Lanthanide ytterbium complexes

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 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 basis for applying the LIS quantitatively to problems in stereochemistry depends upon expressions including the term (3 cos2 — l)r-3, where r is the distance from the carbon to the lanthanide ion and the angle d is defined by the symmetry axis of the complex and the vector from the lanthanide ion to the carbon in question. This application depends on a LIS imposed entirely by the pseudocontact mechanism. It has been shown that the contact mechanism is important for europium and praseodymium complexes in 13C NMR for distances up to four bonds from the site of complexation, and that ytterbium complexes interact with 13C nuclei largely, if not entirely, by the pseudocontact process. (12, 13)... 

The transamination of the anionic amido ytterbium complex LiYb (N Pr2)4 with aryloxo-functionalised NHC imidazolium salt precursors affords bis-aryloxo-NHC monoamido ytterbium(III) complexes (Scheme 32)71 The complexes are isostructural with one another in the solid state and exhibit average ytterbium-NHC bond distances of 2.487 A (R = Me) and 2.535 A (R = Pr). These are comparable to the monodentate lanthanide(III)-NHC bond lengths where the ligand possesses and amido tether, though a direct comparison with like-for-like metal centre is not possible. The longer ytterbium-NHC bond length where R = Pr is likely as a result of the increased bulk of the ligand. 

Some salient points to note are (i) the model is in accord with the experimental results, (ii) nuclei at a and yS positions to the coordinating atom show deviations from the model due to contributions of other shift mechanisms. Thus the best procedure for the elucidation of molecular structure by using lanthanide reagents consists of (i) to obtain the relative magnitude of geometrical function, G for different ligand nuclei from the slopes of A bT versus 1 / T plots, (ii) since temperature dependence of ytterbium complexes conforms to the model, use of ytterbium complexes is prudent, (iii) in cases where the temperature dependence is interfered with effects due to chemical equilibrium or exchange, data for a number of lanthanides at room temperature may be obtained and plots of equation... 

The synthesis and characterization of the first bis- and tris[ 1 -(tu-alken-l-yl)indenyl] lanthanide complexes (Ln = Gd, Er, Y, Lu) have been reported. l-Allyl-2,4,7-trimethyl-lFI-indene and l-(3-buten-l-yl)-4,7-dimethyl-lH-indene were prepared from (2,4,7-trimethylindenyl)lithium and allyl chloride or from (4,7-dimethylindenyl)lithium and 4-bromo-l -butene.677 The reactions of the trichlorides of gadolinium, erbium, yttrium, lutetium, and ytterbium in molar ratios in TFIF produce the bis(l-allyl-2,4,7-trimethylindenyl)lanthanide chloride complexes L2LnCl(THF) (Ln = Gd, Er), bis( 1 -buten-1-yM,7-dimcthyIindcnyI)lanthanide complexes (Ln = Y, Lu) or the heterometallic complexes bis(l-buten-l-yl-4,7-dimethylindenyl)Yb(/r-Cl)2Li(THF)2 (Scheme 180).677... 

The analogous ytterbium complex was also prepared ana round to be isostructural. However, attempts to prepare o-bonded derivatives of the larger lanthanides were unsuccessful. 

Most of the lanthanide porphyrin complexes are paramagnetic, except those of lanthanum and lutetium which are diamagnetic. They all show paramagnetic behaviors in the NMR spectra similar to those of the lanthanide 0-diketonate complexes, except that the shift capabilities are less.4 Some of these paramagnetic lanthanide complexes (i.e., that of ytterbium) show Curie behavior at low temperature. Diamagnetic lutetium shows some ring current phenomena.4... 

Mikami s group has also demonstrated the advantage of the fluorous super-Lewis acids such as lanthanide tris(perfluorooctanesulfonyl)methide and perfluorooctane-sulfonimide complexes with respect to temperature-dependent solubility [13bj. For example, these complexes can be re-used for the Friedel-Crafts acylation reaction without fluorous solvents [Eq. (11)]. After the reaction mixture of anisole has been heated with acetic anhydride in 1,2-dichloroefhane in the presence of ytterbium perfluorooctanesulfonimide (10 mol%) at 80 °C for 6 h, the mixture is allowed to stand at -20 °C for 30 min to precipitate the ytterbium complex. The liquid phase is decanted and the residual lanthanide complex is re-used without isolation. No loss of activity is observed for the catalyst recovered. The total isolated yield of the product, which is combined from the three runs, is 78%. 

An enantioselective version of the above reactions has been reported. Lewis acids such as Yb(OTf)3 can profoundly affect the stereochemical outcome of the carbonyl ylide 1,3-dipolar cycloadditions [137]. This provided an indication to effect asymmetric carbonyl ylide cycloaddition using a chiral Lewis acid. The first example of such asymmetric induction using the chiral lanthanide catalysts has been reported [138,139]. For example, the reaction of diazoacetophenone 89 with benzyloxyacetaldehyde, benzyl pyruvate and 3-acryloyl-2-oxazoHdinone in the presence of chiral 2,6-bis(oxazolinyl)pyridine ligands and scandium or ytterbium complexes furnished the corresponding cycloadducts 165-167 with high enantioselectivity (Scheme 53). 

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