Enolate ions geometry

Phosphorylation of enolate ions by dialkyl or diaryl phosphorochlori-dates gives exclusive O-phosphorylation and it appears that the product geometry in acyclic systems is determined by the polarity of the solvent and... 

Stereochemistry of the spiroannelation was determined by geometry of the enolate ion which forces the ring methyl into an axial conformation as completion of the ring occurs trans to that methyl. 

A stereoconvergent reaction without any correlation between the geometry of the enolate and simple diastereoselectivity occurs when fluoride ions are used to induce an aldol addition of enolsilanes to aldehydes. For example, both a 99 1 and a 9 91 mixture of the following (Z)/( )-enolsilane lead predominantly to the formation of the. un-adduct in a highly selective manner, when the addition is mediated by tris(diethylamino)sulfonium difluorotrimethylsili-conate27,28. 

Whether or not the highly electropositive alkali metals or magnesium form an ionic instead of a covalent bond to the oxygen of the enolate is less important. Even if there is a contact ion pair of the metal cation and the oxygen anion, the geometry of the six-membered chair transition state, as outlined above, will be maintained. 

Much attention has been devoted to the examination of chiral enolate systems in which metal ion chelation may play an important role in establishing a fixed stereochemical relationship between the resident chirality and the enolate moiety. This has resulted in the conclusion that enolate geometry is critical in the definition of 7r-l acial selection. The following sections discuss this effort in several different chemical systems. 

The metallation should proceed via the formation of a chelated tetrahedral magnesium enolate complex, with a (Z)-geometry. The conformational rigidity would be enforced by chelation of both the imide enolate and bis(sulfonamide) ligand to the tetrahedral magnesium ion. 

The alkylation of ( )-spirolactones (34a) and (34b) with higher diastereoface selectivity has been modelled by geometry-optimized ab initio 4-31G calculations which suggest that approach of the electrophile occurs at an angle of ca 80° to the plane of the enolate and with some displacement away from the oxygen linked to the metal ion.41 Asymmetric a-methylation of phenylalanine derivatives has been achieved with 82% ee and retention of configuration in the absence of any external chiral source.42... 

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