Lanthanide ions catalysis

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

Some of these processes can be controlled and used for the preparation of chemicals of industrial interest, such as 5-(hydroxymethyl-2-furaldehyde, HMF). HMF is the product of a triple dehydration of fructose, which is itself one of the first degradation products of sucrose. The preparation and the uses of HMF have been extensively studied and reviewed by Lichtenthaler.342 It can be obtained under various degradative conditions, including acid catalysis and catalysis by lanthanide ions.343 Processes for the production of HMF on the multi-ton scale have been developed as well as many uses, notably polycondensations.344,345 The polymerization of HMF has been shown to produce complex... 

Ligands for the lanthanides must strongly chelate lanthanide ions but not inactivate them as catalysts. Coordination sites must be available for catalysis and the metal ion should retain a high degree of Lewis acidity. An overall positive charge on the complex may aid in catalysis, as discussed in subsequent paragraphs. Macrocyclic complexes of the lanthanides abound (24). Few lanthanide macrocyclic complexes, however, are inert to metal ion release in water. For example, perhaps the most well-known class of lanthanide macrocyclic compounds are the crown ethers. Crown ether complexes are synthesized under anhydrous conditions and are known to hydrolyze in water. 

When the reaction was conducted at room temperature under the catalysis of Yb[(-)BNP]3, the asymmetric induction was improved to 73% ee. The effect of the central metal ion of the chiral catalysts on the optical yield of the product, 2-phenyl-2,3-dihydro-4H-pyran-4-one, is shown in Fig. 2. The degree of enanti-oselection is highly sensitive to and dependent on the ionic radius of lanthanide ions [31]. 

Diene (14) reacted with a series of aldehydes under BFs-OEtz catalysis in CH2CI2 to give predominantly trans products (Table lO). " Aldol-type products, such as p-hydroxy enones, are isolated (along with dihydropyrones) from the reaction mixtures. Using TFA as a catalyst, the p-hydroxy enones are, as previously described, converted into dihydropyrones. The stereoselectivity of these reactions is consistent with a Mukaiyama-aldol reaction rather than a Diels-Aider cycloaddition. The stereochemistry of the P-hydroxy enones is also consistent with the observation that the (Z)-alkoxysilane reacts with the aldehyde in an extended transition state to give anti (threo) aldol products (Scheme 16). In the cases using ZnCh or lanthanide ions as catalysts aldol products have not been detected. 

In 1992, the authors found that various lanthanide(III) salts are active for the hydrolysis of phosphodiester linkages in DNA (Matsumoto and Komiyama, 1992). Even linear (non-supercoiled) DNA was hydrolyzed at pH 7. This catalytic activity was quite important and attractive, since none of the catalysts previously reported could hydrolyze linear DNAs (several catalysts could hydrolyze supercoiled plasmid DNA, but they were inactive for the hydrolysis of linear DNAs vide injra). Non-lanthanide ions, e.g., Mg(II), Zn(II), Co(II), Al(III), Fe(III), showed no measurable activity, and thus the remarkable catalysis for the hydrolysis ofDNA is restricted to lanthanide ions. These findings built a bridge between biochemistry and rare earth chemistry. 

Why is Ce(IV) so active for DNA hydrolysis What factors differentiate this metal ion from other lanthanide ions and non-lanthanide ions Do the f-oibitals of Ce(IV) take significant roles in the catalysis These questions are critically important for practical applications of this catalysis and also from the viewpoints of pure rare earth chemistry. In order to answer them, core-level photoelectron spectroscopy (Shigekawa et al., 1996), as well as EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge stracture) measurements (Shigekawa et al., 1999), were carried out. The spectroscopic analysis was simplified by using diphenyl phosphate (DPP) in place of DNA, and the EXAFS and the XANES measurements were carried out on the samples frozen in liquid nitrogen. 

Lanthanides, in catalysis Rare earths, in catalysis Ion size contraction, lanthanides Lanthanolds, in catalysis... 

Aryls Alkyl Homogeneous Catalysis The Electronic Stmcture of the Lanthanides Variable Valency Solvento Complexes of the Lanthanide Ions Lanthanides Coordination Chemistry The Divalent State in Solid Rare Earth Metal Halides Lanthanides Comparison to 3d Metals Trivalent Chemistry Cyclopentadienyl Tetravalent Chemistry Organometallic Organic Synthesis. 

The rate of catalysis by hen egg-white lysozyme has been determined in the presence of various metal ions(Co +, Zn +, and eight of the trivalent lanthanide ions). The inhibition data were fitted to several models of the interactions of the metal ion with the enzyme, and the degrees of fit were compared. The models ranged in complexity from a single inhibitory total binding site on the enzyme to the presence of two non-independent and non-equivalent inhibitory metal binding sites, but the simplest models appeared to be the most valid. 

Most lanthanide ions are paramagnetic, a property first exploited in the mid-1950s when it was found that the dysprosium(III) ion accelerated the rate of decarboxylation of phenylmalonic acid (Pitzer and Gelles, 1953). This magnetic catalysis was considered a minor effect (Gelles and Pitzer, 1955), and not until 1960 was this phenomenon applied to the resolution of the inadvertent overlap of NMR spectral resonances (Jackson et al., 1960). 

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