3-ketoenolate

The closely related tris complex of malonaldehyde (170) has been synthesized by the reaction of 1,1,3,3-tetramethoxypropane with anhydrous CrCl3 in an ether slurry.745 The complex was purified by sublimation and contains the simplest structural unit capable of enolizing to form a /3-ketoenolate chelate. 

Ketoenolates have not been isolated for the tripositive elements in the attempted preparation of Pu(MeCOCHCOMe)3, oxidation to plutonium(IV) occurred very readily.17 Complexes with ethers appear to be restricted to THF solvates apart from UC13-THF, a useful starting material for uranium(III) preparative chemistry,18 the only other examples are solvated organometallic compounds, such as Cp3Np-3THF and Cp3PuTHF. 

Table 90 Complexes of Transplutonium Actinide(III) /3-Ketoenolates with P-Oxides...

In an approach to CC-1065 (Section 7.21.12), Sundberg used the internal cyclization of a 3-ketoenolate anion with a stabilized carbocation to construct the carbocyclic ring <85JOC425>. 

These /3-ketoenolate ions form very stable chelate complexes with most metal ions. The commonest ligand is the acetylacetonate ion (acac) , in which R = R" = CH3 and R = H. A general abbreviation for /3-ketoenolate ions in general is dike. 

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. 

Polycyclic phenolic humic substances produce phenol carboxylates (oxidation of adjacent rings) of the salicylate type and possibly 3-ketoenolates (El(L) = -0.08 V for both acac" and sal2 )... 

Since the oxygen atoms as well as the alpha carbon on the /3-keto-enolate ring can function as bases to coordinate with metal ions (5, or to hydrogen bonding solvents (14)f it should be recognized that /3-keto-enolate complexes may have a stereochemistry quite different from that expected—based on the stoichiometry of the complex. However, by a suitable choice of a /3-ketoenol ligand, such as 2,2,6,6-tetramethyl-3,5-heptanedione (dipivaloylmethane), H-DPM, intermolecular effects can be reduced or eliminated with the result that the metal complex formed has the stereochemistry expected for the monomeric acetylacetonate complex of that metal ion. For example, nickel(II) acetylacetonate is trimeric as a solid and in solution (11), but the dipivaloylmethane complex is a monomeric, undoubtedly planar species, in solution, as well as in the solid state (37), This stereochemistry occurs with the monomeric acetylacetonate in dilute solution or in the vapor phase, as indicated by absorption spectra and electron diffraction (SO). 

Table 1. Possible Numbers of Face (F) and Edge (E) Oxygen Atoms for Polymeric, Octahedral /3-Ketoenolates ...

More common is the use of a metal salt, such as halide, hydroxide, oxide, sulfate, carboxylate, and so on. A base can also be added, to convert the /3-diketone to the /3-ketoenolate anion, which usually has greater solubility in aqueous media. 

Transplutonium(IV) complexes, 1219 halogeno, 1219 3-ketoenolates, 1219 oxides, 1219 tetrahalides, 1219... 

Metal 3-ketoenolate complexes. J. P. Fackler, Prog. Inorg. Chem., 1966, 7, 361-425 (414). 

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