1.3- Diketones metal enolates

The decarboxylation of allyl /3-keto carboxylates generates 7r-allylpalladium enolates. Aldol condensation and Michael addition are typical reactions for metal enolates. Actually Pd enolates undergo intramolecular aldol condensation and Michael addition. When an aldehyde group is present in the allyl fi-keto ester 738, intramolecular aldol condensation takes place yielding the cyclic aldol 739 as a main product[463]. At the same time, the diketone 740 is formed as a minor product by /3-eIimination. This is Pd-catalyzed aldol condensation under neutral conditions. The reaction proceeds even in the presence of water, showing that the Pd enolate is not decomposed with water. The spiro-aldol 742 is obtained from 741. Allyl acetates with other EWGs such as allyl malonate, cyanoacetate 743, and sulfonylacetate undergo similar aldol-type cycliza-tions[464]. 

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... 

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 transition d-metal enolate complexes generally exhibit no luminescence, and as a consequence there are few studies reported in the literature about the photoluminescence properties of these compounds. Indeed, the d-metal -diketonate complexes are... 

Three main types of redox reactions of keto compounds leading to the formation of metal enolates have been reported (i) two-electron reduction of diketones or a,(f-unsaturated ketones or esters (equation 1), (ii) oxidative addition reactions (equation 2) and (iii) threefold deprotonation of diketoamines followed by a two-electron oxidation of the trianion by the metal (equation 3). 

Stable metal complexes can be favorably formed when a bidentate metal-binding site is available, such as a- and -diketone moieties which are the tautomeric forms of a- and /3-ketoenols. Some /S-diketonate complexes of paramagnetic lanthanides such as Pr(III), Eu(III) and Yb(III) have been extensively utilized as paramagnetic shift reagents for structural assignment of molecules with complicated stereochemistry prior to 2D techniques in NMR spectroscopy. Their syntheses and application are discussed in separate chapters in this volume. The examples below provide some dynamic and structural basis for better understanding of metal enolates in biomolecules and biochemical processes. 

Metal enolates found varied application in chemical analysis. An outstanding group are certain lanthanide enolates used as shift reagents in NMR spectroscopy. The analytical methods discussed in Section IV are based on formation of a metal enolate for separation, detection, identification and determination of metal ions or the use of a metal enolate as ancillary reagent to improve analytical quality. Of special relevance in analytical chemistry are the metal /3-diketonates, M(dik) , derivatived from deprotonated /3-diketones (dikH),... 

This part of the chapter gives an overview of different types of metal enolates, mainly -diketonates, with emphasis on the properties that can be of interest in analytical applications. The analytical applications of rare earth /3-diketonate complexes were reviewed . 

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. 

This section describes the formation of metals and metal alloys from metal enolate precursor sources (/3-diketonates, /3-ketoiminates) where chemical vapor deposition, spin coating and electrochemical deposition processes are used to generate blanket, surface selective or patterned surface structures. 

Recently, phosphane and phosphite copper(I) carboxylates have become favored over copper(I) / -diketonates, since these species produce copper films of high purity with excellent electrical properties at low temperatures. However, at the time this review is being written Cu(I) / -diketonates are commercially available, e.g. Cu(hfac)( -ViSiMe3) (lOo, CupraSelect ) and Cu(hfac)(mhy) (10k, Gigacopper ), making this class of metal enolate precursors still the most important in industrial applications. 

In contrast to metal enolates, enol silyl ethers are covalent compounds. It is easier to accommodate two or more enol silyl ether structures within the same molecule, whereas it would be more difficult to generate the corresponding enolate structures because of their strongly basic and reactive characters. 2,3-But-anedione has been converted to 2,3-bis(trimethylsiloxy)butadiene by a number of methods the best appears to be TMS triflate and EtiN in benzene. Likewise, 1,3-diketones have been converted to the... 

Condensation Reactions. Traditionally, intermolecular aldol condensation reactions have been performed under equilibrating conditions using weaker bases than r-BuOK in protic solvents. Since the mid-1970s, new methodology has focused on directed aldol condensations which involve the use of preformed Lithium and (jroup 2 enolates, (Troup 13 enolates, and transition metal enolates. Although examples of the use of f-BuOK in intramolecular aldol condensations are limited, complex diketones... 

Diketones can be synthesized by treatment of metal enolates with AcCl. O-Acetylation is often a significant side reaction, but the amount can be minimized by choosing a counterion that is bonded covalently to the enolate such as copper or zinc, and by using AcCl rather than AC2O. Proton transfer from the product p-diketone to the starting enolate is another common side reaction. Alternative procedures for effecting C-acetylation... 

Because the stmcture of 1,3-diketones comprise a methylene group between two activating carbonyls, equiUbrium is shifted toward the enol form. The equihbrium distribution varies with stmcture and solvent (303,306) (Table 13). The enol forms are cycHc and acidic and form covalent, colored, soHd chelates with metals ... 

A modification of the K-R reaction was introduced by Mozingo. This method involved reacting an o-hydroxyacetophenone with an ester in the presence of metallic sodium to form a 1,3-diketone. Treatment of the diketone with an acid then delivered the chromone via an intramolecular cyclization reaction. This method was applied to the preparation of 2-ethylchromone (21). 0-hydroxyarylketone 22 was allowed to react with ethyl propionate (23) in the presence of sodium metal.The resulting sodium enolate was then quenched with acetic acid to deliver the 1,3-diketone 24. Upon heating 24 in glacial acetic acid and hydrochloric acid, 2-ethylchromone (21) was delivered in 70-75% overall yield. 

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