Lanthanide-induced reactions

Nucleotides in the form of metal ion complexes are involved in a variety of enzymatic reactions either as substrates or as cofactors. These may also be viewed as monomers of DNA and RNA. Lanthanide complexes of nucleotides have been extensively studied because (i) the conformation of nucleotides in solution can be elucidated from lanthanide induced NMR chemical shifts and line-broadenings [40] and (ii) lanthanide nucleotide complexes may act as competitive inhibitors in enzymatic reactions [88] and hence can be used as paramagnetic probes in the mapping of their binding site on the enzyme [89]. 

An approach based on samarium diiodide-promoted reactions of the iminoketone 279 with aldehydes provides access to a series of substituted or fused pyrroles bearing at least two phenyl groups, for instance, the system 280 (Equation 86) <2001T4881>. In an alternative lanthanide-catalyzed route, a series of pyrroles were constructed from imines and nitroalkenes in the presence of Sm(fPrO)3 <1999T13957>. A set of 1-dimethylaminopyrroles have also been obtained by TiCU-induced reactions between 2-acetoxypropanal hydrazones with silyl enol ethers <1995TL8007>. 

The Kobayashi modification of the Mukaiyama aldol, in which lanthanide Lewis acids are used in aqueous solution, is a very attractive high-yielding green process, but the role of water is not understood.A computational smdy of the Eu +-catalysed reaction 0 between TMS cyclohexenolate and benzaldehyde seeks to probe the possibilities. Does water act as proton source Does it stabilize TMS dissociation Does it stabilize the syn-TSl These questions are addressed and answered using the AFiR method (artificial force-induced reaction) to probe the energy surfaces for the two most likely europium clusters, [Eu(H20)g] and [Eu(H20)9]. ... 

Confirmation that cw-addition of alkyl copper complexes occurs in these reactions has been obtained by studies of lanthanide-induced shift H n.m.r. spectra of the products. Corey has now introduced (3-methyl-3-methoxybut-l-ynyl)copper in THF as a relatively inexpensive reagent for the generation of mixed cuprates (Gilman s reagents), which enable coupling reactions to occur in high yield with alkyl-lithium reagents. ... 

Reactions.—A new method for the determination of absolute configuration in chiral secondary alcohols is based on the changes in chemical shifts observed in their n.m.r. spectra on glycoside formation. Characteristic shift changes for the glycosidic carbon of the sugar, and for C-a and C-/3 of the alcohol, have been summarized as rules. Measurement of lanthanide-induced shifts for the CF3 resonance in the n.m.r. spectra of a-methoxy-a-trifluoromethylphenyl-acetate (MTPA) esters of secondary alcohols has been reported as an... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

According to this extended silylamide route, completely exchanged Ln(tritox)3(THF) are formed easily and in high yield for the smaller lanthanides. THF dissociation in solution probably induces the reaction of Ln(bdsa)3(THF)2... 

Lanthanide chelates have been used as aids in studying the products of hydrogenation reactions monitored through parahydrogen-induced polarization (PHIP). Carrying out... 

Danishefsky and co-workers pioneered the use of chiral lanthanide complexes as catalysts in organic reactions. They found out that Eu(hfc)3, which is used as an NMR shift reagent, promoted hetero Diels-Alder reactions [30] of aldehydes with siloxydienes and induced enantiomeric enrichment (Sch. 1) [31]. Suitable substituents on the dienes were introduced to improve the extent of asymmetric induction. The best result was obtained in the reaction of benzaldehyde with l-methoxy-2-methyl-3-(trimethyl-siloxy)- , 3-butadiene using 1 mol % Eu(hfc)3 the enantiomerie excess was, however, moderate (58%). The authors maintained that the major advantage of lanthanide catalysis lay in the survival of otherwise labile systems used as adducts. 

The lanthanides react slowly with cold water (more rapidly with hot water) to form hydrogen gas, and readily bum in air to form oxides. Oxides are substances in which a metal and oxygen have chemically combined to form a compound. For example, samarium and oxygen combine to form the compound samarium oxide. Yttrium has a natural protective oxide coating, making it much more resistant. The lanthanides form compounds with many nonmetals, such as hydrogen, fluorine, phosphorous, sulfur, and chlorine, and heating may be required to induce these reactions. 

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