Ligands enol/keto forms

Spectral evidence" indicates an equilibrium between tetrahedral and octahedral Co" in iViV-dimethylacetamide and the equilibrium constant for [Co (tet)]/[Co (oct)] is reported at various temperatures. The complexes of acetylhydrazine (A), [CoA3]X2 (X = Cl or Br) and [Co(NCS)2A2]H20 and the tri-N-deuterio-analogue[Co(NCS)2(Ad3)2]D20 have been isolated and examined by i.r. Cationic complexes of JV-acyl hydrazines have been isolated with ligands in their keto-form, RCO-NH-NH2 however, the ligands also react in their enol form, RC(OH) = NNH2, forming neutral complexes (R = Me, Pr", Pr , or Ph). ... 

Different tautomers of / -diketones may react with metal ions at quite different rates. For example, the keto form of thenoyltrifiuoroacetone does not react at all the enol form reacts by two pathways, one independent and one inversely dependent on hydrogen ion concentration. Deprotonation of the ligand is evidently an important, rate determining factor.46 Both tautomers of acetylacetone react with iron(III). The rate constants for reaction of the keto form with Fe3+ and FeOH2+ are 0.29 and 5.4 M-1 s-1 respectively and the rate constants for the reaction of undissociated enol form with Fe3+ and FeOH2+ are 5.2 and 4.4 x 103 M-1 s-1 respectively.47 Rate constants for the reaction of the enol and keto tautomers of acetylacetone with Ti3+ are 2.4 x 103 and 0.7 M-1 s-1 respectively.48... 

Neutral diketone complexes are not generally available and so it is of interest to find Mg(C104)2 2acacH 2H20. The keto form of the Ugand has been shown to predominate in the complex in solution, by use of H NMR, and althou exchange between free and complexed ligand is fast, the keto-enol tautomerization is slow. ... 

The optically active Schiff bases containing intramolecular hydrogen bonds are of major interest because of their use as ligands for complexes employed as catalysts in enantioselective reactions or model compounds in studies of enzymatic reactions. In the studies of intramolecularly hydrogen bonded Schiff bases, the NMR spectroscopy is widely used and allows detection of the presence of proton transfer equilibrium and determination of the mole fraction of tautomers [21]. Literature gives a few names of tautomers in equilibrium. The OH-tautomer has been also known as OH-, enol- or imine-form, while NH tautomer as NH-, keto-, enamine-, or proton-transferred form. More detail information concerning the application of NMR spectroscopy for investigation of proton transfer equilibrium in Schiff bases is presented in reviews.42-44... 

Jorgensen s group44a carried out the reaction using the anhydrous form of chiral bis(oxazoline) coordinated copper complex. Complex 106 containing 83 as the chiral ligand was found to be the most effective. As shown in Scheme 5-32, the asymmetric hetero Diels-Alder reaction of //.y-unsaturated a-keto esters with acyclic enol ethers results in products with excellent yield and enantioselectivity. 

S-Diketones, by virtue of their keto-enol tautomerism (Scheme 21), are weak acids and in most cases bind nickel(II) in the deprotonated enol form, acting as uninegative chelating ligands. 

In the above structure, if Ph3P is replaced by a bidentate ligand such as bipyridyl, there results a C-rj, -acetylacetonate complex in which palladium is tr-bonded to a terminal CH2 group. These presumably exist as tautomeric keto and enol forms (equation l).53... 

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). 

Titanium chelates are formed from tetraalkyl titanates or halides and bi- or polydentate ligands. One of the functional groups is usually alcoholic or enolic hydroxyl, which interchanges with an alkoxy group, RO, on titanium to liberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, CC-hydroxycarboxylic acids and oxalic acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

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