Lanthanide enolates ligands

Since tropolones and 3-hydroxy-4-pyrones are taken in this chapter to be enols, we now cite their binding as enolato ligands to lanthanum and all the other trivalent lanthanides (save the radioactive promethium) " " . Likewise, we note such studies for complexes with enolato ligands derived from 3-acetyl-4-hydroxycoumarin, dehydroacetic acid and their oximes, and with the aromatic enediolates, squarate and croconate. Periodic trends in thermodynamic parameters were reported and analyzed in these studies. 

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. 

Phosphine Chalcogenides as Ligands. - The complexation of lanthanide and actinide ions by phosphine oxide ligands remains an active area, and a theoretical assessment of the coordination of phosphine oxides (and phosphate esters) by trivalent lanthanide ions has appeared." Trivalent lanthanide complexes of the functionalised enol phosphine oxide (265), (and a related phosphonate), have been describedComplexes of thorium(iv) with bis(di-phenylphosphino) ethane dioxide and bis(diphenylphosphinoyl)amide have also been characterised." Calixarene systems which bear phosphine oxide... 

By the reaction of theenolatc of camphor with carboxylic acid esters or chlorides, 1,3-diketones [better formulated as enols. such as (hydroxymethylene)camphor] are obtained. When trifluo-roacetic acid or heptafluorobutanoic acid are used, the corresponding diketones (abbreviated as tfc or hfc, respectively) have been successfully used as ligands for lanthanides and these are used as chiral shift reagents in NMR spectroscopy12. The complex Eu(hfc)3 [derived from ( + )-camphor)] 3 was used as a chiral catalyst for enantioselective Diels-Alder-type cycloadditions of aldehydes to dienes (Section D.l.6.1.1.1.2.4.). 

The inorganic lanthanide triflate complexes Ln(OTf)3 (made in aqueous solution) have been shown by Kobayashi to be efficient Lewis-acid catalysts for hydroxy-methylation (using commercial aqueous formaldehyde solutions) of silicon enolates in aqueous medium (water -i- THF) or even in water alone in the presence of a surfactant. In these reactions, activation proceeds by coordination of the aldehyde oxygen atom by the Ln center that is a strong Lewis acid due to its hard character. Among the lanthanide triflates, ytterbium triflate was found to be the most active catalyst, but scandium triflate can sometimes also be efficiently used. Enantio-selective versions are also known in the presence of chiral macrocyclic ligands. The water-soluble catalyst is recovered in water after extraction of the organic products. 

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