Alkali metal enolates

The mechanistic considerations of a catalytic asymmetric Michael reaction suggest that the reaction of a alkali metal enolate derived from a malon-ate derivative with an enone should lead to an intermediary aluminum enolate. Is it possible that such an A1 enolate could then be trapped by an... 

Conversion of tight ion pairs into crown ether-separated ion pairs leads in many cases to increased basicity. For example, Dietrich and Lehn (1973) have shown that a homogeneous solution of sodium t-amyloxide in benzene is unable to deprotonate triphenylmethane, whereas the reaction occurs rapidly in the presence of [2.2.2]-cryptand [37]. In THF or diethyl ether, alkali metal enolates do not react with triphenyl- or diphenylmethane (Pierre et al.,... 

The present procedures illustrate general methods for the use of preformed lithium enolates5 as reactants in the aldol condensation6 and for the quenching of alkali metal enolates in acetic anhydride to form enol acetates with the same structure and stereochemistry as the starting metal enolate.7 The aldol product, [Pg.55]

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... 

Tetr 32 2979 (1976) (regiospecific preparation of ketone enolates and synthetic uses) 33 2737 (1977) (structure and reactivity of alkali metal enolates)... 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymerization of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkali metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Useful initiating systems for acrylate polymerization have been prepared from complexes of ester enolates with alkali metal alkoxides (104,105). 

Titanium enolates. These enolates have generally been prepared by transmet-talation of alkali-metal enolates or silyl enolate ethers. Surprisingly, Evans et al. find that a titanium enolate can be prepared directly from the oxazolidinone 1 by reaction with TiCl4 (1 equiv.) in CH2C12 and shortly thereafter with ethyldiisopropyl-amine (or triethylamine) at 0°. The enolate may actually be an ate complex (2a)... 

Alkali metal enolates of ordinary ketones and aldehydes tend to fail to undergo the Tsuji-Trost allylation.37 However, it has been found that their enoxyborates and zinc enolates52 readily undergo the Pd catalysed allylation which proceeds with net retention just like the other examples of the... 

Silyl enolates undergo Pd-catalyzed arylation with aryl halides in the presence of an additive such as Bu3SnF,327 CuF2,328 or ZnF2.329 The arylation of silyl enolates has higher functional group tolerance than that of alkali metal enolates. The ZnF2-promoted arylation is valuable for efficient, stereoselective synthesis of a-arylated esters and imides (Equation (89)). 

Bidentate /3-diketonates usually have symmetric structure, and many crystal structures show sets of equal M—O, C—C and C—O bonds. Alkali metal enolate structures are symmetric as well as those for the Pd, Rh and A1 enolates. Few structures show unequal M—O distances these enolate complexes (M = Ge, Sn and Sb) are asymmetric as the metal atom is not located at equal distances from the nearest oxygen atoms . 

The reaction of [Cp (PMe3)M( -All)]+ (M = Rh, Ir) with alkali metal enolates has been demonstrated to be a reversible nucleophilic addition. There is a kinetic preference for addition to the central carbon of the allyl ligand to give the 3-(C-enolato)metallacyclo-butane 53. The thermodynamically more stable product is the lower valent metal-olefin jt complex, 54. 

Trichlorotitanium enolates are directly prepared from a ketone, TiCU, and a tertiary amine [122,123] and undergo aldol reactions with aldehydes [124-129], ketones [129], and imines [130,131], Intramolecular condensation with esters is also known [132-137], Although these reactions, based on a titanium enolate [16], which often results in high diastereoselectivity in aldol and related reactions [122], will not be discussed in detail in this article, the success of the alkylation of this titanium enolate with SNl-active electrophiles should be discussed owing to the high Lewis acidity of the metal center [123], Equation (37) shows stereoselective alkylation with an orthoacetate, which is usually inert to alkali metal enolates [138], Aminoalkylation of trichlorotitanium enolates with (a-chloroalkyl)amine has been performed analogously [139,140],... 

A precocious explanation of the complex role of alkali metal enolates was presented in a manuscript published in 1971. A paragraph from this paper is reproduced below. It represents the manuscript s authors explanation for the counterintuitive observation that more highly substituted (i.e. more sterically hindered) enolate anions undergo alkylation reactions faster than less highly substituted (i.e. less sterically hindered) enolates. 

A few comments concerning the crystallization of carbanions are in order. These comments are based upon the personal experience developed in our own laboratory and also upon observations noted in the literature in the course of crystallizing enolate anions. Although alkali metal enolate anions are relatively unstable compounds, they have been prepared in the solid state, isolated, and characterized by IR and UV spectroscopy in the 1970s. Thus the ot-lithiated esters of a number of simple esters of isobutyric acid are prepared by metallation of the esters with lithium diisopropylamide in benzene or toluene solution. The soluble lithiated esters are quite stable at room temperature in aliphatic or aromatic hydrocarbon solvents and are crystallized out of solution at low temperature (e.g. -70 °C.). Alternatively the less soluble enolates tend to precipitate out of solution and are isolated by centrifugation and subsequent removal of the solvent. Recrystallization from a suitable solvent can then be attempted. The thermal stability of the lithiated ester enolates is dramatically decreased in the presence of a solvent with a donor atom such as tetrahydrofuran. 

Fluorinations. Electrophilic fluorination of alkali metal enolates with (PhSOj) 2NF is facile. Benzylic a,a-difluoronitriles, sulfonates, and phosphonates are similarly prepared. Monofluorination of benzylphosphonates is achieved by prior silylation. ... 

Davis and coworkers have shown that A -sulfonyl oxaziridines are potent oxidants [741], In order to induce asymmetry, these authors prepared rigid, bulky oxaziridines bearing the bomane skeleton. Oxaziridines 2.82 (X = H, Cl or Me) are very interesting reagents for the asymmetric oxidation of alkali metal enolates to a-hydroxyketones [147, 742-746], In some cases, the presence of chlorine or methoxy substituents on reagents 2.82 is necessary to observe a high asymmetric induction [747, 748], Both enantiomers are available, and these chiral reagents may be recycled. 

Alkali Metal Enolates by Deprotonation of Carbonyl Compounds... 

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