Chiral lanthanide complexes

Lanthanide shift reagent An organic complex of a lanthanide element that has the property of shifting the NMR signals of a compound with which it can form a co-ordination complex. Chiral lanthanide shift reagents shift the peaks of the two enantiomers to different extents. 

Few investigations have included chiral lanthanide complexes as catalysts for cycloaddition reactions of activated aldehydes [42]. The reaction of tert-butyl glyoxylate with Danishefsky s diene gave the expected cycloaddition product in up to 88% yield and 66% ee when a chiral yttrium bis-trifluoromethanesulfonylamide complex was used as the catalyst. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

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... 

Kobayashi s chiral lanthanide complex 63 has been used for asymmetric Diels-Alder reactions, and very good results have been obtained (see Section 5.4.2). This kind of complex is also effective in asymmetric 1,3-dipolar reactions.87 The chiral ligand is prepared in situ by mixing Yb(OTf)3,... 

Similar differentiation between enantiomers by means of NMR can also be achieved by the use of chiral lanthanide shift reagents (243). Tris-[3-(heptafluoropropylhydroxymethylene)-d-camphorato] -europium was used for the first time (244) for determining the enantiomeric content of benzyl methyl sulfoxide 34. The enantiomeric composition of the partially resolved methyl p-tolyl sulfoxide 41 was estimated using tris-[3-(r-butylhydroxymethylene)-c -camphorato]-europium (245). Another complex of europium, tris-[3-(trifluoro-methylhydroxymethylene)-c -camphorato] europium (TFMC), in contrast to those mentioned above, was effective in the differentiation of various enantiomeric mixtures of chiral sulfinates (107), thiosul-finates (35), and sulfinamides (246). 

Optically pure (3i )(—)-linalyl acetate was detected in the oils of clary sage Salvia sclarea). Salvia dominica, lavender and lavandin using H-NMR spectroscopy with a chiral lanthanide shift reagent, Eu(hfc)3. This enantiomer was also detected in the oils of lavender, lavandin and bergamot using complexation gas chromatography on Ni(hfc) 2, and... 

In 1995, Mikami reported that chiral lanthanide complex 35 catalyzed the enan-tioselective hetero-Diels-Alder reachon of Danishefsky s diene and various glyoxy-lates [121]. Interestingly, the addition of water to the reachon mixture resulted in increases in both chemical yield and enanhoselechvity. It was later reasoned that... 

In the following section covalently bound derivatives are mainly discussed. Complexes are described only if they have a defined structure. Other methods of NMR discrimination of enantiomers are described elsewhere in this series. They involve mostly chiral solvents7,8 or chiral lanthanide shift reagents9,10. 

Inanaga, J. Furuno, H. Hayano, T. Asymmetric catalysis and amplification with chiral lanthanide complexes. Chem. Rev. 2002,102, 2211-2225. 

Scheme 8C.5. Asymmetric carbonyl-ene reaction catalyzed by chiral lanthanide complexes.

Kobayashi reported an asymmetric Diels-Alder reaction catalyzed by a chiral lanthanide(III) complex 24, prepared from ytterbium or scandium triflate [ Yb(OTf)3 or Sc(OTf)3], (Zf)-BINOL and tertiary amine (ex. 1,2,6-trimethylpiperidine) [30], A highly enantioselective and endose-lective Diels-Alder reaction of 3-(2-butenoyl)-l,3-oxazolidin-2-one (23) with cyclopentadiene (Scheme 9.13) takes place in the presence of 24. When chiral Sc catalyst 24a was used, asymmetric amplification was observed with regard to the enantiopurity of (/ )-BINOL and that of the endoadduct [31 ]. On the other hand, in the case of chiral Yb catalyst 24b, NLE was affected by additives, that is, when 3-acetyl-l,3-oxazolidin-2-one was added, almost no deviation was observed from linearity, whereas a negative NLE was observed with the addition of 3-pheny-lacetylacetone. 

The use of aqueous chiral lanthanide complexes in the determination of the enantiomeric purity of chiral a-hydroxy acids has also been assessed by H NMR [21], Large lanthanide induced shifts, chemical shift non-equivalence and an apparent absence of kinetic resolution in complex formation is observed upon addition of racemic lactate to [Yb.3a]3+ (Figure 1). The lactate CH3 resonances are clearly resolved for the... 

The last category was concerned with miscellaneous subjects, while citing some chirogenic porphyrin-based systems. Representative reviews include chiral lanthanide complexes by Aspinall [41], coordination chemistry of tin porphyrins by Arnold and Blok [42], photoprocesses of copper complexes that bind to DNA by McMillin and McNett [43], nonplanar porphyrins and their significance in proteins by Shelnutt et al. [44], cytochrome P450 biomimetic systems by Feiters, Rowan, and Nolte [45] and phthalocyanines by Kobayashi [46,47]. 

H.C. Aspinall, Chiral lanthanide complexes coordination chemistry and applications, Chem. Rev. 1807-1850, 2002. 

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