Lanthanide achiral

To achieve catalytic enantioselective aza Diels-Alder reactions, choice of metal is very important. It has been shown that lanthanide triflates are excellent catalysts for achiral aza Diels-Alder reactions [5]. Although stoichiometric amounts of Lewis acids are often required, a small amount of the triflate effectively catalyzes the reactions. On the basis of these findings chiral lanthanides were used in catalytic asymmetric aza Diels-Alder reactions. The chiral lanthanide Lewis acids were first developed to realize highly enantioselective Diels-Alder reactions of 2-oxazolidin-l-one with dienes [6]. 

A closely related method does not require conversion of enantiomers to diastereomers but relies on the fact that (in principle, at least) enantiomers have different NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which case transient diastereomeric species may form). In such cases, the peaks may be separated enough to permit the proportions of enantiomers to be determined from their intensities. Another variation, which gives better results in many cases, is to use an achiral solvent but with the addition of a chiral lanthanide shift reagent such as tris[3-trifiuoroacetyl-Lanthanide shift reagents have the property of spreading NMR peaks of compounds with which they can form coordination compounds, for examples, alcohols, carbonyl compounds, amines, and so on. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of many such compounds to different extents. 

Marks and coworkers developed a series of cyclopentadienyl-lanthanide complexes. In the initial investigations on achiral catalysts 36a and 36b (Fig. 29.21), TOFs greater than 100000 IT1 were observed in the hydrogenation of 1,2-disub-stituted unfunctionalized alkenes [48]. 

Achiral lanthanide shifting reagents may be used to enhance the anisochrony of diastereomeric mixtures to facilitate their quantitative analysis. Chiral lanthanide shift reagents are much more commonly used to quantitatively analyze enantiomer compositions. Sometimes it may be necessary to chemically convert the enantiomer mixtures to their derivatives in order to get reasonable peak separation with chiral chemical shift reagents. 

In the presence of a catalytic amount of chiral lanthanide triflate 63, the reaction of 3-acyl-l,3-oxazolidin-2-ones with cyclopentadiene produces Diels-Alder adducts in high yields and high ee. The chiral lanthanide triflate 63 can be prepared from ytterbium triflate, (R)-( I )-binaphthol, and a tertiary amine. Both enantiomers of the cycloaddition product can be prepared via this chiral lanthanide (III) complex-catalyzed reaction using the same chiral source [(R)-(+)-binaphthol] and an appropriately selected achiral ligand. This achiral ligand serves as an additive to stabilize the catalyst in the sense of preventing the catalyst from aging. Asymmetric catalytic aza Diels-Alder reactions can also be carried out successfully under these conditions (Scheme 5-21).19... 

A more recent report by Sibi and co-workers displayed the utility of chiral lanthanide Lewis acids for addition-trapping reactions [150]. An exhaustive screening of lanthanide Lewis acids and several chiral ligands revealed that Y(OTf)3 and proline derived ligand 138 was optimal (data not shown). Upon further optimization it was discovered that achiral additives 139 and 212 increased ee s (Scheme 56, entries 2 and 3). Bulkier radicals were found to decrease the enantioselectivity (entries 4 and 5). Also, larger aryl substituents on the ligand gave similar ee s as observed for 138 (compare entries 1, 6, and 7). 

Differential stability of these solvates has also been demonstrated by NMR through use of an achiral lanthanide shift reagent in conjunction with TFAE. Incremental addition of Eu(fod>3 to a solution of (R)-TFAE and the dinitrolactone shifts the resonances of the (5)-enantiomer more rapidly downfield than those of the (/ )-enantiomer. Nonequivalence increase in this manner arises by a preferential disruption of the least stable R, S) solvate. In the case of the nonnitrated parent, addition of the LSR gradually attenuates nonequivalence, as both solvates (of approximately equal stability) are equally disbanded. 

Chemical shift differences dj, s<5 are usually small. They can be enhanced by the addition of an achiral paramagnetic lanthanide shift reagent84. Thus, Eu(fod)3 (see Section 3.1.4.2.) altered the magnitude, and occasionally the sense of spectral nonequivalence, of racemic sulfoxides in the presence of nonracemic perfluoroalkylcarbinols85. 

The development of achiral lanthanide shift reagents preceded that of chiral LSRs. It was demonstrated86 in 1969 that paramagnetic Eu(thd)3 and Eu(fod)3 (Tablet), as well as the... 

Europium(III), and particularly ytterbium(III) shift reagents, induce downfield proton resonance shifts while the praseodymium(III) analogs cause upfield shifts. Lanthanide chelates of fluorinated /3-diketonates are more soluble in organic solvents, and they form more stable association complexes with donor molecules, than do LSRs with nonfluorinated ligands. Thus Eu(fod)3 is the preferred achiral LSR for weak nucleophiles89. 

Absolute configuration of complexes. 495-496 Absolute electronegativity, 351 Absolute hardness. 351 Absorption spectra of lanthanide and actinide ions. 604-607 Acceptor number CAN). 370 Achiral molecules, and point groups, 64 Acids. 2... 

In 1975, it was reported that while lanthanide shift reagents could not be used directly to simplify the NMR spectra of alkenes, when coupled with silver salts substantial shifts could be induced.232 Since then, a number of studies have reported the use of both chiral and achiral lanthanide(III)-silver(I) binuclear shift reagents,233-237 where the ligands were generally fluorinated /3-diketones. 

Acetyl ligands, in niobium complexes, C-H BDEs, 1, 298 Achiral phosphines, on polymer-supported peptides, 12, 698 Acid halides, indium compound reactions, 9, 683 Acidity, one-electron oxidized metal hydrides, 1, 294 Acid leaching, in organometallic stability studies, 12, 612 Acid-platinum rf-monoalkynes, interactions, 8, 641 Acrylate, polymerization with aluminum catalysts, 3, 280 Acrylic monomers, lanthanide-catalyzed polymerization,... 

Although some of the lanthanides form compounds with oxidation states other than + 3, the vast majority of stable species involve the trivalent state. Due to the nature of the weak bonding f electrons, complexes in solution are normally quite labile, and as described below, the preparation and the isolation of pure enantiomers containing a central lanthanide ion and achiral ligands are extremely rare. The coordination number of lanthanide(III) ions is somewhat variable and depends on the size and charge of the coordinated ligands, and the size of the lanthanide ion that varies slightly... 

In addition to achiral precatalysts the chiral lanthanide metallocenes (R)-[Me2SiCp" ( — )-menthylCp ]SmCH(SiMe3)2 and (S)-[Me2SiCp" ( - )-men-thyl Cp ]SmCH(SiMe3)2 have been employed [71]. The hydrocarbyl derivatives have been shown to mediate the enantioselective hydrosilylation of 2-phenyl-l-butene by PhSiH3 with exclusive 1,2-addition and with N, 50h 1. In this case enantioselection proceeds with 68% ee ((R) product) and 65% ee ((S) product) for the (R)-Sm and (S)-Sm catalysts (70% enantiopure), respectively. 

There are many hundreds of published reports on the use of achiral lanthanide tris(/3-diketonates) as NMR shift reagents. Essentially any substrate with an oxygen, nitrogen or sulfur atom is a potential candidate for analysis with lanthanide shift reagents. These include sulfur- and phosphorus-containing functional groups that have oxygen atoms. Carboxylic acids and phenols were observed to decompose lanthanide chelates of dpm, whereas solutions with chelates of fod were stable for several days and suitable for study. 

Achiral lanthanide chelates have been used in conjunction with other chiral NMR deriva-tizing or solvating agents. The addition of the lanthanide enhances enantiomeric discrimination and/or causes shifts in the spectrum that show a characteristic trend with absolute configuration. 

Achiral lanthanide chelates can also be added to CSAs such as arylperfluoroalkyl-carbinols, the ethyl ester of 3,5-dinitrobenzoyl-L-leucine (25) , the 3,5-dinitrobenzoyl derivative of 1-phenylethylamine, Af-(l-(l-naphthyl)ethyl)trifluoroacetamide (26) and a series of l-(l-naphthyl)ethyl urea derivatives of amino acids (27) to enhance the enantiomeric discrimination. With sulfoxide or lactone substrates , the europium ion preferentially associates with the substrate in the bulk solution. Provided the enantiomers have different association constants with the CSA, the isomer that shows the weaker association with the CSA shows the larger lanthanide-induced shifts. Low concentrations of lanthanide relative to the substrate and CSA lead to enhancements of enantiomeric discrimination in the NMR spectrum. If the concentration of lanthanide is too high, binding of the substrate to the lanthanide strips the substrate from the chiral solvating agent and diminishes the chiral discrimination in the NMR spectrum. 

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