Catalyst activities, lanthanide complexes

Scheme 12) [20a]. Shibasaki et al. [20b] used a chiral in situ generated lanthanide complex (64) as catalyst. The optically active lanthanide complex 66 is postulated as the basic intermediate, activating the nitromethane as shown in 67. However, in the case of the Mukaiyama aldol addition, lanthanide Lewis acids still give moderate ee values. 

Danishefsky et al. were probably the first to observe that lanthanide complexes can catalyze the cycloaddition reaction of aldehydes with activated dienes [24]. The reaction of benzaldehyde la with activated conjugated dienes such as 2d was found to be catalyzed by Eu(hfc)3 16 giving up to 58% ee (Scheme 4.16). The ee of the cycloaddition products for other substrates was in the range 20-40% with 1 mol% loading of 16. Catalyst 16 has also been used for diastereoselective cycloaddition reactions using chiral 0-menthoxy-activated dienes derived from (-)-menthol, giving up to 84% de [24b,c] it has also been used for the synthesis of optically pure saccharides. 

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. 

There has also been some interest in NHC-lanthanide complexes as polymerisation catalysts. Indenyl and fluorenyl functionalised NHC complexes of structures 14 and 15 (Fig. 4.5) were evaluated for isoprene polymerisation following activation... 

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 a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

Lactones, via indium compounds, 9, 686 Lactonizations, via ruthenium catalysts, 10, 160 Ladder polysilanes, preparation and properties, 3, 639 Lanthanacarboranes, synthesis, 3, 249 Lanthanide complexes with alkenyls, 4, 17 with alkyls, 4, 7 with alkynyls, 4, 17 with allyls, 4, 19 with arenes, 4, 119, 4, 118 and aromatic C-F bond activation, 1, 738 bis(Cp ), 4, 73... 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

The pentamethyl cyclopentadiene lanthanide complexes containing hydrocarbyl substituents have been studied extensively for their applications in homogeneous catalysis and C-H activation. The well-known catalyst of the Ziegler-Nutta type Cp2LnMe(Et20) is typical of the large number of compounds [155] that have been studied. Solvent-free electrophilic alkyl derivatives serve as precursors of the majority of the compounds which have been studied. 

The potential use of non-solvated lanthanide cyclopentadienyl hydride complexes as catalysts in alkene C-H bond activation, hydrogenation of alkynes led to synthesis of aluminum hydride organo lanthanide complexes. Examples of such complexes with polymeric structure and chain structure have been characterized [251]. 

Although the development of a variety of Lewis acids has enabled the reahzation of a wide range of catalytic asymmetric reactions, most of the catalysts have limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activates only one side of the substrate in an intermolecular reaction, whereas the latter not only can activate both sides of the substrate but also can control the orientation of the substrate. If this kind of synergistic cooperation could be realized in synthetic asymmetric catalysis, it would open up a new field in asymmetric synthesis, and a wide range of applications might well ensure. In this section we discuss asymmetric two-center catalysis promoted by chiral lanthanide complexes with Lewis acidity and Brpnsted basicity [44,45]. 

When certain cyclodipeptides are used as catalysts for the enantioselective formation of cyanohydrins, an autocatalytic improvement of selectivity is observed in the presence of chiral hydrocyanation products [80]. A commercial process for the manufacture of a pyrethroid insecticide involving asymmetric addition of HCN to an aromatic aldehyde in the presence of a cyclic dipeptide has been described [80]. Besides HCN itself, acetone cyanohydrin is also used (usually in the literature referred to as the Nazarov method), which can be activated cata-lytically by certain lanthanide complexes [81]. Acetylcyanation of aldehydes is described with samarium-based catalysts in the presence of isopropenyl acetate cyclohexanone oxime acetate is hydrocyanated with acetone cyanohydrin as the HCN source in the presence of these catalytic systems [82]. 

Most of the lanthanide complexes that readily polymerize ethylene are inactive in propylene polymerization [48]. Thus, the complexes [ (Me3Si)2NC (NPr )2 2Ln(/i-H)]2 (Scheme 25) were also tested in catalysis of propylene and styrene polymerization. The yttrium derivative had low activity in propylene polymerization. Over a period of 2 h the monomer absorption reached 58 mol per mole of catalyst, whereupon the catalytic activity was lost. Complexes with Ln = La, Sm, Dd, and Yb were even less active and became inactive after 15-20 min. In styrene polymerization, only derivatives of smallest lanthanide metals showed catalytic activity. The Lu complex initiated polymerization of styrene (20°C, neat styrene, 5% of Lu complex), and 90% conversion was reached in 6 days. The polystyrene obtained had a high molecular weight Mn = 811,000gmor = 1250,000gmoF ), a narrow molecular-weight... 

Anionic bridged bis(amidinate) lithium lanthanide complexes have been found to be efficient catalysts for the amidahon of aldehydes with amines under mild conditions (Scheme 56). The achvity was found to follow the order of yttrium < neodymium < europium ytterbium. The catalysts are available for the formahon of benzamides derived from pyrrolidine, piperidine, and morpholine with good to excellent yields. In comparison with the corresponding neutral complexes, the anionic complexes showed higher achvity and a wider range of scope for the amines. A cooperation of the lanthanide and lithium metals in this process was proposed to contribute to the high activity of these catalysts [66,67]. 

Sc(OTf)3 and Yb(( ) l r), are quite valuable catalysts of the aza-DA reaction of 102 [204] (Scheme 10.113). Wifh these catalysts, three-component coupling of aldehydes, anilines, and 102 proceeds smoothly [304]. Sc(OSO2C8Fi7)3 enables an efficient aza-DA reaction in supercritical CO2 [305]. Cationic lanthanide complexes, [(C5Me5)2Ce][BPh4] and fhe corresponding Sm and La complexes, have high catalytic activity in the HDA reaction of 102 with aromatic aldehydes [306]. 

Described in this paper is a model system - one in which well-characterized lanthanide complexes exhibit high catalyst activities for ethylene polymerization but where the corresponding oligomerization of propene is sufficiently slowed so that stepwise insertion of the olefin can be studied quantitatively and all important intermediates observed or isolated. Emphasized in this paper is the effect of added Lewis acids and bases on the rate of olefin insertions, and comparison between ethylene and propene reactions. The catalysts, of general structure M(ri -Cp )2CH3 L (M = Yb, Lu ... 

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