Structure metal enolates

One of the first careful studies of the influence of chirality proximal to ketone enolates is illustrated in eq. [95] (113). Condensation of the enolate 126 (M = Li) with propanal (THF, -100 C) afforded a modest bias for the (5,i )-diastereomeric aldol adduct 127 (127 128 = 57 43). The influence of the metal center in this condensation has recently been examined. The boryl enolate 126 [M = B(n-C4H9)2l afforded a ratio 127 128 = 64 36 in pentane (-78°C) (6a, 113). Similar studies designed to probe the dependence of diastereoface selection on metal enolate structure have been carried out with metal enolates 129 (eq. [96], Table 32). 

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 . 

C02-Bridged bimetallic zirconocene complexes have been formed from 1 and metallocarboxylic acids [229]. Reachon of 1 with metal enolates Cp(CO)3WCHR COX (X = OEt, Me, Ph) gives Cp(CO)3WCH(R )CH(R)OZrCp2(Cl). The structure for R = H and R = Me was solved by an X-ray analysis and the chemical reactivity of these organometallic products have been studied [230]. 

Cp 2Sm(jU-H)]2, (188), affords very high-molecular-weight PMMA with very low polydispersities (typically < 1.05).453-456 At — 95 °C the polymer formed is highly syndiotactic (95% rr triad). Isolation and X-ray analysis of (189), the 1 2 complex of (188) and MMA, provides strong support for the participation of a metal-enolate as the active site. (189) behaves in an identical manner to the hydride precursor, converting 100 equivalents MMA to polymer with Mn= 11,000 and Mw/Mn= 1.03.457 The successful structural characterization of (189) provides support for intermediates proposed earlier.458,459... 

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

Scheme 8), which also control the enolate stereochemistry in amide systems. The influence of metal ion structure on the stereochemical outcome of the aldol process again underscores the importance of metal ligand effects in the enhancement of aldol stereoselection. 

In cyclopropane carboxylates the ring strain influences the acidity of the a-carbon, thus the enolates are more difficult to prepare and once made, are more reactive than in the higher-inem-bered rings. These enolates probably do not have an enolate structure, but rather are a-metal-lated species. 

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]

Preliminary fluorination experiments using optically active A -fluoro compounds ( —)-2b and ( + )-2c show that there is reaction with various metal enolates (Table 16) generated under standard reaction conditions to give the anticipated a-fluoro carbonyl compounds with enantiomeric excesses depending strongly on the structure of the metal enolate.119... 

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)... 

Current understanding of the reaction suggests that an unprecedented mechanism is operating. Unlike in classical Lewis acid catalysed reactions [28], the metal complex does not activate the carbonyl moiety but is understood to enhance the degree of enolisation and thus create the necessary nucleophilic enol structure for reaction with the fluorinating agent [29]. 

In Figure 13.1, the enolate structures are shown with the charge on the heteroatom and with the heteroatom in association with a metal ion. The metal ion stems from the reagent used in the enolate formation. In the majority of the reactions in Chapter 13, the enolate is generated by deprotonation of C,H acids. The commonly employed bases contain the metal ions Li , Na , or K . Therefore, in Chapter 13, we will consider the chemistry of lithium, sodium, and potassium enolates. 

The position of the lithium cation in the enolates has been the object of much debate. It is now well established that the cations of the strongly electropositive metals of groups I, II and III stand closer to the oxygen than to the carbon atom while this metalotropy is more balanced with transition metal enolates. The structure of the lithium enolates in vacuum, in the solid state as well as in solution is discussed in detail in the next section of this chapter. 

Mixed-metal enolate has also been found. The core of the lithium sodium enolate of pinacolone forms an open stack structure in which two edges of the face-sharing cubes are absent. 

Metal enolates are widely used as building blocks in modem organic synthesis. A thorough understanding of their structure and reactivity is important, particularly since many of these... 

Metal enolates of carbonyl compounds are important nucleophiles in C—C bondforming reactions for the synthesis of nonfluorinated compounds. However, the metal enolates of fluorinated carbonyl compounds have been severely limited to a-F metal enolates, which can be stabilized by chelate structures containing the M—F moiety. In sharp... 

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