Lanthanoid complex

Shibasaki M., Sasai H. Asymmetric Catalysis With Chiral Lanthanoid Complexes... 

Keywords chiral lanthanoid complexes, rare earth complexes, carbocyclic ring construction via intramolecular Diels-Alder reaction... 

The neglect of the ligand field in Eq. (10.1) leads one to expect no satisfactory account of the experimental magnetism of lanthanoid complexes either. It is an empirical fact, however, that Eq. (10.1) accounts extremely well for observed magnetic moments in most lanthanoid compounds. We compare typical experimental moments for lanthanoid complexes with those calculated from Eq. (10.1) in Fig. 10-3. Significant discrepancies occur for/ and/ species and we will comment on these shortly. 

Altogether, we can say that the success of Eq. (10.1) in reproducing the magnetic moments of lanthanoid complexes is due entirely to the very small magnitude of the ligand-field splittings and so, in turn, to the contracted nature of the/orbitals. 

Overall, then, the magnetic moments of all lanthanoid complexes are well reproduced without reference to the ligand field inter alia, we can infer that the ligand-field splittings in /-block complexes are no greater than about kT at room temperature. 

Part of the absorption spectrum of an aqueous solution of neodymium(iii) -configuration/ - is shown in Fig. 10-4. The situation shown there is quite typical of the whole of the lanthanoid series i.e. we could have chosen any/" configuration equally well to illustrate the main characteristics of the spectra of lanthanoid complexes. We shall focus on three main features splittings, band widths and absolute excitation frequencies. 

Shibasaki M, Grdger H (1999) Chiral Heterobimetallic Lanthanoid Complexes Highly Efficient Multifunctional Catalysts for the Asymmetric Formation of C-C, C-O and C-P Bonds. 2 199-232... 

Asymmetric Mukaiyama aldol reactions have also been performed in the presence of Lewis-acid lanthanoid complexes combined with a chiral sulfonamide ligand. Similar enantioselectivities of about 40% ee were obtained for all... 

Groger, H., Saida, Y., Sasai, H., Yamaguchi, K., Martens, J., and Shibasaki, M., A new and highly efficient asymmetric route to cyclic a-amino phosphonates the first catalytic enantioselective hydrophosphonylation of cyclic imines catalyzed by chiral heterobimetallic lanthanoid complexes, /. Am. Chem. Soc., 120, 3089, 1998. 

Shibasaki, M., Asymmetric Catalysis with Chiral Lanthanoid Complexes, 22, 201. 

In the first of these techniques the lanthanoid complex (33) (5-8 mol%) is used as the organometallic activator in cumene hydroperoxide or tert-butyl hydrogen peroxide-mediated oxidation of chalcone (epoxide yield 99 % 99 % ee) or the ketone (34) (Scheme 20)[1001. 

In fact, even approximate cubic symmetry seems to be rare for lanthanoid or actinoid element complexes.50 In low symmetry the number of crystal field parameters necessary to account for the system can be quite large. On the other hand, the spectra of lanthanoid complexes contain many... 

A series of trivalent lanthanoid complexes, scandium, and ytterbium tris-(R)-(-)-1,1 -binaphthyl-2,2 -diyl phosphonate, have been introduced as new chiral and stable Lewis acids for the asymmetric hetero Diels-Alder reaction of Danishefsky s diene and aldehydes. 2,6-Lutidine was found to be an effective additive to improve the enantioselectivity up to 89% ee [66] (Eq. 8A.42). 

Actinide and lanthanoid complexes have been employed for hydrogenation reactions, for which they often generate dramatic rate increases and high numbers of turnovers42. Many of these complexes exhibit good selectivity for preferential reduction of the less hindered alkene in situations where more than one is present in a substrate (Scheme l)43. 

Asymmetric catalytic addition of dialkylphosphites to a C=0 double bond is a powerful method, and probably the most general and widely applied, for formation of optically active a-hydroxy phosphonates [258], The basic principle of this reaction is shown in Scheme 6.108. Several types of catalyst have been found to be useful. The transition-metal-catalyzed asymmetric hydrophosphonylation using chiral titanium or lanthanoid complexes was developed by several groups [259, 260], The most efficient type of chiral catalyst so far is a heterobimetallic complex consisting... 

Golding, P. T., Sellars, P. J., and Wong, A. K. (1977) Resolution of racemic epoxides on g.l.c. columns containing optically active lanthanoid complexes. J. Chem. Soc., Chem. Comm., 570-571. 

Asymmetric Catalysis with Chiral Lanthanoid Complexes... 

IV. Previous examples of asymmetric catalysis with chiral lanthanoid complexes... 

C. Catalytic, asymmetric reduction of ketones with a chiral lanthanoid complex... 

The third part of this chapter reviews previously described catalytic asymmetric reactions that can be promoted by chiral lanthanoid complexes. Transformations such as Diels-Alder reactions, Mukaiyama aldol reactions, several types of reductions, Michael addition reactions, hydrosilylations, and hydroaminations proceed under asymmetric catalysis in the presence of chiral lanthanoid complexes. 

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