Lanthanide complexes adducts

Under this section are considered lanthanide complexes of /3-diketones and their adducts, but work specifically in the area of lanthanide shift reagents is dealt with in Section 39.2.9. Of course, the majority of lanthanide shift reagents are lanthanide /3-diketonates, and when they function as shift reagents they do so by forming adducts in solution. Furthermore, interest in shift reagents has directly stimulated a considerable amount of fundamental research on lanthanide /3-diketonates and their adducts. Much of this fundamental work was, therefore, carried out in the early 1970s. AH this means that the division between this section and Section 39.2.9 is not entirely clear cut. 

Cyclocondensation of 2-siloxy dienes and aldehydes is catalyzed by 1 mol % of a soluble lanthanide complex, Eu(hfc)3, and gives the hetero-Diels-Alder adduct in up to 58% ee (Scheme 107) (263). Upon treat-... 

Several of the lanthanide complexes MX3, where X = NSi2Me6, have been shown to give 1 1 adducts MX3L where M = La, Eu and Lu and L = OPPh3.145 The central MN30 unit has virtual C3t symmetry. Evidence has also been presented for the formation of unstable 1 2 adducts MX3L 2 where L = OPMe3.144... 

The homoleptic lanthanide(III) benzamidinates 20-23 can be regarded as analogues of the well known tris(cyclopentadienyl)lanthanide complexes (C5H5)3Ln [7, 8]. One of the most characteristic reactions of the homoleptic cyclopentadienyls is the formation of 1 1 adducts with Lewis bases such as ethers, nitriles, esters etc. [7, 8], Recently it was discovered that the homoleptic lanthanide benzamidinates [PhC(NSiMe3)2]3Ln (20) form similar adducts with THF and nitrile ligands such as acetonitrile or benzonitrile [59]. The molecular and crystal structures of two benzonitrile adducts (26g, h) have been determined by X-ray diffraction. Figure 8 depicts the molecular structure of the europium... 

The solid-state structure of the ethylaluminum oxide metallocene complex (Fig. 30) shows two trivalent samarocene units connected by an [(A EtsO ]2-ethylalumoxane ( EAO ) unit. The latter was described as an adduct of two molecules of triethylaluminum with two [AUfoO]- anions [116]. An unusual asymmetric fx-r q1 (side-on) ethyl coordination mode was observed, which previously has been found only in a small number of lanthanide complexes, i.e., (CsMeshYM/zy 1-Et)AlEt2(THF) [ 144], (C5Me5) 2Sm(THF) (fx-rj1 ijx-Et) AlEt3 (Fig. 9) [115] and (C5Me5)2Sm(THF) (/i-r]1 X-Et) (/z-Cl)AlEt2 [116]. 

The first lanthanide-NHC complexes were isolated by Arduengo and coworkers in 1994.62 A stable carbene displaces THF in bis(pentamethylcy-clopentadienyl)-samarium-THF to form the samarium(II)-NHC complex 55 (Scheme 29). The addition of a second equivalent of NHC resulted in the isolation of the bis(NHC) adduct 56. Compound 56 was characterised in the solid state by single crystal X-ray diffraction and exhibits samarium-NHC bond distances of 2.837(7) A and 2.845(7) A, which are longer than the M-C bond in a-bonded alkyl lanthanide complexes. 

The lanthanide triflates may also be obtained by the addition of triflic acid to lanthanide carbonates in ethanol or acetonitrile [18]. Filtration and concentration of the filtrate yield a solid salt which is washed with solvent. Lanthanide salts of poorly coordinating anions such as PFg, BF4 or BiCftHs) have been also used in the synthesis of lanthanide complexes as starting materials. Adducts of acetonitrile with europium tetrafluoroborate (BF4) and hexafluorophosphate (PF ) have been prepared [19]... 

The discussion so far has centred on lanthanide complexes with axial symmetry. When lanthanide adducts are devoid of axial symmetry, the dipolar shift is given by the equation [18]... 

Although pyridine and other amine-type ligands form adducts with lanthanide diketonates, few other lanthanide complexes have been made. However, the first homoleptic lanthanide ammine complexes, [Yb(NH3)g] [Cu(S4)2] NH3, [Yb(NH3)s] [Ag(S4)2]-2NH3,and[La(NH3)9] [ 0(84)2],have been synthesised by reactions in aqueous ammonia. ... 

Tris(indenyl)lanthanide complexes of all rare earth metals can be prepared analogously from the lanthanide trichlorides with alkali or magnesium indenyl salts. All five-membered rings are -bonded to the metal. They form base adducts with... 

The enhancement of the extraction of Ln(III) ions with ttfaH in cyclohexane or benzene by the addition of Cr(acac)3 was investigated . The equilibrium analysis suggested that the effect of Cr(acac)3 could be ascribed to the formation of a binuclear 1 1 La(ttfa)3-Cr(acac)3 adduct. The formation constants of adducts along the lanthanide series decreased with the decrease of the ionic radii among the light lanthanides and were constant for their heavy counterparts. UVV, IR and H NMR spectroscopic studies were also performed to explain the molecular structural differences between the light and heavy lanthanide complexes. 

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. 

Table 1 Comparison of tris(indenyl)lanthanide complexes and their THF adducts...

A conceptually different [4-1-2] cycloaddition catalyzed by a chiral lanthanide complex has been disclosed. The inverse electron demand Diels Alder reaction of 3-methoxycarbonyl-2-pyrone (67) and enol ethers or sulfides [135] was catalyzed by a chiral ytterbium(III) triflate-binaphthol complex in the presence of diisopropylethylamine (Scheme 51) [136]. Thermal decarboxylations of bicyclic lactones such as 68 are known to yield dienes which may undergo subsequent pericyclic reactions [137] thus, the adducts of this process are potentially useful chiral building blocks. The nature of the substituent on the 2k component was found to be crucial for the realization of high enantioselectivity. 

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