Lanthanide isopropoxides

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

Treatment of scandium, yttrium and the lanthanides with refluxing isopropyl alcohol in the presence of HgCl2 (10—3—10 4 equivalents) leads to the metal isopropoxide, which can then be crystallized from the excess isopropyl alcohol (equation 3).17,18... 

A large number of alkoxides of the lanthanides have been synthesized.169 Although the methoxides and ethoxides are involatile and insoluble, the isopropoxides will sublime and dissolve in organic solvents. A combination of mass spectral and cryoscopic measurements indicate a tetrameric structure for these compounds.170... 

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]. 

Fig 1 Chemical structure of initiators used in ROP of lactones and lactides.n) stannous oc-toate, b) aluminum isopropoxide, c) lanthanide isopropoxide. Lanthanum atoms are represented by gray circles and oxygen atoms by white circles. The black circle represents the bridging oxygen atom connecting all lanthanum atoms. Alkyl groups are omitted for clarity... 

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]. 

For the first time, catalytic activity of lanthanide isopropoxides was detected in the TBHP (tert-butylhydroperoxide) assisted oxidation of allylic alcohols to epoxyalcohols (Eq. 26) [231], For example epoxy geraniol was obtained in up to 96% yield by using YbfOiPr in the presence of molecular sieves (4 A) [232]. 

Catalytic ring opening of epoxides and aziridines was also observed (Eq. 27). The acetone cyanohydrine reaction provided j8-hydroxy nitrile and / -amino nitriles, with the lanthanide isopropoxides exhibiting a higher reactivity than Et3N [233]. 

Transesterification [236] and the ester exchange reaction [237] were reported to be efficiently catalyzed (Eqs. 30 and 31). Either of the exchange reactions are sensitive to steric constraints of the substrates and to metal ion size. For example, transesterification is most applicable to primary alcohols. Increased catalytic activity in the presence of larger lanthanide centers is explained by enhanced coordinative unsaturation and increased basicity of the alkoxide complexes. Strong basicity of the lanthanide isopropoxides is considered to catalyze effectively the transhydrocyanation from acetone cyanohydrin to sev-... 

Insertion complexes of lanthanide isopropoxides with isocyanate can act as carbon dioxide carrier for the carboxylation of active methylene compounds , as exemplified in Scheme 25 [241], The catalytic formation of urethane is based on insertion of isocyanate into the La-OtBu bond of La3(OfBu)9(THF)2 [242]. 

Scheme 25. Lanthanide isopropoxides involved in carbon dioxide transport...

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

Compounds of the type Ln[Al(OC3H7)4]3 have been reported [205]. The compounds are monomeric in benzene and a structure consists of a six-coordinate lanthanide ion in which one half of the isopropoxide ions form bridges of the type Ln-O-Al. 

The complexes formed between lanthanide isopropoxides and 2-hydroxy-1-naphthyli-dene-n-butylamine and 2-hydroxy- 1-naphthylideneaniline in benzene show the formation of different products with different molar ratios of the reactants. In these complexes the ligand is coordinated in the anionic form to the lanthanide [275], It has been pointed out earlier in the synthesis section that it is necessary to synthesize these complexes in anhydrous media because of the weaker donor capacity of the ligands. 

Lanthanide isopropoxides, usually written Ln(OPr )3, but more likely to be oxo-centred clusters Ln50(0Pr )i3, are used, not just as starting materials for the synthesis of catalysts such as the naphthoxides but also as catalysts in their own right. They have been used in the Meerwein-Ponndorf-Verley reaction, where carbonyl compounds are reduced to alcohols, recent studies having shown that the reaction takes place exclusively by a carbon-to-carbon hydrogen transfer. 

Lanthanide aluminates (LnAlOJ are of interest as buffer layers in controlling the overgrowth of various perovskite films and as alternative gate dielectrics to Si02. Using the singlesource heterometallic lanthanide aluminum isopropoxide precursors [LnAl(OPr )6(HOPr )]2... 

Lanthanide isopropoxides were introduced as the first-generation alkoxide-type precatalysts (Structures 1-3) [133]. They proved to be more effective in the catalytic ring-opening of epoxides and aziridines than Et3N [134]. The acetone cyanohydrin reaction provided 5-hydroxynitriles and /3-aminonitriles. Strong basicity of the lanthanide isopropoxides is considered to catalyze the transhydrocya-nation effectively from acetone cyanohydrin to several aldehydes and ketones [135]. YbBu3 exhibited similar catalytic activity in this reaction. 

Heterobimetallic isopropoxides of two lanthanide metals (Sm and Yb) in the divalent state were prepared recently (75) by the following reaction ... 

The close relationships in the structural features of homo- and heterometallic alkoxides can be exemplified by those of Al Al(0-i-Pr)4 3 (150) and Ln Al(0-/-Pr)4 3 (18). The molecular weights of the latter in benzene correspond to their empirical formulas. All of these compounds can be distilled in the range of 200-180°C/0.1 mm, with a lowering of the boiling point as was expected from increasing the covalent character which results from lanthanide contraction in the series. The tetrameric aluminum isopropoxide A1 A1(0-/-Pr)4 3 or A1(0-/-Pr)3 4, however, disproportionates and distills as a dimeric vapor around... 

A number of double isopropoxides of lanthanides with A1 have been prepared, by the reactions ... 

Lamellar nanohybrids composed of Ln203 layers regularly separated from each other by organic layers of intercalated benzoate molecules can be obtained by a one-pot procedure (Karmaoui et al., 2006), the benzyl alcohol route. Lanthanide isopropoxides are simply dissolved in benzyl alcohol and reacted at high temperature (250-300 °C), resulting in the isolation of nanoparticles of 50-nm mean size. Eu -doped nanohybrids have better radiance characteristics than the standard phosphor Y203 Eu while both yttrium- and gadolinium-based nanomaterials doped with Nd display intense NIR luminescence, with a Nd( F3/2) lifetime of 49 ps in the case of the yttrium nanohybrid (Sa Ferreira et al., 2006). 

Thus, although the yttrium isopropoxide certainly has its merits, the limited availability of different yttrium alkoxides also limits the array of macromolecular structures that can be prepared. An approach to more flexible systems has been developed, in which the active initiator is generated in situ The concept is based on sterically very crowded tri(2,6 i-t-butylphenoxy) lanthanides that can not initiate lactone polymerization, but do exchange with sterically less crowded alcohols to generate alkoxides that turn into active initiators for e.g. lactide, E-caprolactone or 5-valerolactone polymerization. In this way, any alcohol can effectively be used to functionalize a polyester chain as shown in Fig 2 (vide supra). In a similar way the in situ formation of such a catalyst/initiator system from the commercially available [tris(hexamethyldisilyl)amide]Yttrium with isopropanol can be performed . 

Samarium, neodynium, and yttrium isopropoxide have been used in combination with benzyl alcohol, which is a transfer agent, for the -CL polymerization. The catalytic activity is higher compared with aluminum and zirconiiun coim-terparts. The apparent propagation rate constant first increases with the alco-hol/metal molar ratio up to an optimum value, and then decreases according to an egg-shaped curve (20,57). Once again, the use of a catal5d ic amount of lanthanide alkoxides is a valuable strategy to decrease the polyester contamination by metallic residues. 

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