0-Keto-enolates, metal

Divalent transition metal 3-keto-enolate complexes as Lewis acids. D. P. Graddon, Coord. Chem. Rev., 1969, 4,1-28(117). 

With an electrophilic transition metal complex, it is believed that the hydration of an alkyne occurs through a trans-addition of water to an 72-alkyne metal complex (Scheme 15, path A),380 although the m-pathway via hydroxymetallation has also been proposed (path B).381,382 However, distinguishing between the two pathways is difficult due to the rapid keto-enol tautomerization that renders isolation of the initial water adduct challenging. 

The reaction of metal ion M"+ with the keto, enol tautomeric mixture of acetylacetone (acacH) in acidic aqueous solution has been treated by a similar approach to that outlined above (see Prob. 16). 

If the polyelectrolyte can coordinate strongly to a metal ion, marked deceleration effects can be noted, as, for example, in the reactions of Ni + and Co + with pad in the presence of polyphosphates. Modifications of equilibria constants in these micelles must also be recognized as contributing to rate change, e. g., ligand pK or keto-enol equilibria may be altered. 

These equations do not provide complete definition of the reactions that may be of significance in particular solvent extraction systems. For example, HTTA can exist as a keto, an enol, and a keto-hydrate species. The metal combines with the enol form, which usually is the dominant one in organic solvents (e.g., K = [HTTA]en i/[HTTA]]jet = 6 in wet benzene). The kinetics of the keto -> enol reaction are not fast although it seems to be catalyzed by the presence of a reagent such as TBP or TOPO. Such reagents react with the enol form in drier solvents but cannot compete with water in wetter ones. HTTA TBP and TBP H2O species also are present in these synergistic systems. However, if extraction into only one solvent (e.g., benzene) is considered, these effects are constant and need not be considered in a simple analysis. 

A commonly used nucleophile has been water. Although initial attack affords a hydroxy-carbene derivative, ready cleavage of the Ca—Cp bond resulting from formal keto-enol tautomerism occurs to give either the acyl or the metal carbonyl (usually cationic) and the corresponding organic fragment (Equation 1.13) ... 

Triketones are homologues of 1,3-diketones and in these, too, keto-enol tautomerism has been probed by H NMR spectroscopy.567,568 Triketones and tetraketones may coordinate to one or more metal ions per molecule. In the latter case the metal centers are held in such close proximity that, in some cases, interesting magnetic effects may be observed. Structural and magnetic properties of polynuclear transition metal jS-polyketonates have been thoroughly reviewed.569,570... 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

The keto-enol tautomerism rates for oxaloacetate and zinc(II)-oxaloacetate have also been investigated using acetate buffer solutions.339,357 Thus for the equilibrium shown in equation (24) the value of fcf for the reaction oxacketI12 +OAc is 6.6x 10 3 M"1 s l, while for Zn(oxac)keto+ OAc , fcf= 25 M-1 s 1. The rate acceleration at 25 °C is ca. 4 x 103. Metal ion-promoted enolization is considered in detail in Section 61.4.20. 

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). 

The effects of cationic and zwitterionic micelles on the keto-enol tautomerism of 2-phenylacetyl-furan and -thiophene (73, X = O, S) have been studied in aqueous media.285 While the micelles perturb the equilibrium only slightly, the apparent acidity of one or other tautomer is increased, as the micelles have an affinity for the enolate. The systems also show lowered water rates at the minima of their pH-rate profiles, allowing an otherwise undetectable metal ion catalysis to be observed. 

It has been established that zinc-catalysed decarboxylation of (3-keto-acids involves a preliminary metal-promoted keto-enol tautomerism, as shown.272... 

High yields of the monofluorinated product at short reaction times were obtained that exceeded the performance of standard batch laboratory processing [309]. Yields of 72% were achieved at 99% conversion [309,310], The metallic construction material interacts with the reaction and impacts the keto/enol equilibrium to the advantage of the enol species that is fluorinated faster. 

Some General Observations on the Syntheses of Metal /3-keto-enolate Compounds... 

This section on metal ft-keto-enolates was assembled in cooperation with Prof. John P. Fackler.)... 

The syntheses of very volatile fluorine-containing metal 0-keto-enolates led Sievers and co-workers to use the volatility in the development of chromatographic techniques for metalion microanalyses.7... 

High-coordination-number complexes of 0-keto-enolates continue to be obtained with the metals such as zirconium(IV),8 hafnium(IV),8 cerium(IV),9 and the lanthanons(III),10 the last being tetrakis anionic species. At least one example of a volatile tetrakis 0-keto-enolate salt has been reported,11 Cs[Y(CF3-COCHCOCF3)4]. The ionic charge on the 0-keto-enolate complex has been shown to produce12 a high field nuclear magnetic resonance for anions and low field shifts for cations, relative to the positions observed for the neutral species. 

Triketones and similar polyketones exhibit keto-enol tautomerism and have been found to coordinate to one or more metal atoms (24). Such complexes can bring metal atoms in close proximity, resulting in interesting magnetic properties (see Magnetism of Transition Metal Ions). 

The hydrogen atom on the a-carbon is activated by the C=0 groups, and a conjugate system is formed. Under appropriate conditions the enolic hydrogen atom of /3-diketone can be replaced by a metal ion (M"+) to produce a six membered chelate ring (1) thereby shifting the keto-enol equilibrium favoring the enol form. 

This chapter is primarily concerned with keto-enol equilibrium and the chemistry of dissociated enols (enoxides) and lithium enolates. The acid-base aspects of the chemistry of other metal enolates (e.g. silyl enol ethers "", boron enol ethers" " " , aluminium , tin " , gallium , bismuth , zinc " ", rhodium , palladium " , manganese ", copper , nickel , magnesium " , titanium " , molybdenum , zirconium" " and ammonium" " enolates) have been reported elsewhere. 

The presence of S-carbonyl groups with at least one proton on the carbon between them allows a keto/enol tautomerism to occur and, under appropriate conditions, the eno-lic proton can be removed. The S-5-tricarbonyl compounds are the higher analogues of the / -diketonates and can take triketone, monoenol and dienol forms in their tautomeric equilibrium (equation 86) accordingly, they can behave as bidentate or tridentate ligands to form metal chelate complexes. ... 

In this manner, both C- and 0-metallo derivatives of keto-enol systems were shown not to be in tautomeric ketone enolate equilibrium (in benzene), thus differing from ketones and enols themselves where ketone enol equilibria exist. Metal enolates, however, are transformed by substitution to the derivatives of the ketone species, while our C-mercury derivatives of ketones or aldehydes have been shown to produce the enol compounds. A conventional scheme (in which ketones, not their metallic derivatives, are shown) thus becomes somewhat unsatisfactory ... 

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