Enolates reactivity

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

Enolate reactivity depends on the electrophile. Enolates generally form CC bonds with carbon electrophiles, and OSi bonds with silicon electrophiles. 

Enolate Reactivity at the Anomeric Site Based on Ester Stabilization... 

The enolate reactivity associated with this approach to C-glycoside synthesis was, however, first developed with octulosonic acid derivatives, such as 283 [114], 284 [115], and 285 [116], and a series of examples involving aldehyde and halide-based electrophiles are shown in Scheme 74. Related studies involving stereoselective protonation of this class of exocyclic enolate have also been described [13] and Scheme 75 illustrates this with an example of reductive samariation (using ethane-1,2-diol, but no HMPA) of an anomeric acetate 286 [117]. 

Carbon Versus Oxygen Alkylation and Enolate Reactivity... 

Highly coordinated tin enolates, reactivity, 9, 362 High-molecular weight polystannanes, preparation and properties, 12, 381... 

However, as the C-6 position is a a-carbonyl position, the preparation of various penem-based antibiotics becomes easily accessible with the aid of enolate reactivity. The example depicted in Scheme 33 is a mild and efficient method for the preparation of l-hydroxyalkyl(aryl) penems 59 from 6 -brornopcncrns <2004JOC5850>. 

Now for a reaction with nitrogen as an electrophile that illustrates enol reactivity and reminds us that tautomerism applies to functional groups other than the carbonyl. Let us suppose you have a... 

To facilitate an analysis of enolate reactivity and as an aid to the rationalization of the stereochemical outcome of the aldol reaction, a consideration of the enolate structure is necessary. For convenience, the following classification of transition metal and lanthanide metal enoiates will be used here T) -0-bound metal enoiates (1) of carbonyl compounds T) -C-bound metal enoiates (2) and ij -metal enoiates (3) of... 

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