Lanthanum alkoxides

Enantioselective trimethylsilylcyanation of benzaldehydes has been achieved using a lanthanum alkoxide of a chiral binaphthol as catalyst. ... 

Shibasaki et al. have reported an asymmetric nitroaldol reaction catalyzed by chiral lanthanum alkoxide 18 to produce an optically active 2-hydroxy-1-nitroalkane with moderate-to-high enantiomeric excesses (Scheme 8B1.10) [27]. Apparently this novel catalyst acts as Lewis base. The proposed reaction mechanism is shown in Scheme 8B1.11, where the first step of the reaction is the ligand exchange between binaphthol and nitromethane. This reaction is probably the first successful example of the catalytic asymmetric reaction promoted by a Lew i s base metal catalyst. Future application of this methodology is quite promising. 

Another chirally modified Lewis acid catalyst used in the borane reduction is the chiral lanthanum alkoxide [114]. The reaction of lanthanum triisopropoxide with enantiopure binaphthol gave a catalyst system for the borane reduction of ketones. Reduction of 6 -methoxy-2 -acetonaphtone gave the corresponding secondary alcohol in 100% yield with 61.8% ee (S) [114]. 

The actinides and lanthanides or f-block metal alkoxides are the last group. According to the report of Nawaratna and co-workers, yttrium isopro-poxide has been catalyzed transesterification reaction of fatty acids (reaction 7.13) [78]. Hatano et al. have used this strategy in the presence of lanthanum alkoxides for transestrification of carboxylic esters and preparation of new esters [79]. 

The first example of enantioselective trimethylsilylcyanation of aldehydes catalyzed by chiral binaphthol and binaphthol-modified lanthanum alkoxides shows an obvious effect of substituents at the 3,3 -positions of BINOL. The 3,3 -bis(methoxyethyl)-BINOL had the advantage over simple BINOL to give (S)-products in excellent yields with 73% ee (119). Chiral lanthanum and samarium complexes catalyze the Michael addition of thiols to a, )3-unsaturated carbonyl compounds (120). 

Alkoxides of Yttrium, Lanthanides, and Actinides 3.4.4.1 Yttrium and lanthanum alkoxides... 

A number of yttrium and lanthanum alkoxides have been investigated by and spectral studies by the research groups of Evans and Bradley ef in an attempt to provide preliminary evidence for the structures of... 

Lanthanum alkoxides have been the subject of considerable research in the 1990s. The first chloride-free alkoxo compound to be structurally characterized by X-ray crystallography was [La3(/u,3-OBu )2(/r-OBu )3(OBu )4(Bu OH)2] having a cluster of octahedra (Fig. 4.25). ... 

Lanthanum alkoxides have been used in the catalysed transhydrocyanation from acetone cyanohydrin to other aldehydes or ketones and also in the reactions of silyl... 

The promising results obtained with Sn(Oct)2, Al(Oi-Pr)3 and Zn(Lact)2 have stimulated the evaluation of a number of homoleptic complexes featuring alkoxy and carboxy ligands [2bj. As representative examples, trivalent yttrium and lanthanum alkoxides Ln(OR)3 (Ln = La, Y and R = i-Pr, n-Bu) have proved to be much more active than the related aluminum alkoxides, and to efficiently promote the... 

Lanthanide compounds such as yttrium and lanthanum alkoxides have been reported to yield high-molecular-mass polyesters under mild conditions. The yttrium alkoxide-initiated polymerization of CL proceeded rapidly at room temperature [27-29], while the use of bulky groups reduced the transesterification reaction such that polymers with a narrow molecular mass were obtained (Scheme 11.3). For example, the bulky phenoxide ligands of the yttrium or lanthanide catalyst were exchanged for the smaller alcohol (2-propanol), followed by coordination and insertion of the monomer (CL) [27]. 

Asymmetric amplification has also been observed in lanthanum-catalyzed nitro-aldol reaction, Shibasaki used a chiral lanthanum complex 15 prepared from LaCl3 and dilithium alkoxide of chiral BINOL for the enantioselective aldol reaction between naphthoxyacetaldehyde 14 and nitromethane (Scheme 9.11) [26]. When chiral catalyst 15 was prepared from BINOL with 56% ee, the corresponding aldol adduct 16 with 68% ee was obtained. This result indicates that the lanthanum 15 complex should exist as oligomer(s). 

Modification by acetylacetone is a powerful route, that allows precursor solutions to be stabilized. Interaction of titanium alkoxides with acetylacetone was extensively studied and reviewed in [1391,86]. Study ofreactions, occurring on interaction of Zr(OPrn)4 and Ti-Zr alkoxide mixture with acetylacetone, was performed in [1448] and allowed the authors to simplify the technique for preparation of precursor solution for PZT films application and to overcome the requirement of prolonged refluxing, which certainly decreases reproducibility. After dissolution of titanium and zirconium alkoxides in methoxyethanol, acetylacetone is added to form stable zirconium and titanium stock solutions. The introduction of acetylacetone allowed aqueous lead acetate (and lanthanum acetate for PLZT films) solutions to be added to mixed titanium and zirconium solutions. No reaction steps involving elevated temperatures or distillation or long reaction times are required. The solution could be used both immediately on mixing or after storage for several months. Such solutions were successfully used for application of ferroelectric films. 

Synthetic strategies to alkoxide complexes have been covered in full by previous reviews [14]. The silylamide route proved to be an advantageous method of preparation, especially in the case of homoleptic derivatives [15]. The group (IIIA) elements - scandium, yttrium and lanthanum - are considered as lanthanides on the basis of their general chemical similarity to the true lanthanides. 

More recently, a very efficient yttrium-based catalyst, yttrium 2-methoxyeth-oxide, has been applied successfully for /f-butyrolactone polymerisation which proceeded easily at room temperature [99], It is worth mentioning that rare-earth metal alkoxides (derived from yttrium and lanthanum) exhibit outstanding efficiency as catalysts for the polymerisation of cyclic esters such as e-caprolactone [132] and lactide [133]. 

The structures of three of the compounds have been determined they show essentially octahedral coordination of the lanthanide with bond angles around 90°. Average lanthanum-carbon distances are 2.563(18) A (Ho) 2.57(2) A (Er), and 2.53(2)A (Lu). More complicated lanthanide methyl species have been synthesized by another route, involving reaction of main-group methyls, Lewis acids, with lanthanide alkoxides " and amides, " a process of the type implicated in the lanthanide-catalyzed polymerization of conjugated dienes... 

It was proposed that a Lewis acid lanthanum center controls the direction of the carbonyl function and activates the enone while the sodium alkoxide forms enolate intermediates and regenerates the catalyst by hydrogen abstraction (Scheme 6). Other Ln/alkali metal combinations, including La/Li, show negligible asymmetric induction, yet give almost racemic products in excellent yield. In contrast, alkali-metal free BINOL ester enolate complexes catalyze Michael reactions with high enantioselectivities, albeit at lower temperatures. 

With this arrangement of ligands, each lanthanum is seven coordinate and each sodium is five coordinate. In general, a coordination number of 6 or less is the most common for yttrium, lanthanide alkoxide, and aryloxide complexes (4a, 6). A rare example of an alkoxide complex that contains seven-coordinate yttrium is the cyclic decamer [Y(OC2H4OMe)3]10 (see Section V.C.l Fig. 56). 

Cyclization. On treatment with La[N(SiMe3)2]3, a 4-alkynol forms the alkoxide which cyclizes to give 2-methylenetetrahydrofuran the isomer with endocyclic double bond is also formed from 3,4-pentadienol. A modified trisfhexamethyldisilazide) of lanthanum (and of several other rare earth metals) (1) catalyzes intramolecular hydroamina-tion of alkynylamines and the products also undergo hydrosilylation. ... 

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