Lanthanide alkoxides preparation

Chirally modified lanthanide alkoxides [prepared in situ from Ln(0-/-Bu)3 and chiral ligands] give higher asymmetric induction in the reduction of acetophenone (for La. 32%ee) than the normally used aluminum or alkali metal alkoxides208. However, the decrease of ee values with increasing reaction time shows that kinetic control is not maintained with 2-propanol as the reducing agent. 

Bis(ethylacetoacetonate)-lanthanide(III) alkoxides, represented by structure (314), also initiate the well-controlled ROP of CL.895 Mn increases linearly with conversion (with Mw/Mn<1.10 throughout), and increasing [M]0/[I]o- Kinetic analysis implies a first order dependence on the lanthanide initiator, consistent with a non-aggregated active site. Block copolymers with moderately narrow polydispersities (1.25-1.45) have also been prepared using these initiators. NMR spectroscopy confirms well-controlled block sequences suggesting that these initiators are less susceptible to transesteriflcation than other lanthanide alkoxides. Initiation occurs exclusively at the alkoxide bond, and the tris(ethylacetoacetonate) analogs are inactive under the same conditions. 

Yttrium isopropoxide and yttrium 3-oxapentoxide initiators were the first lanthanide alkoxides described in the literature for the ROP of e-CL [93]. The discovery of lanthanide-based initiator systems allowed the block copolymerization of e-CL with compounds such as ethylene [94], tetrahydrofuran [95], L-LA [96], trimethylene carbonate [97], and methyl methacrylate [98]. This type of initiator has also been used to prepare poly((3-butyrolactone)s [99,100]. 

Lanthanide alkoxide complexes can be prepared using a number of methods. The key difference lies in the nature of lanthanide starting materials. These include elemental metals, halides, alkoxides, amides, carboxylates, hydrides, and organometallic species [1, 11], The organic ligands come from aliphatic alcohols, phenols, or their metal salts. 

Namy, J. L., Souppe, J., Collin, J., Kagan, H. B. New preparations of lanthanide alkoxides and their catalytical activity in Meerwein-Ponndorf-Verley-Oppenauer reactions. J. Org. Chem. 1984,49, 2045-2049. 

Other small-scale laboratory procedures have been developed for the direct synthesis of the more reactive THF adducts, avoiding inconvenient high temperature treatment [59-62]. For example, the preparation of LnCl3(thf)x from metal powder and hexachloroethane is facilitated by sonication [Eq. (1)] [59]. Additional metal-based synthetic routes include the redox transmetallation with mercury(II) halides [Eq. (2)] [60] and the reaction with trimethylsilyl chloride and anhydrous methanol [Eq. (3)] [61]. Ammonia has been employed as an alternative donating solvent in the synthesis of lanthanide alkoxides starting from lanthanide chlorides [63]. 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

Lanthanide-doped inverse photonic crystals have been reported.282 The lattices were prepared by infilling self-assembled polystyrene sphere templates with a mixture of zirconium alkoxide and europium at 450 °C, the polystyrene spheres were burnt out leaving hollow spheres of air, and the infilled material was converted to Zr02 Eu3+. The PL properties of the resulting photonic lattice were reported.282 The possibility of including phosphors into photonic lattices could lead to many... 

Some well-defined scandium alkoxides have been synthesized and studied (together with lanthanide analogues). The compounds are [Sc(OR)3], Sc(OR)3(THF), Sc(OR)3(tppo) and (RO)2Sc(u-OR)2Na(THF), where R = 2,6-di-t-butyl-4-methylphenate. [Sc(OR)3] was prepared by reaction of Sc N(SiMe3)2 3 with ROH in hexane at 20°C followed by reflux, or by reaction of NaOR and ScCl3 in boiling THF, or by sublimation of Sc(OR)3(THF). The latter and... 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

Shibasaki et al. also developed a barium complex (BaB-M, 14, Scheme 5) for the aldol reaction of acetophenone (la), making use of the strongly basic characteristic of barium alkoxide. The catalyst was prepared from Ba(0-z-Pr)2 and BINOL monomethyl ether, and the products were obtained in excellent yield with up to 70% ee (Scheme 6) [8], Shibasaki et al. attempted to incorporate a strong Bronsted base into the catalyst and developed a lanthanide heterobime-tallic catalyst (15) possessing lithium alkoxide moieties, which promoted the aldol reaction with up to 74% ee (Scheme 6) [9]. Noyori and Shibasaki et al. reported a calcium alkoxide catalyst (16) that was prepared from Ca[N(SiMe3)2]2,... 

It is necessary to mention in particular the application of carboxylates of lanthanides instead of halides in the reactions with Li, Na, and K, alkoxides for the preparation of M(OR)3 -derivatives of almost all the lanthanides patented by Ozaki. The methoxides and n-buthoxides were thus obtained by interaction of formates with NaOR, the ethoxides by that of propionates with LiOEt, n-and isopropoxides by reaction of acetates or bensoates with LiOPr, t-buthox-ides by that of oxalates with KOBu [1246]. In addition to carboxylates for the interaction with NaOR using the easily accessible anhydrous Ln(OCOCCl3)3 [1494, 1159] was proposed. The adducts of Ln(N03)3 with glycols or polyethers were used for the preparation ofphenoxides [73]. 

Metals. Lanthanide metals are also considered as valuable precursors. For example, alkoxides derived from cheap and low-boiling-point alcohols have been alternatively synthesized from metals in the presence of HgQ2 as catalyst [133]. Representative and specific methods of preparation include transmetalla-tion reactions (Eq. 7-9) [134], using ammonia solutions of ytterbium and europium as synthetic reagents (Eq. 10) [135] and the generation of thiolate complexes from disulfides (Eq. 11) [136],... 

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. 

We have noted the affinity of lanthanides for water and this affinity is extended to alcohols to form alcoholates [180]. Alcoholates are generally unstable. Lanthanide triisopropoxide has been synthesized and characterized [204], The lanthanide triisopropoxide may be used as a starting material in the preparation of other alkoxides by alcoholysis. Triisopropoxides of lanthanides are reported to be in dimeric and tetrameric forms as evidenced by mass spectrometry. 

Answer 4.15 (i) The chemicals are readily available. The chloride is fairly insoluble in organic solvents and it is possible that chlorine is retained in the product, (ii) There is no need to make the alkali metal alkoxide, as the alcohol is the starting material. The metal may need careful cleaning and there may be the need for heating and a catalyst. One product is a gas. (iii) The lanthanide silylamide has to be prepared first. Because of the bulky nature of the ligand, it may be inert to substimtion, but there are no problems with chloride retention and the reaction should be clean. 

This silylamide route has attracted enormous attention as pure alkoxide complexes may be obtained directly [23]. The success of this reaction hinges upon the volatility of the amine byproduct that drives the reaction to completion [24]. Apotential concern is that the amide starting complexes are prepared by the reaction of lanthanide halides with alkali metal amides. High-purity, halide-free amides are thus critical to the success of this synthetic method. 

As with aliphatic alkoxides, aryloxides have also been used extensively for the preparation of lanthanide complexes. Among the many attractive features is the ease of manipulation of the... 

Although so far utilized (64a) only for the preparation of aryloxide derivatives, ether adducts of the lanthanide nitrates, such as [Ln(N03)3-Me0(CH2CH20)4Me], may also prove to be convenient starting materials for their alkoxide analogues by reactions with alkali alkoxides. 

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