Metal 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. 

The classical aldol addition, which is usually run in protic solvents, is reversible. Most modern aldol methodologies, however, rely on highly reactive preformed metal enolates, whereby proton donors are rigorously excluded. As a consequence, the majority of recent stereoselective aldol additions are performed under kinetic control. Despite this, reversibility and, as a consequence, an equilibration of yrn-aldolates to a t/-aldolates by retro-aldol addition, should not be excluded a priori. 

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]. 

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)... 

The complex is additionally stabilised by co-ordination of the phenoxide, and possibly the carboxylate, to the metal ion, illustrating the utility of chelating ligands in the study of metal-directed reactivity. We saw in the previous section the ways in which a metal ion may perturb keto-enol equilibria in carbonyl derivatives, and similar effects are observed with imines. The metal ion allows facile interconversion of the isomeric imines. The first step of the reaction is thus the tautomerisation of 5.28 to 5.29 (Fig. 5-56). Finally, the metal ion may direct the hydrolysis of the new imine (5.29) which has been formed, to yield pyridoxamine (5.30) and the a-ketoacid (Fig. 5-57). 

Aldol reactions of both (E)- and (Z)-ketene acetals are highly susceptible to KOBuc catalysis. In the presence of 5 mol% of KOBuc, aldol reactions proceeded to completion within minutes at -78 °C < 1994JA7026>. A double-label crossover experiment, devised to probe the nature of the silicon group transfer in the alkoxide-catalyzed aldol reaction, suggested that free metal enolates are the true reactive species adding to the aldehydes. 

Several new catalytic asymmetric protonations of metal enolates under basic conditions have been published to date. In those processes, reactive metal enolates such as lithium enolates are usually protonated by a catalytic amount of chiral proton source and a stoichiometric amount of achiral proton source. Vedejs et al. reported a catalytic enantioselective protonation of amide enolates [35]. For example, when lithium enolate 43, generated from racemic amide 42 and s-BuLi, was treated with 0.1 equivalents of chiral aniline 31 followed by slow addition of 2 equivalents of ferf-butyl phenylacetate, (K)-enriched amide 42 was obtained with 94% ee (Scheme 2). In this reaction, various achiral acids were... 

Over the past 30 years, the widespread use of Sml2 in organic synthesis has brought the chemistry of Sm(III) enolates to the fore as many processes using the reagent involve the formation and reaction of these organometallic species. Although Sm(III) enolates can be exploited as nucleophiles in a number of reactions, their reactivity is still poorly understood and they appear to behave differently to more conventional metal enolates for example, there are few... 

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... 

Interest in zinc enolates , compared to alkaline or alkaline earth enolates, is growing due to their ideal combination of reactivity and tolerance of many functional groups. The form and structural motifs of complexes depend on the type of metal, solvents and bulkiness of the ligands. Generally, enolates of electropositive metals prefer O-coordination however, even C-coordinated metal enolates usually rearrange to O-bonded ones. 

Although several research groups have been interested in transition metal enolates to use the metal centre as a potential site of asymmetry in the design of chiral catalysts, examples of well defined redox reaction involving middle to late transition elements and lanthanides are scarce in the literatnre. Based on Pearson s theory of hard and soft acids and bases", it has been proposed that combining a hard ligand with a soft late transition metal centre may lead to weak metal-heteroatom links, resnlting in reactive late metal-heteroatom bonds. 

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

Many metal enolate a-hydroxylations have been performed with racemic fra r-2-phenylsulfonyl-3-phenyloxazir-idine 33 because it is more reactive than the enantiomerically pure camphor-derived A -sulfonyloxaziridines (Equation 11). Examples are given in Table 20. 

This, and many other results (80-88), led us to the problem of the dual reactivity of metal enolates. We prepared (89) the simplest enolates such as CH2=CHOLi and CH2=CHONa by reacting the mercury derivative of acetaldehyde with the respective metal. 

Dual reactivity of metal enolates had been known for years, e.g., for the sodium derivative of acetoacetic ester. It was rationalized in two essentially different ways (1) first, the enolates were thought to be tautomeric [Hiickel (90)] (2) later they were supposed [Arndt, Eistert (91)] to be ionic and to exist as mesomeric ions... 

In recent organic synthesis, stereoselective aldol condensations has been performed under two different conditions. Under the influence of acid, stabilized enol derivatives, enolsilanes (M = SiMe3), can condense with aldehydes or acetals in a stereoselective fashion [Eq. (12)]. In this reaction the role of the acid is to activate aldehydes or acetals. Alternatively, under basic conditions, the same process can be carried out directly with aldehydes and reactive, preformed metal enolates (M = Li, MgL, ZnL, AIL2, BL2, etc.) of defined geometry. 

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