Magnesium enolates structure

Synthesis, structure, and transformations of a-pyrone derivatives, viz., xanthy-rones, glaucyrones, and chelated magnesium enolates 98T8243. 

The enolate structure of 17 is deduced from the IR data of the reaction medium as a result of the presence of absorption bands at 1490 cm for the C=C bond and 1665 cm for the C=0 bond of the ester group, characteristic for an internal coordination of the enolate magnesium atom with the ester C=0 . 

All prepared magnesium enolates 17 are stable in refluxing diethyl ether. Deuteriation, and reactions with various electrophiles confirm their structure (see section HI). It is noteworthy that the lithiated carbanion-enolate analogue, directly obtained by deprotonation of an a-ketoester 18 with lithiated bases (LDA, for example), is not stable and immediately degrades in the medium, whatever the temperature. Comparatively, the magnesium chelate 17 shows a higher stability, which allows its preparation and synthetic applications. 

In addition to the structural effects due to the geometry of a substituted magnesium enolate, the stereochemistry of the reaction with a chiral aldehyde can be controlled, as described in equation 85. The aldol reaction based on the addition of magnesium enolate 56 to aldehyde 55 has been applied to the synthesis of monensin. The chiral center in the aldehyde induces the preferential approach of one diastereotopic face of the aldehyde by... 

Aldol-Type Addition. Aldol-type addition of the magnesium enolate of (R)-(+)-7-butyl 2-(p-tolylsulfinyl)acetate, prepared with 7-butylmagnesium bromide, with aldehydes and ketones afforded, after desulfurization with Aluminum Amalgam, p-hydroxy esters in very high diastereoselectivity (eq Two chiral centers are created in the first step with very high diastereoselectivity (mainly one diastereomer is formed). A model M based on the structure of the sulfinyl ester enolate (determined by C NMR) and on electrophilic assistance of magnesium to the carbonyl approach, was proposed to explain and predict the absolute configuration of the two created chiral centers. ... 

Before commencing this discussion, it is appropriate to consider briefly the issue of kinetic versus thermodynamic control in the reactions of preformed Group I and Group II enolates and to summarize the structure-stereoselectivity generalizations that have emerged to date. It is now welt established that preformed lithium, sodium, potassium and magnesium enolates react with aldehydes in ethereal solvents at low temperatures (typically -78 °C) with a very low activation barrier. For example, reactions can often be quenched within seconds of the addition of an aldehyde to a solution of a lithium enolate. ... 

The magnesium enolates generated by ECA using the copper-L24 catalytic system can be easily trapped with 1-alkyl-l-nitroolefins (Scheme 44) [76]. After the trapping, an in sim Nef reaction [77] takes place, generating the corresponding 1,4-diketones 22. These Michael adducts 22 can be then derivatised towards notable bicyclic structures, with relevance in natural products. 

An important modification of the aldol reaction involves the use of boron enolates. The boron enolates react with aldehydes to give aldols. A cyclic transition state is believed to be involved, and, in general, the stereoselectivity is higher than for lithium or magnesium enolates. The O—B bond distances are shorter than those in metal enolates, and this leads to a more compact structure for the transition state. This should magnify the steric interactions which control stereoselectivity. 

After almost half century of intensive, fundamental, and fruitful investigations of enolate structures, there is now clear evidence indicating that enolates of groups 1, 2, and 13 metals - lithium and boron being the most relevant ones - exist as the O-bound tautomers 1 the same holds in general for silicon, tin, titanium, and zirconium enolates [4]. Numerous crystal structure analyses and spectroscopic data confirmed type metalla tautomer 1 to be the rule for enolates of the alkali metals, magnesium, boron, and silicon [5]. 

At a glance, the descriptors Z and E might seem to be appropriate for O - metal-bound enolates like 6. Indeed, E/Z nomenclature causes no problems when the configuration of preformed enolates derived from aldehydes, ketones, and amides has to be assigned, because the O-metal residue at the enolate double bond has the higher priority. However, application of the E/Z descriptors to ester enolates leads to the dilemma that enolates with different metals but otherwise identical structures will be classified by opposite descriptors, as illustrated by lithium and magnesium enolates 9 and 10, respectively the former would have to be termed Z, and the latter E (Scheme 1.4). 

Figure 3.2 (a) Structure of dimeric THF-solvated lithium enolate of p-fluorophenyl benzyl ketone. Copied from Ref [7]. (b) Structure of a c/s-configured magnesium enolate of t-butyl ethyl ketone. Copied from Ref [8a]. 

The first diastereoselective and enantioselective allylic alkylation of cyclohexanone (through the magnesium enolate 18a) with diphenylallyl acetate 19a was reported in 2000 by Braun and coworkers [16a]. (7J)-BINAP (23) served as the optimum chiral ligand, and the alkene 20 was obtained as an almost pure diastereomer with an enantiomeric excess of 99% ee. The relative configuration was proven by the crystal structure analysis the absolute configuration was assigned unambiguously by chemical correlation. A first diastereoselective and enantioselective Tsuji-Trost reaction of a lithium enolate derived from... 

Lithium, magnesium, and aluminum enolates appear to afford comparable levels of kinetic aldol diastereoselection for a given enolate of defined structure. 

The increasing interest in enolization reactions mediated by magnesium amides led to new investigations for structural features of these reagents . 

Ketones and nitriles are rather soft bases their coordination onto electron-deficient sites on oxides is, therefore, relatively weak. One may, however, expect an improved specificity of chemisorption due to their softness. Unfortunately, however, these substances very easily undergo chemical transformations at oxide surfaces. Thus, carboxylate structures are formed on adsorption of acetone on alumina (194, 245-247), titanium dioxide (194), and magnesium oxide (219, 248, 249). Besides, acetone is also coordinated onto Lewis acid sites. A surface enolate species has been suggested as an intermediate of the carboxylate formation (248, 249). However, hexafluoroacetone also leads to the formation of trifluoroacetate ions (219). The attack of a basic surface OH ion may, therefore, be envisaged as an alternative or competing reaction path ... 

The key idea of the Zimmerman-Traxler model is that aldol additions proceed via six-membered ring transition state structures. In these transition states, the metal (a magnesium cation in the case of the Ivanov reaction) coordinates both to the enolate oxygen and to the O atom of the carbonyl compound. By way of this coordination, the metal ion guides the approach of the electrophilic carbonyl carbon to the nucleophilic enolate carbon. The approach of the carbonyl and enolate carbons occurs in a transition state structure with chair conformation. C—C bond formation is fastest in the transition state with the maximum number of quasi-equatorially oriented and therefore sterically unhindered substituents. 

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