Tantalum alkoxides preparation

A combination of DAT and a metal alkoxide other than titanium alkoxide serves as a poor catalyst for the epoxidation of allylic alcohols. However, the combination of DAT and silica-supported tantalum alkoxides (2a) and (2b) prepared from Ta(=CHCMe3)(CH2Cme3)3 and silica(5oo) shows high enantioselectivity in the epoxidation of E-allylic alcohols, though chemical yields are not very great (Scheme 4).3... 

Mesoporous oxides from elements other than sihca have been reported as early as 1994. Cieslaetal. [169] found that metals such as Sb, Fe, Zn, Pb,W, and Mo also form mesoporous oxides. However, many of the mesophases obtained were lamellar and were not porous after template removal (calcination). Antonelli and Ying reported the transformation of titanium, niobium, and tantalum alkoxides into stable mesophases [170]. Subsequently, mesoporous oxides based on zirconium, hafnium, and manganese have been synthesized (for a recent review on these materials see [171]). Bagshaw and Pinnavaia [172] prepared mesoporous alumina with worm-like pores and a specific surface area of more than 400 m g . Mesoporous alumina with surface areas above 700 m g have been reported by Vaudry et al. [173]. 

Figure 27.10 Ferroelectric strontium bismuth the larger grain size of the surface microstruc-tantalate (Sro BBi2.2Ta209) thin films prepared tures of the films prepared from the tantalum from the sols of Figure 27.9 (a) tantalum pen- pentaglycolate sols and the larger values of taethoxide sols and (b) tantalum pentaglyco- the remanent polarization (5Pr) obtained in late sols. SEM images of the top surfaces of these films compared with the films prepared the films are shown, as well as the ferroelectric from the tantalum alkoxide-based sol [47]. hysteresis loops measured in these films. Note...

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

Vinyl ethers have also been prepared by addition of alkoxides to acetylene,6 7 6 elimination from halo ethers and related precursors,6 8 and vinyl exchange reactions.6 Reaction of an electrophilic tungsten carbenoid with methylene phosphorane or diazomethane also produces vinyl ethers.9 Enol ethers have resulted from the reaction of some tantalum and niobium carbenoids with esters,10 and the reaction of phosphoranes with electrophilic esters.4... 

Preparation of heterometallic alkoxides between pairs of metals as similar to each other as aluminum and gallium (137) or even niobium and tantalum (138) were described. However, the formation constants of the latter derivative were found to be rather low, precluding its isolation in view of the dynamism of the equilibrium. 

The alkali alkoxide method has been extended to the preparation of alkoxides of the hexanuclear niobium and tantalum cluster units, e.g., [M6Xi2](OMe)2.4MeOH (where M = Nb or Ta and X = Cl or Br), and M2[Ta2Cli2](OMe)6.6MeOH. 

Thomas prepared tantalum and niobium penta-alkoxides by the method of Eq. (2.101). For example, the alcoholysis of tris-(dialkylamido) monoalkylimidotan-talum yielded pentaalkoxides very conveniently ... 

The bismuth aUcoxide was insoluble in methoxyethanol because of its own strong association however, it easily dissolved in methoxyethanol containing strontium alkoxide in Sr(OR)2 Bi(OR)3 molar ratios of >0.5 (Katayama and Sekine, 1991). As previously reported, the strong association of the bismuth alkoxide can be destroyed by the formation of a strontium-bismuth double alkoxide. The formation of the strontium-bismuth double alkoxide is discussed next. Also, the bismuth alkoxide was insoluble in strontium-tantalum or strontium-niobium double alkoxide solutions. The strontium-bismuth double alkoxide formed and then reacted with the tantalum or niobium alkoxide to prepare uniform alkoxide complex solutions (Kato et al., 1998). 

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