoselective aldol condensations was described by Reetz and Thornton. 1· 2 More recently we have shown that they add regio-and stereoselectively to unsaturated carbonyl compounds in a 1, 4 fashion and offer various advantages over their lithium counterparts. 3 In particular, ketone enolate Ti complexes proved to be more regioselective than the parent Li enolates, allowing highly selective 1, 4 addition even to unhindered,/9-enones. From a stereochemical point of view, the reaction outcome depends on the configuration of the starting materials: when adding to an E enone, E enolates give rise to syn compounds and Z enolates to the anti isomers (Scheme 1). The same trend is displayed by the parent lithium enolates, which, however, are less stereoselective.Although the synthetic value of titanium ate enolate complexes has been demonstrated, very little is known about their structure. Most researchersagree that the reaction of lithium enolates with Ti (OiPr) 4 is not likely to result in the substitution of one of the alkoxy ligands by the enolate, but rather in the formation of a pentacoordinated titanium species. However, no structural data are available in the literature on the reaction product. A second important question concerns the configuration of the enolates andwhether it is conserved during the reaction with Ti (OiPr) 4. For the related case of ClTi-(OiPr) s, Thornton presented some evidence indicating that enolate isomerization during the titanation process is unlikely. 4 However, the very low reactivity of Ti ate enolates with chlorosilanes has precluded the usual determination of their stereostructure via the correspond-ing enolsilanes. This is obviously a point of major concern when trying to rationalize the stereochemical results obtained both in aldol condensation and Michael addition reactions. A betterknowledge of the structure of these complexes would also be of help in designing efficient chiral