Enolates of Alkali and Alkaline Earth Metals

In view of the enormous chemical diversity of the enolate-bound metals that cover the range in the periodic table from lithium to tin, it is not surprising to realize enormous differences in enolate structures. As a consequence, structural studies have to answer very different questions concerning mainly the type of bonding, stereochemistry, the role of ligands at the metal, aggregation, and solvation. 

Modern Enolate Chemistry From Preparation to Applications in Asymmetric Synthesis, First Edition. Manfred Braun. 

Koten s group and revealed a tetrameric structure of the lithium enolate of 2-(dimethylamino)methylacetophenone, wherein the metal obtains a fourfold coordination by chelation of the amino group [6]. 

The aggregate 2 features a ladder structure, wherein two of the lithium atoms coordinate to the oxygen atoms of the silyloxy groups despite the latter s steric bulk [9]. 

The successive replacement of the enolate by aldolate moieties en route from an enolate tetramer to an aldolate tetramer might involve severe reconstruction of the skeleton. Thus, a mixture of pinacolone, its lithium aldolate with pivalaldehyde, and the enolized aldolate dianion was recently reported to cocrystallize in a 1 1 3 ratio as a heptalithium cage compound missing any cube-shaped unit [15]. Rather complex crystal structures aldolate-enolate aggregates were also found for calcium enolates [8b]. 

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We recently disclosed that salts of alkali and alkaline earth metals effectively promote fhe aldol reaction of DMS enolates (Scheme 10.34) [101]. For example, fhe CaCl2-catalyzed reaction of propiophenone DMS enolate with aldehydes proceeds smoothly in DMF at 30 °C with high reaction efficiency. In the metal salt-catalyzed aldol reaction fhe counter anion of fhe metal salt plays a crucial role in rate acceleration. The activity of metal salt increases with increasing intrinsic nucleophilicity of fhe counter anion TfO metal salts work as Lewis bases to activate DMS enolates. To our surprise, the... 

The coordination of /3-diketonates and related species to alkali and alkaline earth cations has long been recognized. Combes first prepared Be(acac)2 in 1887,255 and the contribution of Sidgwick and Brewer concerning the nature of dihydrated alkali metal /3-diketonates plays a seminal role in the development of this area of coordination chemistry.19 A review concerning the structure and reactivity of alkali metal enolates has been written, 6 and sections on alkali and alkaline earth /3-diketonates can be found within more generalized accounts of /3-diketone complexes.257,258... 

Alkaline Earth Metals. In the early literature, similar to alkali earth metals, alkaline earth metals were usually used to form the enolates, especially magnesium enolates, in the aldol reaction (128,129). Inspired by heterogeneous MgO solid base catalysis (130), alkaline earth metals have been applied successfully as homogeneous catalysts in the aldol reaction (Scheme 23). In 1998, Corey and co-workers described a new magnesium-catalyzed diastereoselective anti-aldol reaction as a key step for the total synthesis of lactacystin (131). The metal requirement is strict, since several salts were tested, for example, TiCl4, SnCL, ZnCl2, and Cu(OTf)2, only Mgl2 functioned as an effective catalyst (Scheme 24). 

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